Global Warming

Why Global Warming Would be Good for You

A Shortened version of this article entitled "Why Global Warming Would be Good for You" appeared in The Public InterestWinter 1995 without footnotes, tables, the chart or references. The complete version was published later in 1995 in the Hoover Institution Working Paper series as

GLOBAL WARMING: A Boon to Humans and Other Animals

Thomas Gale Moore
Senior Fellow
Hoover Institution

Climate extremes would trigger meteorological chaos -- raging hurricanes such as we have never seen, capable of killing millions of people; uncommonly long, record-breaking heat waves; and profound drought that could drive Africa and the entire Indian subcontinent over the edge into mass starvation. ... Even if we could stop all greenhouse gas emissions today, we would still be committed to a temperature increase worldwide of two to four degrees Fahrenheit by the middle of the twenty-first century. It would be warmer then than it has been for the past two million years. Unchecked it would match nuclear war in its potential for devastation.[1]

-- Senate Majority Leader George J. Mitchell

Senator Mitchell's forecast and his history are both wrong. Warmer periods bring benign rather than more violent weather. Milder temperatures will induce more evaporation from oceans and thus more rainfall -- where it will fall we cannot be sure but the earth as a whole should receive greater precipitation. Meteorologists now believe that any rise in sea levels over the next century will be at most a foot or more, not twenty.[2] In addition, Mitchell flunks history: around 6,000 years ago the earth sustained temperatures that were probably more than four degrees Fahrenheit hotter than those of the twentieth century, yet mankind flourished. The Sahara desert bloomed with plants, and water loving animals such as hippopotamuses wallowed in rivers and lakes. Dense forests carpeted Europe from the Alps to Scandinavia. The Midwest of the United States was somewhat drier than it is today, similar to contemporary western Kansas or eastern Colorado; but Canada enjoyed a warmer climate and more rainfall.

Raising the specter of disaster as well, Vice President Al Gore has called the threat of global warming "the most serious problem our civilization faces."[3] In fact, he has styled those who dispute it as "self-interested" and compared them to spokesmen for the tobacco industry who have questioned the relation of smoking to cancer. But Gore is misinformed; many disinterested scientists, including climatologists with no financial interest other than preventing wasteful expenditures of society's limited resources, question the evidence and the models that underlie the warming hypothesis.

In fact, the evidence supporting the claim that the earth has grown warmer is shaky; the theory is weak; and the models on which the conclusions are based cannot even replicate the current climate. It is asserted, for example, that over the last hundred years the average temperature at the earth's surface has gone up by 0.5deg. Centigrade or about 1deg. Fahrenheit. Given the paucity of data in the Southern Hemisphere, the evidence that in the United States, with the best records, temperatures have failed to rise; the British naval records that find no significant change in temperatures at sea since the mid-1800s; and that the reported increases occurred mainly prior to 1940 -- before the rapid rise in CO2 -- the public is entitled to be wary. Moreover, even the National Academy of Sciences is skeptical of the validity of the computer models and warns that the modeling of clouds -- a key factor -- is inadequate and poorly understood.[4]

The dire forecasts of global warming hinge on a prediction that human activity will provoke a continued upsurge in atmospheric carbon dioxide. Many environmentalists believe that the burning of fossil fuels, the release of methane from agricultural activities, and the escape of other chemicals into the air over the next few decades will lead to an effective doubling of greenhouse gases sometime in the next century. Although fluctuations in CO2 correlate with climate shifts, the record cannot distinguish whether they followed the temperature changes or preceded them.[5] Theory suggests either is possible.

What is well known is that climate changes. The world has shifted from periods that were considerably warmer -- during the Mesozoic era when the dinosaurs thrived the earth appears to have been about 18deg. Fahrenheit warmer than now -- to spells that were substantially colder, such as the Ice Ages when huge glaciers submerged much of the Northern Hemisphere.[6] One paleoclimatologist estimated that, during the Precambrian period, the polar regions were about 36deg.F colder than they are in the contemporary world.[7] During the last interglacial, about 130,000 years ago or about when modern man was first exploring the globe, the average temperature in Europe was at least 2deg. to 5deg.F warmer than at present.[8] Hippopotamuses, lions, rhinoceroses and elephants roamed the English countryside. Areas watered today by the monsoons in Africa and east Asia enjoyed even more rainfall then. Indeed during the last 12,000 years, that is since the end of the last glacial period, the globe has alternated between times substantially warmer and epochs that were noticeably cooler.

An examination of the record of the last twelve millennia reveals that mankind prospered during the warm periods and suffered during the cold ones. Transitions from a warm to a cold period or vice-versa were difficult for people who lived in climates that were adversely affected yet benefited others who inhabited regions in which the weather improved. On average, however, humans gained during the centuries in which the earth enjoyed higher temperatures. In writing about the effect of climate change on human development, Senator and now Vice-President Al Gore admits:

The archaeological and anthropological records indicate that each time the ice retreated [during the ice ages], the primitive peoples of the Eurasian landmass grew more populous and their culture more advanced. ... Then, 40,000 years ago, the so-called cultural explosion of tools and jewelry may have coincided with an unusually warm millennium in Europe.[9]

Expected Effects of Global Warming

Although most of the forecasts of global warming's repercussions have been dire, an examination of the likely effects suggests little basis for that gloomy view. Climate affects principally agriculture, forestry, and fishing. Manufacturing, most service industries, and nearly all extractive industries are immune to climate shifts. Factories can be built in northern Sweden or Canada or in Texas, Central America, or Mexico. Banking, insurance, medical services, retailing, education and a wide variety of other services can prosper as well in warm climates (with air-conditioning) as in cold (with central heating). A few services, such as transportation and tourism, may be more susceptible to weather. A warmer climate will lower transportation costs: less snow and ice will torment truckers and automobile drivers; fewer winter storms -- bad weather in the summer has less disruptive effects and is over quickly -- will disrupt air travel; a lower incidence of storms and less fog will make water transport less risky. Hotter temperatures will leave mining and the extractive industries largely unaffected; they might even benefit oil drilling in the northern seas and mining in the mountains. A warmer climate could, however, change the nature and location of tourism. Many ski resorts, for example, might face less reliably cold weather and shorter seasons. Warmer conditions would mean that fewer northerners would feel the need to vacation in Florida or the Caribbean. On the other hand, new tourist opportunities might develop in Alaska, northern Canada and other locales at higher latitudes or in upper elevations.

A rise in world-wide temperatures will go virtually unnoticed by inhabitants of the advanced industrial countries. In his 1991 address to its members, the President of the American Economic Association asserted: "I conclude that in the United States, and probably Japan, Western Europe and other developed countries, the impact on economic output [of global warming] will be negligible and unlikely to be noticed."[10] As modern societies have developed a larger industrial base and become more service oriented, they have grown less dependent on farming, thus boosting their immunity to temperature variations. Warmer weather means, if anything, fewer power outages and less frequent interruptions of wired communications.

Only if warmer weather caused more droughts or lowered agricultural output would even Third World countries suffer. Should the world warm -- and there is little evidence or theory to support such a prognostication -- most climatologists believe that precipitation would increase. Although some areas might become drier, others would become wetter. Judging from history, Western Europe would retain plentiful rainfall, while North Africa and the Sahara might gain moisture. The Midwest of the United States might suffer from less precipitation and become more suitable for cattle grazing than farming. On the other hand, the Southwest would likely become wetter and better for crops.

A warmer climate would produce the greatest gain in temperatures at northern latitudes and much less change near the equator. Not only would this foster a longer growing season and open up new territory for farming but it would mitigate harsh weather. The contrast between the extreme cold near the poles and the warm moist atmosphere on the equator drives storms and much of the earth's climate. This difference propels air flows; if the disparity is reduced, the strength of winds driven by equatorial highs and Arctic lows will be diminished.

Warmer nighttime temperatures, particularly in the spring and fall, create longer growing seasons, which should enhance agricultural productivity. Moreover, the enrichment of the atmosphere with CO2 will fertilize plants and make for more vigorous growth. Agricultural economists studying the relationship of higher temperatures and additional CO2 to crop yields in Canada, Australia, Japan, northern Russia, Finland, and Iceland found not only that a warmer climate would push up yields, but also that the added boost from enriched CO2 would enhance output by 17 percent.[11] Researchers have attributed a burgeoning of forests in Europe to the increased CO2 and the fertilizing effect of nitrogen oxides.[12] Professor of Climatology Robert Pease writes that we may now be living in an "icehouse" world and that a warming of about two degrees Celsius, which is what his model indicates,

may actually make the earth more habitable. The higher temperatures combined with more carbon dioxide will favor plant and crop growth and could well provide more food for our burgeoning global populations. Geologic history reveals that warmer global temperatures produce more, not less, precipitation, a fact reflected by a recent scientific investigation that shows the Greenland ice-cap to be thickening, not melting. So much for the catastrophic prediction that our coastlines will be flooded by a rise in sea level from polar meltwaters.[13]

The United States Department of Agriculture in a cautious report reviewed the likely influence of global warming on crop production and world food prices. The study, which assumed that farmers fail to make any adjustment to mitigate the effects of warmer, wetter, or drier weather -- such as substituting new varieties or alternative crops, increasing or decreasing irrigation -- concludes that:

The overall effect on the world and domestic economies would be small as reduced production in some areas would be balanced by gains in others, according to an economic model of the effects of climate change on world agricultural markets. The model ... estimates a slight increase in world output and a decline in commodity prices under moderate climate change conditions.[14] [Emphasis added.]

Economists Robert Mendelsohn, William D. Nordhous, and Daigee Shaw researched the relationship of climate to land values in the United States.[15] After holding land quality, the proximity to urban areas and the nearest coast, and income per capita constant, they found that climate explained over two-thirds of the value of crop lands. They concluded that for the lower-48 states, a rise in average temperature of about 5deg.F and an 8 percent increase in rainfall stemming from global warming would, depending on their model, reduce the value of output between 4 and 6 percent or boost the value of output slightly. This result ignored the effect of increased CO2 on farm output. It is also consistent with the Department of Agriculture study that suggests the U.S. might see a slight fall in output while the rest of the world increased production.

Forestry is another sector that is potentially subject to change due to an increase in world temperatures. Canadian agricultural economists have examined the effect of a doubling of CO2 on forestry production. They concluded that increased carbon dioxide would boost productivity by 20 percent and that overall the harvest of timber in Canada would climb by about 7.5 percent.[16]

Historical Evidence

History provides the best evidence for the effect of climate change on humans, plants and animals, but a few researchers have challenged its relevance. David Rind, a climate modeler and NASA scientist, has questioned the applicability of past warming episodes to the modern issue of climatic alteration caused by increased CO2 concentrations.[17] He attributes the origin of past periods of warmth and cold to shifts over time in the orbital position of the earth which impose more or less energy on the poles, as contrasted to a general world-wide warming that might result from the addition of man-made greenhouse gases. [See Appendix A on factors determining climate]. He also argues that the swiftness in warming that would occur following increased levels of CO2 is unprecedented in history. On the latter point, he ignores other research, such as that by a German academic, Burkhard Frenzel, who writes, "During the Holocene, very rapid changes of climate occurred. According to dendroclimatology [tree ring analysis applied to climatology], they often lasted about 20 to 30 years, or [were] even as brief as 2 to 3 years."[18] Other climate historians have found that a rapid cooling in the late glacial period -- about 11,000 years ago --took about 100 to 150 years to complete and realized about 5deg.F variation in temperature within 100 years, more than is being forecast for the next century.[19]

Although changes in the earth's orbital position may easily have played a role in warming the earth after the last Ice Age, the effect was world-wide rather than concentrated in northern latitudes. Ice retreated in the Southern as well as in the Northern Hemisphere. Moreover, in the subsequent warming, from around 7,000 to 4,000 years ago, the climate around the world appears to have improved. Although the evidence for warming in the Southern Hemisphere is weaker, even if higher temperatures had been localized in one hemisphere or one continent, the effect on human beings would still tell us about the benefits or costs of climatic change. Dr. Rind argues that greenhouse warming would raise winter as well as summer temperatures while past warmings, driven by orbital mechanics, have raised summer temperatures alone. Even though his models suggest that these past warmings should have boosted temperatures solely in June, July, and August, the evidence, albeit a little tenuous for the three thousand year period of Climatic Optimum, supports warmer winters. For the Little Climate Optimum that coincided with the High Middle Ages, researchers have found strong support for mild winters.

Moreover, at a recent conference the Russians have put forward the hypothesis that past climate changes support the proposition that the cause of the warming or cooling is irrelevant; the pattern has been the same.[20] This conclusion, disputed by some, is based on a large number of past shifts in average weather conditions dating back millions of years. The Russians contend that the climate models overstate the amount of temperature change at the equator and understate it at the poles.

Measurement of Human Well-being

Since statistics on the human condition are unavailable except for the most recent centuries, I shall use indirect methods to demonstrate the influence of climate on man's well-being. A growth in the population, major construction projects, a significant expansion in arts and culture, all indicate that society is prosperous. If the population is expanding, food must be plentiful, disease cannot be overwhelming, and living standards must be satisfactory. In addition, if building, art, science, and literature are vigorous, the civilization must be producing enough goods and services to provide a surplus available for such activities. Renaissance Florence was rich; Shakespeare flourished in prosperous London; wealthy Vienna provided a welcome venue for Haydn, Schubert, Mozart, and Beethoven.

Clearly climate is far from the only influence on man's well-being. Governments that extort too much from their people impoverish their countries. A free open economy stimulates growth and prosperity. War and diseases can prove catastrophic. On the other hand, a change in climate has frequently been a cause of war or aided the spread of disease. A shift to more arid conditions, for example, impelled the Mongols to desert their traditional lands to invade richer areas. A cold wet climate can also confine people to close quarters, which can abet contagion. Moreover, a shift towards a poorer climate can lead to hunger and famine, which make disease more virulent.

Throughout history climatic changes probably forced technological innovations and adaptations. The shift from warm periods into Ice Ages and back again likely accelerated the evolution of modern man. Each shift would have left small groups of hominoids isolated and subject to pressures to adapt to new weather conditions. These shifts, especially to the more adverse conditions created by the spread of extreme cold, would put strong selection pressure on the human forebears that ultimately led to modern man. Even after Homo Sapiens started spreading across the earth, climate shifts fostered new technologies to deal with changed circumstances.

The influence of climate on human activities has declined with the growth in wealth and resources. Primitive man and hunter-gatherer tribes were at the mercy of the weather, as are societies which are still almost totally bound to the soil. A series of bad years can be devastating. If, as was the usual case until very recently, transportation is costly and slow, even a regionalized drought or an excess of rain can lead to disaster, although crops may be plentiful a short distance away. Thus variation in the weather for early man had a more profound influence on his life and death than do fluctuations in temperature or rainfall in modern times when economies are more developed. Since the time of the Industrial Revolution, climate has basically been confined to a minor role in human activity.

Climate History

Since its origins, the earth has experienced periods significantly warmer than the modern world -- some epochs have been even hotter than the most extreme predictions of global warming -- and times much colder than today. Today's cool temperatures are well below average for the globe in its more than four billion year history.[21] During one of the warmest such eras the dinosaurs roamed the earth and a rich ecological world flourished.

Studies of climate history show as was mentioned above that sharp changes in temperatures over brief periods of time have occurred frequently without setting into motion any disastrous feedback systems that would lead either to a runaway heating that would cook the earth or a freezing that would eliminate all life. In addition, carbon dioxide levels have varied greatly. Ice core data exhibit fluctuating levels of CO2 that do not correspond to temperature changes.[22] Most past periods display a positive relationship between CO2 and temperature, however, with a relationship roughly corresponding to that of the Global Climate Models.[23] During interglacial periods high latitudes enjoyed temperatures that were about 5deg. to 11deg.F warmer than today.[24] Middle latitudes experienced temperatures only about 4deg. to 5deg.F warmer. These warmer periods brought more moisture to the Northern Hemisphere with the exception during the Holocene of central North America. At the time of the medieval warm period, temperatures in Europe, except for the area around the Caspian Sea basin, were 1deg. to 3deg.F higher and rainfall more plentiful than today.[25]

This historical evidence is consistent with only some of the forecasts of the computer climate models. Most climate estimates indicate that a doubling of CO2 would generate greater rainfall in middle latitudes, and history shows that warm climates do produce more wet weather.[26] As has been found in the historical record, land temperatures should increase more than water thus strengthening monsoons. The models also predict that sea-surface temperatures in the tropics would be higher with increased CO2 but evidence from the past evinces no such relationship.[27]

Carbon dioxide concentrations may have been up to sixteen times higher about 60 million years ago without producing runaway greenhouse effects.[28] Other periods experienced two to four times current levels of CO2 with some warming. Scientists have been unable to determine whether the warming preceded or followed the rises in carbon dioxide. For virtually all of the period from around 125 million to about 75,000 years ago, CO2 levels were markedly higher than now.

The prevailing view among climatologists is that the Climatic Optimum -- 9,000 to 4,000 years ago -- resulted from orbital mechanics which increased summer radiation in the Northern Hemisphere, although winters received less heat than they do in the modern world.[29] The warmer summers melted the northern glaciers over several millennia. Warmer lands in the interior of northern continents and cooler oceans expanded the monsoons further north to bring greater rainfall to the Sahara, Arabia and southern and eastern Asia.[30] North of the monsoon area, the climate was drier than today. Anatolia, Northwestern Africa, parts of China and northern Japan experienced less rainfall.[31] By 4000 B.C., however, a slackening of the trade winds had produced warmer Atlantic ocean water off northwestern Africa, and as a consequence the Middle East, including Greece and modern Turkey, were enjoying more reliable rain.

If orbital variations produced the Climatic Optimum, the Southern Hemisphere should have been cooler. Between 10,000 B.C. and 7000 B.C., however, winter temperatures (June, July, August) below the equator warmed to higher levels than now while summer temperatures (December, January, February) were cooler than the modern world.[32] Rainfall over South America, Australia and New Zealand apparently was lighter than the present. Although the Southern Hemisphere moved out of the Ice Age with the Northern Hemisphere, its climate since then has not tracked well weather patterns north of the equator.[33] Data based on vegetation suggest that annual temperatures in New Zealand were coldest between 20,000 and 15,000 years ago, warmed subsequently and peaked between 10,000 and 8,000 years before the present -- somewhat earlier than they did in the Northern Hemisphere.[34] Temperatures appear to have been falling over the last 7,500 years. By 1500 B.C., the climate was quite similar to today's.[35]

Whether the whole globe warmed or not during the period 7,000 to 4,000 years ago is really irrelevant to the question of how hotter temperatures affect humans. If the Northern Hemisphere warmed, and there is good evidence that it did, then comparing how people survived in that portion of the globe provides information about how higher global temperatures would influence mankind.

Modern man apparently evolved into his current genotype between 40,000 and 200,000 years ago, probably in Africa during an Ice Age.[36] Around 150,000 years ago the extent of ice coverage reached a maximum, followed around 130,000 years before the present (YBP) by a rapid deglaciation.[37] The warm interglacial era, during which temperatures may have exceeded those forecast under a doubling of greenhouse gases, lasted about 15,000 years until the onset of renewed glaciation at 115,000 YBP. Over the next 100,000 years the glaciers fluctuated with the climate, but at no time did the average temperature equal the level of the previous interglacial epoch or reach the warmth of the last 10,000 years.[38]

In the thousands of years of the last Ice Age preceding the current warm epoch, man existed as a hunter-gatherer in a world that looked quite different from today's. Herds of large animals such as bison, mammoths, and elk roamed a largely treeless savanna in Europe. These beasts made easy prey for human hunters that enjoyed as a consequence a rich diet of wild animal meat plus, in season, local fruits and vegetables. It was during the Ice Age that the level of the ocean fell sufficiently that Asian peoples were able to migrate across what is now the Bering Strait but then was dry land. Most archaeologists date the first arrivals of humans in the Americas from around 15,000 years ago, although some have claimed evidence for an earlier arrival. No doubt the lower sea levels during the Ice Age also facilitated the arrival of the aborigines in Australia some 35,000 years ago.

Climatologists consider that the last Ice Age ended about 12,000 to 10,000 years ago when the glaciers covering much of North America, Scandinavia and northern Asia began to retreat to approximately their current positions. In North America the glacial covering lasted longer than in Eurasia because of topographic features that delayed the warming. Throughout history warming and cooling in different regions of the world have not been exactly correlated because of the influence of oceans, mountains, prevailing winds, and numerous other factors. Nevertheless, across the Northern Hemisphere large temperature shifts have occurred roughly together -- perhaps in some areas they have lagged other zones by a century or more. The correspondence between warming and cooling in the Northern Hemisphere and that in the Southern is less well known and may be less well correlated because of the predominance of water south of the equator and the existence of Antarctica.

Human progress, a few improvements in hunting tools and some cave art, was incredibly slow during the Ice Age -- a period whose length dwarfs the centuries since. Over the last 12 millennia of interglacial warmth, however, modern man has advanced rapidly. The growth in technology and living standards required a climate that was more hospitable than existed throughout that frozen period.

During the last Ice Age humans survived through hunting and gathering. Initially archeologists believed that these tribes, which typically consisted of 15 to 40 people, eked out a precarious existence.[39] Many modern archeologists, however, feel, based on studies of the few bands of hunter-gatherers that survived into the twentieth century, that they normally found plentiful foods in their forays and would rarely have been hungry. Modern primitive people, however, may not have been typical of earlier groups. The ones that did face food pressures would have adopted farming while those that found plentiful supplies in their environment would be less concerned with new ways of acquiring sustenance.[40] Food pressures could have arisen from either a change in climate that made previous ways-of-life untenable or an expansion of population in the region that began to overwhelm the natural supply.

As the earth warmed with the waning of the Ice Age, the sea level rose as much as 300 feet; hunters in Europe roamed through modern Norway; agriculture developed in the Middle East. For about 3,000 to 4,000 years the globe enjoyed what historians of climate call the Climatic Optimum period -- a time when average world temperatures -- at least in the Northern Hemisphere -- were significantly hotter than today. At its height between 4000 B.C. and 2000 B.C., H.H. Lamb, a leading climate historian, judges that the world was 4deg. to 5deg. Fahrenheit warmer than the twentieth century.[41] During the relatively short period since the end of glaciation the climate has experienced periods of stability separated by "abrupt transition."[42] Lamb calculates that at its coldest, during the Mini Ice Age, the temperature in central England for January was about 4.5deg.F colder than today.[43] He also concludes that in the central and northern latitudes of Europe during the warmest periods, rainfall may have been 10 to 15 percent greater than now and during the coldest periods of the Mini Ice Ages, 5 to 15 percent less.[44] On the other hand, cooler periods usually suffered from more swampy conditions because of less evaporation.

If modern humans originated 200,000 years ago, why did they not develop agriculture for the first 190,000 years? Even if Homo Sapiens Sapiens originated only 40,000 years ago, people waited 30,000 years to grow their first crops -- an innovation which yielded a more reliable and ample food supply. Farming developed first in the Middle East, right after the end of the last Ice Age -- a coincidence? The evidence suggests that from 11,000 to 9,000 years ago the climate became warmer and wetter in the Middle East shifting the ecology from steppe to open woodland.[45] This led to the domestication of plants and animals, probably because the warmer, wetter weather made farming possible. From its origins around 8000 B.C., agriculture spread northward, appearing in Greece about 6000 B.C., Hungary 5000 B.C., France 4500 B.C. and Poland 4250 B.C.46 Is it chance that this northward spread followed a gradual warming of the climate that made agriculture more feasible at higher latitudes?

As Anthropologist Mark Cohen writes, "If, as the archaeological record indicates, hunting and gathering was such a successful mode of adaptation over such a long period of time, and if most human populations are as conservative as anthropologists have observed them to be, we are faced with answering the question why this form of adaptation was ever abandoned."[47] He gives estimates of the efficiency of hunting and gathering that indicate that the latter was more efficient than farming -- at least for large game. He reports that when large animals are available, hunting brings 10,000 to 15,000 kilocalories per hour of hunting. However, if large animals are unavailable -- because the environment is poor or because they have all been killed -- hunting of small game will return only a few hundred to 1,500 kilocalories per hour devoted to the effort. Collecting and processing small seeds from such plants as wild wheat may produce only 700 to 1,300 kilocalories for each hour. Shellfish collection can produce 1,000 to 2,000 kilocalories per hour of work. On the other hand subsistence farming produces 3,000 to 5,000 kilocalories per hour devoted to agriculture.[48] This connotes that hunting large animals, when and if they are available, is the most economical method of subsistence, but if these beasts are exterminated or if the humans move to areas without such species, domestication of plants and animals can produce more food for the effort than any other strategy.

Moreover hunter-gatherers can only survive if the density of their population is low. Too many mouths would strain the environment and preclude survival. Once, humans developed farming which could support larger families and a denser population, however, the number of people did explode. Primitive tribes, dependent on hunting, scavenging, and collecting edibles to survive, had to hold their populations below what they would individually have preferred or nature kept them in check through periodic food shortages. A number of twentieth century hunter-gatherers have practiced infanticide and induced abortions to restrict the number and spacing of their children.[49] Constant travel by nomads may increase infant mortality, maternal mortality and produce more miscarriages than a sedentary life and thus have kept the numbers in check. In any case farming solved a major problem for primitive peoples. Once people settled down into fixed abodes, the population apparently ballooned.

Although many people view the current world's huge population with alarm, most ecologists take the size of the population of a species as an indicator of its fitness. By this criterion, the domestication of plants and animals improved greatly Homo Sapiens fitness. This essay is not the place to discuss the capacity of the globe to sustain the number of people expected to populate the world in the next century, but certainly anything that produced greater numbers of people thousands of years ago must have been beneficial for mankind.

Over history the number of humans has been expanding at ever more rapid rates. Around 25,000 years ago, the world's population may have measured only about 3 million.[50] Fifteen thousand years later, around 10,000 B.C., the total had grown by one-third to 4 million. It took 5,000 more years to jump one more million, but in the 1,000 years after 5000 B.C. it added another million. Except for a few disastrous periods, the number of men, women and children has mounted with increasing rapidity. Only in the last few decades of the twentieth century has the escalation slowed. Certainly there have been good times when man did better and poor times when people suffered -- although in most cases these were regional problems. However, as the following chart shows, in propitious periods, that is, when the climate was warm, the population swelled faster than during less clement eras.

This chart is based on a paper by economist Michael Kremer who argues that, until the Industrial Revolution, existing technology limited the size of the population.[51] As innovators discovered new techniques and invented new tools, more people could be fed and housed and the population expanded. Moreover, the greater the number of people, the more innovations would be hit upon. He assumed that every individual had an equal but very small probability of uncovering a new technique or device and that the probability of being an innovator was independent of the size of the population. Therefore the number of inventions would be proportional to the number of people. Thus as the world population expanded -- slowly at first -- the rate of technological innovation escalated and hence the rate of growth of the population that could be sustained. Only in recent times has technological change become so rapid that it has run ahead of population growth, leading to a rising standard of living, which in turn has reduced the birth rate.

Source: Michael Kremer (August 1993): Table 1 and the Author.

Kremer's hypothesis signifies that for most of history the rate of population growth should be proportional to the size of the population. To link his model and data with climate change, I started with his estimate of the world's people in 10,000 B.C. and calculated the rate of growth of the population over the next 5,000 years. For each subsequent period, I also computed the rate of increase in numbers of people. Comparing these expected rates with actual growth revealed eras in which the number of humans has expanded faster than predicted and periods during which the world's people has grown more slowly. The chart then shows the centuries in which the growth rate of the globe's populace has exceeded or fallen short of the rate expected under this simple model. As can be seen, warm periods have done considerably better than cold periods in terms of human expansion. The warmest period since the end of the last Ice Age produced the highest rate of population growth compared to what would have been expected -- in this era agriculture was spreading. Moreover, the Mini Ice Age, which saw the coldest temperatures in the last 10,000 years, underwent the slowest relative population expansion. This chart demonstrates that mankind has prospered in warm periods and the hotter the better!

Another measure of the well-being of humans is how long they live. The life of the hunter-gatherer was not as rosy as some have contended. Life was short -- skeleton remains from before 8000 B.C. show that the average age of death for men was about 33 and that of women 28.[52] Death for men was frequently violent, while many women must have died in childbirth. Since women died so young, they had only around thirteen years in which to bear children. Anthropologists have estimated that on average they could have given birth to less than five live babies, assuming that they bore a child every 22 months.[53] An infant and childhood mortality rate of about 60 percent would have kept the population stagnant.

Table 1
Life Expectancy at Various Periods

EuropeMesolithic, ca. 8000B.C.31.5
AnatoliaNeolithic, ca. 6000B.C.38.2
AustriaBronze Age, ca. 2000B.C.38.0
GreeceClassical, ca. 500 B.C.35.0
RomeClassical, 100 B.C.-A.D. 30032.0
EnglandA.D. 127648.0
Table 1 below shows some relevant data. The warmest periods, the Neolithic, Bronze Ages and England in the thirteenth centuries enjoyed the longest life spans of the entire record. The shortening of lives from the late thirteenth to the late fourteenth centuries with the advent of much cooler weather is particularly notable. Moreover, the rise in life expectancies during the warm period could easily explain the population explosion that took place during that period.

Good childhood nutrition is reflected in taller adults. Skeleton remains collected over wide areas of Eurasia from the period when roving bands shifted from eating large animals and a few plants to smaller prey and a much wider variety of foods attest to a decline in height for both men and women of about five centimeters (two inches).[54] The shorter stature came at the end of the Ice Age when large animals were disappearing. Some archaeologists have found that average age of death for adults also declined during this transitionary period.[55] Studies of bone chemistry from Middle Eastern skeletons indicate a reduction in meat consumption. The new diet although more dependent on grains, fruits, and vegetables must have been less nutritious than the old. As large game animals disappeared with the end of the Ice Age, humans widened the variety of plants in their diet, increasingly consuming vegetable matter that they had ignored for thousands of years either because it was less nutritious, more difficult to secure and process, or less tasty.

Table 2
Average Height of Icelandic Males

Period (A.D.)Mean Height
874-110068 inches
Mini Ice Age
1650-179666 inches
1700-180066 inches
Modern World
1952-195470 inches
Source: Lamb [1977]: 264 from Bergthorsson [1962].

Research on American Indians before the arrival of Europeans also reveals a decline in health between early periods and later.[56] The evidence for the Americas is more mixed, however, than for Europe. Based on the Eurasian studies and those of North American aborigines it seems safe to conclude that health and nutrition were declining before the advent of agriculture and that it may be that agriculture was invented to stave off further decreases in food availability. The absence of agriculture for most North American peoples may have reflected that their nutrition fell less than it did in Europe.

In southern Europe, the shift to agriculture coincides with a reduction in skeleton size of 3 centimeters (1.2 inches) for men and 4 centimeters (1.6 inches) for women.[57] Although some other archaeological studies have found that agriculture led to shorter people, a few have found the reverse. In Israel, for example, one study found that people grew taller with the domestication of animals.[58] Overall the evidence supports the view that the diet may have become less nutritious with the shift from large animal hunting to food production but that its quality initially exceeded that of medieval Europe. Table 2 on heights, however, signifies that food was more plentiful and better during the medieval Period than during the mini Ice Age.

In summary, the evidence overwhelmingly supports the proposition that during warm periods, humans prospered. They multiplied more rapidly; they lived longer; and they apparently were healthier. We now turn to a closer examination of the two major warm epochs.

The First Climatic Optimum

Around 9,000 to 5,000 years ago the earth was much warmer than today; perhaps 4deg.F hotter, about the average of the various predictions for global warming after a doubling of CO2.[59] Although the climate cooled a bit after 3000 B.C., it stayed relatively warmer than the modern world until sometime after 1000 B.C., when chilly temperatures became more common. During this Climatic Optimum epoch, Europe enjoyed mild winters and warm summers with a storm belt far to the north. Not only was the country less subject to severe storms, but the skies were less cloudy and the days sunnier.

Notwithstanding the less stormy weather, rainfall was more than adequate to produce widespread forests. Western Europe, including parts of Iceland and the Highlands of Scotland, was mantled by great woods.[60] The timber, until average temperatures dipped temporarily for about 400 years between 3,500 B.C. and 3,000 B.C., consisted of warmth-demanding trees, such as elms and linden in North America and oak and hazel in Europe. These species have never regained their once dominant position in Europe and America. Not only did Europe enjoy a benign climate with adequate rainfall, but the Mediterranean littoral, including the Middle East, apparently received considerably more moisture than it does today.[61] The Indian subcontinent and China were also much wetter during this Optimal period.[62]

As a Senator, Al Gore, writing on the prospect of further global warming and its potential harm, contended that the temperature rise over the last century has led to increased drought in Africa.[63] To bolster his argument, he presented a chart which shows a drop in rainfall from 1930 to the early 1980s for portions of sub-Saharan Africa. His conclusion, however, is based on a false premise: for most of that period the earth was cooling, not warming! His chart actually implies that further cooling would be undesirable. In fact, history demonstrates and climatology attests that warming should drive the monsoon rains that originate near the equator farther north, possibly as far as the Sahara, contributing to a moister not a drier climate!

Compared to cooler periods in the last few thousand years, the Sahara was much wetter and more fertile during the Climatic Optimum.[64] Cave paintings from the epoch depict hippopotamuses, elephants, crocodiles, antelopes and even canoes.[65] The water level in Lake Chad about 14deg. north of the equator in central Africa was some 30 to 40 meters, that is, 90 to 125 feet higher, than it is today, indicating much greater precipitation. Ruins of ancient irrigation channels in Arabia, probably from the warmest millennia, derived their water from sources well above current water supplies, indicating a wetter climate.[66] A warming would likely lead to similar conditions, not a strengthening of African drought. With the cooling that started after 3000 B.C., North Africa dried up and the abundance of life disappeared.

Research has shown, however, that some portions of the globe did suffer from drier conditions. The Caspian Sea may have been at its lowest level in over 80,000 years during the warmest recent period -- 4,000 to 6,000 years ago -- when it was some 20 to 22 meters -- 66 to 72 feet -- below its modern height.[67]

The Southern Hemisphere seems to have flourished as well during the warm millennia after the most recent Ice Age. Professor Lamb reports that the southern temperate zone enjoyed both warmer weather and more moisture than it does currently.[68] Scholars have found that Australia was consistently wetter than today in both the tropical and temperate regions.[69] Since the end of that epoch, the great deserts of Australia have expanded and the climate has become both cooler and drier. Apparently most of the other great desert regions of the world enjoyed more rainfall during the Climatic Optimum than they do now. Lamb contends that the period of temperature maximum was also a period of moisture maximum in subtropical and tropical latitudes and a good period for forests in most temperate regions.[70] During this warm era, Hawaii experienced more rainfall than in the twentieth century.[71] Even Antarctica enjoyed warmer weather, about 4deg. to 5deg.F higher, and during the summer in some of the mountains the weather was warm enough to produce running streams and lakes which have subsequently frozen.[72] Nevertheless, the basic ice sheet remained intact.

As already mentioned, the invention of agriculture coincided with the end of the last Ice Age and the melting of the glaciers. Archaeologists have found the earliest evidence for husbandry and farming in Mesopotamia around 9000 B.C.73 As the earth warmed, the Middle East became wetter and the Iranian plateau shifted from an open dry plain with roving bands of game to a more wooded environment with less reliable food sources and a diminished supply of large animals. No one really knows how man first domesticated plants and animals, but the coincidence in time and the forcing nature of climate change suggest that the warmer wetter weather, especially in the mountains, may have encouraged new techniques.

The transition from the Ice Age to a warmer climate that led eventually to agriculture is best documented in Europe. During the cold period, most of Europe was a dry plain, an open savanna, in which large herds of reindeer, mammoths, and bison roamed. As has been shown by the cave drawings in France and Spain, the population secured a good living by preying on these ungulates. As the climate warmed and as rainfall increased, forests spread north limiting the habitat for these large mammals. This forced humans into following northward the dwindling herds or developing new sources of food. As the large animals disappeared the local people shifted to exploiting red deer, wild boar and smaller species. Those located near the seas or large rivers found seafood a plentiful source of substance. On the other hand, people who made their living at the edge of the ocean faced seas that were rising about 3 feet each century and which often drowned them when high tides and storms washed over their primitive villages.

The domestication of plants appears to have occurred around the world at about the same time: from 10,000 YBP to 7,500 YBP.74 The earliest well documented employment of agriculture arose in the Middle East. Planting of wheat and barley began in southwest Asia between 8000 B.C. and 7000 B.C. In north China's Shensi Province between 4500 B.C. and 3500 B.C., peasants grew foxtail and millet and raised pigs. Food production in this part of China extends back at least into the sixth millennium B.C. In the Americas domestication of some grains and chili peppers dates from between 7000 B.C. and 6000 B.C.; anthropologists have documented maize in the Tehuacan Valley by 5700 B.C. and production may have started earlier. In South America the evidence suggests that domestication of two species of beans and chili pepper as well in the Andean highlands arose 8,500 years ago. Maize appears in this area only about 3000 B.C. In Africa the evidence implies the cultivation of plants after 3500 B.C. Domestication of cattle occurred in the Sahara about 8,000 years before the present.75

As Professors Ammerman and Cavalli-Sforza put it, "One of the few variables that would seem to be shared is timing: early experiments at plant domestication occurred in southwest Asia, east Asia, and Central America during the period between 8000 B.C. and 5500 B.C."76 The coincidence of the invention of agriculture with a general warming of the climate, an increase in rainfall, and a rise in carbon dioxide levels, all of which would have made plant growth more vigorous and more plentiful, cannot be accidental.

Domestication of plants and animals represented a fundamental shift in man's involvement with nature. Prior to this humans simply took what nature offered. People hunted or scavenged the local animals that happened their way. Women gathered fruits and vegetables that grew wild in their territory. With farming and herding, mankind, for the first time, began to modify his environment. Humans determined what would be grown, which plants would survive in their gardens, which animals would be cultivated and bred, and which would be shunned or eliminated. Homo Sapiens ceased being simply another species that survived by predation coupled with grazing and became a manager of his environment.

The shift from a hunter-gatherer existence to a sedentary one may be the most important innovation in human existence. Prior to this change, humans lived in small groups and moved frequently with the seasons to find new sources of meat, fruit and vegetables. Being mobile meant carrying few goods and only those that were light and not fragile. Thus pottery, which is both heavy and easily breakable, was not part of their culture. Any musical instruments must have been small and portable. Many small children would have been a hindrance as would elderly feeble individuals. Such small groups would have had little opportunity to develop specialization. Virtually all males must have participated in the hunt while all females, not giving birth or caring for infants, must have helped gather edibles. These tribal or family groups could not have supported elaborate priesthoods, bureaucratic governmental structures, or even people who specialized in artistic, cultural or intellectual activities. As a consequence these societies were probably quite egalitarian with only a few, such as the chief or elder and perhaps a medicine man, that stood out from the rest.

The development of agriculture and the establishment of fixed communities led to a population explosion and the founding of cities. Agricultural societies produce enough surplus to support such urban developments, including the evolution of trades and new occupations. A large community could afford to have specialists who made farm tools, crafted pots, and traded within the village and between the locals and outsiders. The people in today's Palestine established the first known city, Jericho, and thus the first step towards specialization -- which lies at the heart of economic advancement -- around 8000 B.C.[77]

Farming required the development of property rights in lands, although initially pastures may have been held in common. Even though in the beginning farm holdings were probably fairly equally distributed, over time some families must have acquired larger holdings than others. This increase in income inequality may offend modern sensibilities, but it provided a major benefit. A wealthy class or a rich ruler could afford to maintain individuals who would create desirable objects, such as art, elaborate pots, and musical instruments, and who could record eclipses, star movements or trade with other centers.

Man's taming of animals and plants represents a movement towards establishing property rights. In a hunter-gatherer's world no one owns the wild beasts or the fruit and grains until they are collected. This can work satisfactorily only as long as demands for the resources are quite limited. But as the literature on the tragedy of the commons shows, once pressures for more grow too large, the resource base can be exhausted. In what is now called North America, many large species, such as horses, were apparently hunted to extinction. Domestication -- privatization of animals and plants -- became the answer to over hunting and over grazing.

In Europe, the Optimum period produced an expansion of civilization with the construction of cities and a technological revolution. The Bronze Age replaced the New Stone Age.[78] The more benign climate with less severe storms encouraged travel by sea.

Trade flourished during this warm period. People from ancient Denmark shipped amber along the Atlantic coast to the Mediterranean. As early as 2000 B.C., the Celts apparently were sailing from Cornwall and Brittany to both Scandinavia and southern Italy. Astrological monuments built around this time, such as Stonehenge, indicate that the skies were less cloudy than now.[79] With the glaciers in the Alps during the late Bronze Age being only about 20 percent of the size of the ice in the nineteenth century, merchants made their way through the Brenner Pass, the dominant link between northern and southern Europe. Northern Europeans exchanged tin for manufactured bronze from the south. Alpine people mined gold and traded it for goods crafted around the Mediterranean. Baltic amber found its way to Scotland.

During the warm period prior to 3000 B.C., China enjoyed much warmer temperatures. In particular midwinters were as much as 9deg.F hotter and rice was planted a month earlier than is now common.[80] Bamboo, valued for food, building material, writing implements, furniture and musical instruments, grew much farther north -- about 3deg. in latitude -- than is now possible.[81] Chinese archaeologists have found evidence in a district near Sian that the climate 5,000 to 6,000 years ago was warmer and wetter than the present.

Prior to around 2500 to 1750 B.C., northwestern India, which is now very dry, enjoyed greater rainfall than it does in the twentieth century.[82] In the Indus Valley, the Harappas created a thriving civilization that reached its apogee during the warmest and wettest periods, when their farmers were growing cereals in what is now a desert.[83] The area was well watered with many lakes. This civilization disappeared around 1500 B.C. at a time when the climate became distinctly drier.[84] The earth was cooling. Historians and archaeologists also attribute the failure of this civilization to poor agricultural techniques that may have exacerbated drought.

Virtually all change can make some worse off and the warming after the last Ice Age is no exception. Although as the population explosion indicates most humans benefited, the growing warmth harmed some people, especially those who lived near the coast or who had earned their living hunting large animals. As the ice sheets melted, the sea level rose sharply and probably peaked around 2000 B.C.85 During the many centuries in which the waters mounted, storms often led to ocean flooding of coastal communities. A few times each century, people were forced to abandon well-established villages and move to higher ground.

Cooler, More Varied, and Stormy Times

From the end of the Optimum period of sustained warmth until around 800 A.D. to 900 A.D., what we know of the world's climate and, in particular, the European varied between periods of warmth and cold. Based on the height of the upper tree lines in middle latitudes' mountains, the temperature record following the peak warm period around 5000 B.C. demonstrates a more or less steady decline right up to the 20th century.[86] As mentioned above, tree ring data for New Zealand indicate that after temperatures reached a maximum around 6000 to 8000 B.C., the climate cooled in that part of the world.

After 1000 B.C. the climate in Europe and the Mediterranean cooled sharply and by 500 B.C. had reached modern average temperatures.[87] The period from 500 B.C. to 600 A.D. was one of varied warmth, although cooler on average than the previous 4,500 years. However, the climate became more clement and somewhat more stable from 100 B.C. to 400 A.D., the period of the Roman Empire.[88] The Italians grew grapes and olives farther north than they had prior to this period. During these centuries of varied weather, Classical Greece flourished and then declined; the Roman Empire spread its authority through much of what is now Europe, the Middle East and North Africa, only to be overrun by barbarians from central Asia whose eruption out of their homeland may have been brought on by a change in the climate.

The cooler climate after the start of the last millennium B.C. appears to have contributed to a southern migration of people from northern Europe.[89] Archaeologists have also found evidence that Greeks adopted warmer clothing after 1300 B.C. The population living in the Alps diminished sharply with the cooler weather and mining ceased. Classical historian Ray Carpenter attributes a depopulation of Greece and Turkey between 1200 and 750 B.C. to long term drought that must have reflected the increased coolness of the climate.[90]

Evidence for a cooler Mediterranean climate from 600 B.C. to 100 B.C. comes from remains of ancient harbors at Naples and in the Adriatic which are located about one meter (three feet) below current water levels.[91] Further support for lower sea levels has been found on the North African coast, around the Aegean, the Crimea, and the eastern Mediterranean. Lower oceans imply a colder world leading to a build-up of snow and ice at the poles and in major mountain glaciers. By 400 A.D., however, temperatures had warmed enough to raise water levels to about three feet above current elevations. The ancient harbors of Rome and Ravenna from the time of the Roman Empire are now located about one kilometer from the sea.[92] Evidence exists for a peak in ocean heights in the fourth century for points as remote as Brazil, Ceylon, Crete, England, and the Netherlands, indicating a world-wide warming.

Changes in the climate in Eurasia appear to have played a major role in the waves of conquering horsemen who rode out of the plains of central Asia into China and Europe. Near the end of the Roman Empire, around 300 A.D., the climate began to warm and conditions in central Asia improved apparently leading to a population explosion.[93] These people, needing room to expand and a way to make a living, invaded the more civilized societies of China and the West. The medieval warmth from around 1000 A.D. to 1300 also seems to have also triggered an expansion from that area. During this second optimum period, the homeland of the Khazars centered around the Caspian Sea enjoyed much greater rainfall than earlier or than it does now. The increased prosperity in this area produced a rapidly rising number of young men that provided the manpower for Genghis Khan to invade China and India and to terrorize Russia and the Middle East.[94]

After 550 A.D. until around 800, Europe suffered through a colder, wetter, and more stormy period. As the weather became wetter, peat bogs formed in northern areas.[95] The population abandoned many lakeside dwellings while mountain passes became choked with ice and snow, making transportation between northern Europe and the south difficult. The Mediterranean littoral and North Africa dried up, although they remained moister than now.

Inhabitants of the British Isles between the seventh and the ninth centuries were often crippled with arthritis while their predecessors during the warmer Bronze Age period suffered little from such an affliction. Although some archaeologists have attributed the difficulties of the dark age people to harder work, the cold wet climate between 600 and 1000 A.D. may have fostered such ailments.[96]

During the centuries after the fall of the Roman empire and with the deterioration of the climate, Greece languished. In 542 A.D., the population was decimated by the plague, aggravated by cold damp conditions; the Black Death struck again between 744 and 747.[97] As a consequence the number of people was sharply reduced. Greece was partially re-populated in the ninth and tenth centuries when the Byzantine Emperors brought Greek settlers from Asia Minor back into the area. For the first time in centuries Greek commerce and prosperity returned -- probably due to an improved climate.[98]

In the ninth century, land hunger and a rising population in Norway and Sweden spurred the Scandinavians on to loot and pillage by sea. Their first descent was on the monastery of Lindisfarne in northern England in 793. This was followed by raids on Seville in Muslim Spain in 844 and later farther into the Mediterranean.[99] In 870 they discovered Iceland and in the next century, Greenland. In 877 they began an invasion of England and conquered from the north to the whole of the midlands -- all of which became a Danish overseas kingdom by the mid-tenth century. At the same time, they stormed France and the king had to cede them Normandy as a fief. They also crossed the Baltic (known as Rus in that time) and sent traders south to Islam and Byzantium.

The High Middle Ages and Medieval Warmth

From around 800 A.D. to 1200 or 1300, the globe warmed considerably and civilization prospered. This Little Climate Optimum generally displays, although less distinctly, many of the same characteristics as the first climate optimum.[100] Virtually all of northern Europe, the British Isles, Scandinavia, Greenland, and Iceland were considerably warmer than at present. The Mediterranean, the Near East, and North Africa, including the Sahara, received more rainfall than they do today.[101] North America enjoyed better weather during most of this period. China during the early part of this epoch experienced higher temperatures and a more clement climate. From Western Europe to China, East Asia, India, and the Americas, mankind flourished as never before.

Evidence for the medieval warming comes from contemporaneous reports on weather conditions, from oxygen isotope measurements taken from the Greenland ice, from upper tree lines in Europe, and from sea level changes. These all point to a more benign, warmer, climate with more rainfall but because of more evaporation less standing water. Not only did northern Europe enjoy more rainfall but the Mediterranean littoral was wetter. An early twelfth century bridge with twelve arches which still exists over the river Oreto at Palermo exceeds the needs of the small trickle of water that flows there now.[102] According to Arab geographers two rivers in Sicily that are too small for boats were navigable during this period.[103] In England at the same time, medieval water mills on streams that today carry too little water to turn them attest to greater rainfall. Although England apparently received more rainfall than in modern times, the warm weather led to more drying out of the land. Support for a more temperate climate in central Europe comes from the period in which German colonists founded villages. As average temperatures rose people established towns at higher elevations. Early settlements were under 650 feet in altitude; those from a later period were between 1,000 and 1,300 feet high; and those built after 1,100 were located above 1,300 feet.[104]

H. H. Lamb counted manuscript reports of flooding and wet years in Italy.[105] He discovered that starting in the latter part of the tenth century, the number of wet years climbed steadily, reaching a peak around 1300. Over the same period northern Europe was enjoying warmer and more clement weather. Not only was the temperature higher than now in Europe during the twelfth and thirteenth century but the population enjoyed mild wet winters. In the Mediterranean it was moist as well with summer thunderstorms frequently reported.[106]

Studies have shown that some areas became drier during these centuries. In particular, the Caspian Sea was apparently four meters -- over 13 feet -- lower from the ninth through the eleventh century than currently.[107] After 1200 A.D. the elevation of the lake rose sharply for the next two or three hundred years.[108] In the Asian steppes, warm periods with fine summers and often little snow in the winter produced lake levels that were low by modern standards.[109] A recent study of tree rings in California's Sierra Nevada and Patagonia concluded that the "Golden State" suffered from extreme droughts from around 900 to 1100 and again from 1210 to 1350 while the tip of South America during the first 200 years also enjoyed little precipitation.[110]

The timing of the medieval warm spell, which lasted no more than 300 years, was not synchronous around the globe. For much of North America, for Greenland and in Russia, the climate was warmer between 950 and 1200.[111] The warmest period in Europe appears to have been later, roughly between 1150 and 1300, although parts of the tenth century were quite warm. Evidence from New Zealand indicates peak temperatures from 1200 to 1400. Data on the Far East is meager but mixed. Judging from the number of severe winters reported by century in China, the climate was somewhat warmer than normal in the ninth, tenth, and eleventh centuries, cold in the twelfth and thirteenth and very cold in the fourteenth. Chinese scholar Chu Ko-chen reports that the eighth and ninth centuries were warmer and received more rainfall, but that the climate deteriorated significantly in the twelfth century.[112] He found records, however, that show that the first half of the thirteenth century was quite clement and very cold weather returned in the fourteenth century.[113] Another historian found on the basis of records of major floods and droughts that between the ninth and eleventh century China suffered much fewer of these calamities than during the fourteenth through the seventeenth.[114] The evidence for Japan is based on records of the average April day on which the cherry trees bloomed in the royal gardens in Kyoto. From this record, the tenth century springs were warmer than normal; in the eleventh century they were cooler; the twelfth century experienced the latest springs; the thirteenth century was average and then the fourteenth was again colder than normal.[115] This record suggests that the Little Climate Optimum began in Asia in the eighth or ninth centuries and continued into the eleventh. The warm climate moved west, reaching Russia and central Asia in the tenth through the eleventh, and Europe from the twelfth to the fourteenth. Some climatologists have theorized that the Mini Ice Age also started in the Far East in the twelfth century and spread westward reaching Europe in the fourteenth.[116]


The warm period coincided with an upsurge of population almost everywhere, but the best numbers are for Europe. For centuries during the cold damp "dark ages" the population of Europe had been relatively stagnant. Towns shrank to a few houses clustered behind city walls. Although we lack census data, the figures from Western Europe after the climate improved show that cities grew in size; new towns were founded; and colonists moved into relatively unpopulated areas.

Historians have failed to agree on why after the eleventh century, the population soared. It might be more enlightening to ask why the population remained stagnant for so long. As John Keegan, the eminent military historian put it: "The mysterious revival of trade between 1100 and 1300, itself perhaps due to an equally mysterious rise in the European population from about 40,000,000 to about 60,000,000, in turn revived the life of towns, which through the growth of a money economy won the funds to protect themselves from dangers beyond the walls".[117]

Although it is impossible to document it, the change in the climate from a cold, wet one to a warm, drier climate -- it had more rainfall, but more evaporation reduced bogs and marshy areas -- seems likely to have played a significant role. In the eighth through the eleventh centuries, most people spent considerable time in dank hovels avoiding the inclement weather. These conditions were ripe for the spread of disease. Tuberculosis, malaria, influenza, and pneumonia undoubtedly took many small children and the elderly -- those over 30.

Written records confirm that the warmer climate brought drier and consequently healthier conditions to much of Europe. Robert Bartlett, citing H.E. Hallam in Settlement and Society, quotes the people of Holland who invaded Lincolnshire in 1189 that "because their own marsh had dried up, they converted them into good and fertile ploughland."[118] Moreover, prior to the twelfth century German settlers on the east side of the Elbe frequently named their towns with mar, which meant marsh, but later colonists did not use that suffix. Bartlett's explanation is that the term had gone out of use, but an alternative one is that the warmer climate had dried up the marshes.[119]

With a more pleasant climate people spent longer periods outdoors; food supplies were more reliable. Even the homes of the peasants would have become warmer and less damp. The draining or drying up of marshes and wetlands reduced the breeding grounds for mosquitoes that brought malaria. In all the infant and childhood mortality rate must have fallen spawning an explosion in population.

From the ninth century, with a climate still quite cool, to the eleventh, medieval Europe was almost totally agricultural. The few cities that existed consisted mainly of religious seats with their support personnel. Even as late as the twelfth century, city dwellers made up less than 10 percent of the population.[120] Trade before the eleventh century was virtually non-existent.[121] People were tied to the land through custom and necessity. The great feudal estates grew what they ate and ate what they grew; they wove their own cloth and sewed their own clothes; they built what little furniture was needed. In a word they were almost entirely self-sufficient. The serfs that tilled the land had inherited rights to enough land to sustain a family. Typically the older son would follow his father. Any other sons either joined the priesthood, became monks, vagabonds, or in later centuries, mercenaries. Given the cold climate before the eleventh century, the lack of medical care, and a restricted diet fostering poor nutrition, few babies lived to adulthood. The problem of an excess of labor was, therefore, nonexistent. The truth is that the population was growing so slowly that a labor shortage persisted and the feudal nobility established laws prohibiting serfs from leaving their land.

Until the twelfth century when the weather became significantly more benign, a Europe fettered by tradition remained cloistered in self-sufficient units. The next two centuries, however, witnessed a profound revolution which, by the end of the fourteenth century, transformed the landscape into an economy filled with merchants, vibrant towns and great fairs. Crop failures became less frequent; new territories were brought under control. With a more clement climate and a more reliable food supply, the population mushroomed. Even with the additional arable land permitted by a warmer climate, the expansion in the number of mouths exceeded farm output: food prices rose while real wages fell. Farmers, however, did well with more ground under cultivation and low wages payable to farm hands.[122]

The rise in the population may have contributed to the spread of primogeniture. Prior to the twelfth century, infant and child mortality coupled with short life expectancy required parents to be flexible in designating their heirs. Robert Bartlett quotes an estimate for the eleventh century that the probability of a couple raising sons to adulthood was only 60 percent.[123]

Although the first sons born on the estates could follow their fathers, other children, especially the men, had to find new opportunities. The crusades furnished an occasion for both the sons of serfs and of the nobility to enrich themselves and even to find new land to cultivate. Others moved to virgin territory in eastern Europe, Scandinavia or previously forested or swampy areas.[124] The Franks and Normans launched invasions of England, southern Italy, Byzantine Greece, and the eastern Mediterranean. In 1130 the Tancred de Hauteville clan, a notable example, founded the Kingdom of Sicily. This family, a classic case of an "over-breeding, land-hungry lesser nobility," consisted of 12 sons from two mothers who, recognizing that their Norman property was inadequate, invaded southern Italy in search of land and riches.[125]

During the High Middle Ages, the Germans advanced across the Elbe to take land from pagan Serbs. The spread of knights and soldiers out of France and Germany demonstrates that the population was multiplying more rapidly in northern Europe than in southern. The rapid rise in numbers north of the Alps fits the improved climate scenario: global or continental warming brought greater temperature change and more beneficial weather to higher latitudes.

The more skilled and enterprising who did not seek their fortune in foreign lands typically flocked to towns and urban centers, becoming laborers, artisans, or traders. Both those who moved to the new cities and those who founded colonies were legally freed of feudal obligations. This new liberty, making risk taking and innovation possible, was essential for those in commerce.

The warmth of the Little Climate Optimum made territory farther north cultivable. In Scandinavia, Iceland, Scotland, and the high country of England and Wales, farming became common in regions which neither before nor since have yielded crops reliably. In Iceland, oats and barley were cultivated. In Norway, farmers were planting further north and higher up hillsides than at any time for centuries. Greenland was 4deg. to 7deg.F warmer than at present and settlers could bury their dead in ground which is now permanently frozen. Scotland flourished during this warm period with increased prosperity and construction.[126] Greater crop production meant that more people could be fed, and the population of Scandinavia exploded.[127] The rapid growth in numbers in turn propelled and sustained the Viking explorations and led to the foundation of colonies in Iceland and Greenland.

The increasingly warm climate was reflected in a rising sea level. People were driven out of the low lands and there was a large scale migration of men and women from these areas to places east of the Elbe, into Wales, Ireland, and Scotland. Flemish dikes to hold back the sea date at least from the early eleventh century. Although Pirenna and Bartlett attribute them to attempts to reclaim land from the sea to provide new areas for farming, the evidence points towards a higher water level that farmers in the low countries had to battle.[128] The earliest texts setting out rights on the reclaimed land fail to mention any obligation to maintain the dikes, although later ones spell out the requirement, suggesting that the problem of holding back the sea became worse over time. Robert Bartlett quotes from a Welsh chronicle on the influx of people from Flanders:

that folk, as is said, had come from Flanders, their land, which is situated close to the sea of the Britons, because the sea had taken and overwhelmed their land ... after they had failed to find a place to live in -- for the sea had overflowed the coast lands, and the mountains [sic] were full of people so that it was not possible for everyone to live together there because of the multitude of the people and the smallness of the land ...[129]

In addition to the land north of the Alps, the warm rainier climate benefited southern Europe, especially Greece, Sicily and southern Italy. All of the Mezzogiorno in the Middle Ages did well.[130] Nicolas Cheetham, a former British diplomat who authored a recent book on Mediaeval Greece, reports that during the first half of the thirteenth century, the plains and valleys of the Peloponnese were fertile and planted with a wide variety of valuable crops and trees. They produced wheat, olives, fruits, honey, cochineal for dyeing, flax for the linen industry and, silk from the mulberry trees. The wealthy in Constantinople prized highly the wines, olives, and fruit from Greece. Thessaly's grain fed the Byzantine Empire.[131] Patras exported textiles and silk of very high quality. Extensive forests, which were full of game, supplied acorns for hordes of pigs. Herders raised sheep and goats in the mountain pastures, while in the valleys farmers kept horses and cattle.[132]

The Mediterranean flourished in the twelfth century. Christian and Moslem lands achieved great brilliance. Cordova, Palermo, Constantinople and Cairo all thrived, engendering great tolerance for contending religions.[133] Christian communities survived and prospered in Moslem Cairo and Cordova. The Rulers of Byzantine countenanced the Moslems and often preferred them to "barbaric" Westerners.

In the West, Charlemagne, creator of the Holy Roman Empire, may have inaugurated the era of the High Middle Ages while Dante, writing The Divine Comedy, may have closed it. In A History of Knowledge, Charles Van Doren contended that: "the ... three centuries, from about 1000 to about 1300, became one of the most optimistic, prosperous, and progressive periods in European history."[134] All across Europe, the population went on an unparalleled building spree erecting at huge cost spectacular cathedrals and public edifices. Byzantine churches gave way to Romanesque, to be replaced in the twelfth century by Gothic cathedrals. During this period construction began on the Abbey of Mont-St-Michel (1017), St. Marks in Venice (1043), Westminster Abbey in London (1045), the Cathedral of our Lady in Coutances (1056), the Leaning Tower at Pisa (1067), the Cathedral of Santiago de Compostela in northern Spain (1078), the Cathedral of Modena (1099), Vézelay Abbey in France (1130), Notre-Dame in Paris (1163), Canterbury in England (1175), Chartres (1194), Rouen's cathedral in France (1201), Burgos' cathedral in Castile (1220), the basilica of Saint Francis in Assisi (1228), the Sainte Chapelle in Paris (1246), Cologne Cathedral (1248) and the Duomo in Florence (1298). Virtually all the magnificent religious edifices that we visit in awe today were started by the optimistic populations of the eleventh through the thirteenth centuries, although many were not finished for centuries. In southern Spain, the Moors laid the cornerstone in 1248 for perhaps the world's most beautiful fortress, the Alhambra. The Franks founded Mistra near ancient Sparta in the middle of the thirteenth century.

It took a prosperous society to launch such major architectural projects. In Europe, building the cathedrals required a large and mostly experienced pool of labor. During the week of June 23 to June 29, 1253, the accounts of the construction at Westminister Abbey, for example, showed 428 men on the job, including 53 stonecutters, 49 monumental masons, 28 carpenters, 14 glassmakers, 4 roofers, and 220 simple laborers.[135] Nearly half of all workers were skilled specialists. Even during the slowest season in November, the Abbey employed 100 workers, including 34 stonecutters. Masons and stonecutters earned the highest wages and usually hired a number of workers as assistants. Master craftsmen moved from job to job around Europe without any concern about national borders -- the first truly European Community. Historians have found than only 5 to 10 percent of the masons and stonecutters were local people, but 85 percent of the men who quarried the stones -- an unhealthy and arduous job -- were from the vicinity.[136] Thus during these centuries a European-wide market flourished in skilled craftsmen.

Economic activity blossomed throughout the continent. Banking, insurance, and finance developed; a money economy became well established; manufacturing of textiles expanded to levels never seen before. Farmers were clearing forests, draining swamps and expanding food production to new areas.[137] The building spree mentioned above was made possible by low wages resulting from a population explosion and by the riches that the new merchant classes were creating. In England, virtually all of the churches and chapels which had originally been built of wood were reconstructed in stone between the twelfth and fourteenth centuries.[138] With the clergy still opposing buying and selling for gain, those who became wealthy often constructed churches or willed their estate or much of it to religious institutions as acts of redemption.[139] They supplied much of the funds to erect the great Gothic cathedrals.

Starting in the eleventh century European traders developed great fairs that brought together merchants from all over Europe. At their peak in the thirteenth century they were located on all the main trade routes and served not only to facilitate the buying and selling of all types of goods but also functioned as major money markets and clearing houses for financial transactions. The fourteenth century saw the waning of these enterprises probably because the weather became so unreliable and poor that transport to and from these locations with great stocks of goods became impractical. Belgian historian Henri Pirenne attributes their decline to war, which may indeed have played a role, but the failure of crops and the increased wetness must have made travel considerably more difficult.[140] Wet roads were muddy rendering it arduous to transport heavy goods. Crop failures made for famines and more vagabonds who preyed on travelers.

During the High Middle Ages, technology grew rapidly. New techniques expanded the use of the water mill, the windmill, and coal for energy and heat. Sailing improved through the invention of the lateen sail, the sternpost rudder and the compass. Governments constructed roads and contractors developed new techniques for use of stone in construction. New iron casting techniques led to better tools and weapons. The textile industry began employing wool, linen, cotton, and silk and, in the thirteenth century, developed the spinning wheel. Soap, an essential for hygiene, came into use in the twelfth century. Mining, which had declined since the Romans, at least partly because the cold and snow made access to mountain areas difficult, revived after the tenth century.

Farmers and peasants in medieval England launched a thriving wine industry south of Manchester. Good wines demand warm springs free of frosts, substantial summer warmth and sunshine without too much rain, and sunny days in the fall. Winters cannot be too cold -- not below zero Fahrenheit for any significant period. The northern limit for grapes during the Middle Ages was about 300 miles above the current commercial wine areas in France and Germany. These wines were not simply marginal supplies, but of sufficient quality and quantity that, after the Norman conquest, the French monarchy tried to prohibit British wine production.[141] Based on average and extreme temperatures in the most northern current wine growing regions of France and Germany compared to current temperatures in the former wine growing regions in England, Lamb calculates that the climate in springs and summers were somewhere between 0.9 and 3.4 degrees Fahrenheit warmer in the Middle Ages.[142]

Not only did the British produce wines during the Little Climate Optimum but farmers grew grapes in East Prussia, Tilsit, and south Norway.[143] Many areas cultivated in Europe were much further up mountains than is possible under the modern climate. Together these factors suggest that the temperatures in central Europe were about 1.8deg. to 2.5deg.F higher than during the twentieth century.

Europe's riches and a surplus of labor enabled and emboldened its rulers to take on the conquest of the Holy Land through a series of Crusades starting in 1096 and ending in 1291. The Crusades, stimulated at least in part by a mushrooming population and an economic surplus large enough to spare men to invade the Muslim empire, captured Jerusalem in 1099 -- a feat not equaled until the nineteenth century. A major attraction of the first crusade was the promise of land in a "southern climate."[144]

Even southern Europe around the Mediterranean enjoyed a more moist climate than currently.[145] In the reign of the Byzantine Emperor Manuel I Comnenus, art and culture flourished and all the world looked to Constantinople as its leader.[146] Under the control of the Fatimid caliphate, Egypt cultivated a "House of Science," where scholars worked on optics, compiled an encyclopedia of natural history, with a depiction of the first known windmills, and described the circulation of the blood. In Egypt block-printing appeared for the first time in the West.[147] The caliphate turned Cairo into a brilliant center of Moslem culture. In Persia, Omar Khayyam published astronomical tables, a revision of the Muslim calendar, a treatise on algebra and his famous Rubáiyát.148

As European commerce expanded, traders reached the Middle East, bringing back not only exotic goods, but new ideas and information about classical times. Drawing on fresh information about Aristotelian logic, St. Thomas Aquinas defined medieval Christian doctrine in his Summa Theologica. Possibly the oldest continuous university in the world was founded in Bologna for the study of the law in 1000 A.D. Early in the twelfth century a group of scholars under a license granted by the chancellor of Notre-Dame began to teach logic, thus inaugurating the University of Paris. Cambridge University traces its foundation to 1209 and Oxford to slightly later in the thirteenth century. Roger Bacon, one of the first to put forward the importance of experimentation and careful research, studied and taught at Oxford in the thirteenth century.

Secular writing began to appear throughout northern Europe. In the twelfth century the medieval epic of chivalry, the Chanson de Roland, was put into writing. Between 1200 and 1220 an anonymous French poet composed the delightful and optimistic masterpiece, Aucassin et Nicolette. An anonymous Austrian wrote in Middle High German the Nibelungenlied.149

The Arctic

From the ninth through the thirteenth centuries agriculture spread into northern Europe and Russia where it had been too cold to produce food before. In the Far East, Chinese and Japanese farmers migrated north into Manchuria, the Amur Valley and northern Japan.[150] As mentioned above, the Vikings founded colonies in Iceland and Greenland, a region that may have been more green than historians have claimed. It was also during this period that Scandinavian seafarers discovered "Vinland" -- somewhere along the East Coast of North America. The subsequent Mini Ice Age cut off the colonies in Greenland from Europe, and they eventually died off. Even today, during this warm period of the late twentieth century, the British climate forecloses large-scale grape production and Greenland is unsuitable for farming.

The Eskimos apparently expanded throughout the Arctic area during the medieval warm epoch.[151] Starting with Ellesmere Land around 900 A.D., Eskimo bands and their culture spread from the Bering Sea into the Siberian Arctic. Two centuries later, these people migrated along the coast of Alaska and into Greenland. During this period the Eskimos' main source of food came from whaling, which had to be abandoned with the subsequent cooling. The Mini Ice Age forced the Thule Eskimos south out of northern Alaska and Greenland. These hardy aborigines had abandoned Ellesmere Land by the sixteenth century.

At the same time that the Eskimos were moving north, Viking explorers were venturing into Greenland, Vinland, and even the Canadian Arctic. Scandinavian sailors found Iceland in 860, Greenland around 930, and reached the shores of North America by 986.[152] By the turn of the millennium, when the waters south-west of Greenland may have been at least 7deg.F warmers than now, Vikings were regularly visiting Vinland for timber.[153] They were received with great hostility by the natives and eventually abandoned contact, although the last trip may have occurred as late as 1347, when a Greenland ship was blown off course.[154] At the height of the warm period, Greenlanders were growing corn and a few cultivated grain. Some archaeologists have found evidence that Vikings from Greenland may have visited remote portions of the Canadian archipelago and even may have sailed through the Northwest Passage to the West Coat of America traveling as far south as the Gulf of California. At least one scientists believes that this visit is the origin of the Aztec belief in the visit of "fair" people from the East.[155]

The Far East

As noted above, the warming in the Far East seems to have preceded that in Europe by about two centuries. Chinese Economist Kang Chao has studied the economic performance of China since 200 B.C. In his careful investigation, he discovers that real earnings rose from the Han period (206 B.C. to 220 A.D.) to a peak during the Northern Sung Dynasty (961 A.D. to 1127).[156] This coincides with other evidence of longer growing seasons and a warmer climate. He explains the fall in worker productivity after the twelfth century as stemming from population pressures, but a change in climate may have played a significant role. Chao reports that the number of major floods averaged fewer than 4 per century in the warm period of the ninth through the eleventh centuries while the average number was more than double that figure in the fourteenth through the seventeenth centuries of the Mini Ice Age.[157] Not only floods but droughts were less common during the warm period. The era of benign climate sustained about 3 major droughts per century, while during the later cold period, China suffered from almost 13 each hundred years.

The wealth of this period gave rise to a great flowering of art, writing, and science. The Little Climate Optimum witnessed the highest rate of technological advance in Chinese history. During the 300 years of the Sung Dynasty, farmers invented 35 major farm implements -- that is, over 11 per century, a significantly higher rate of invention than in any other era.[158] In the middle of the eleventh century, the Chinese invented movable type employing clay pieces.[159]

During the Northern Sung Dynasty Chinese landscape painting with its exquisite detail and color reached a peak never again matched.[160] Adam Kessler, curator of the Los Angeles County Museum of Natural History dates the earliest Chinese blue-and-white porcelain to the twelfth century.[161] The Southern Sung produced pottery and porcelains unequaled in subtlety and sophistication. Literature, history and scholarship flourished as well. Scholars prepared two great encyclopedias, compiled a history of China, and composed essays and poems. Mathematicians developed the properties of the circle. Astronomers devised a number of technological improvements to increase the accuracy of measuring the stars and the year.[162]

Japan also prospered during the Little Climate Optimum. In the Heian Period (794 A.D. to 1192) the arts thrived as emperors and empresses commissioned vast numbers of Buddhist temples. Murasaki Shikibu, perhaps the world's first female novelist, composed Japan's most famous book, The Tale of Genji. Other classical writers penned essays: Sei Shonagon -- another court lady -- wrote Makura-no-Soshi (the Pillow Book). The Japanese aristocracy vied in composing the best poems. All of this attests a prosperous economy with ample food stocks to support a leisured and cultivated upper class.

Over the four hundred years between 800 A.D. and 1200, the peoples of the Indian subcontinent prospered as well. Society was rich enough to produce colossal and impressive temples, beautiful sculpture, elaborate carvings, many of which survive to this day.[164] The Lingaraja Temple, one of the finest Hindu shrines, as well as the Shiva Temple date from this period.[165] Seafaring empires existed in Java and Sumatra, which reached its height around 1180. Ninth century Java erected the vast stupa of Borobudur; other temples -- the Medut, Pawon, Kelasan and Prambanan -- originate in this era. In the early twelfth century, the predecessors of the Cambodians, the Khmers, built the magnificent temple of Angkor Wat.[166] In the eleventh century Burmese civilization reached a pinnacle. In or around its capital, Pagan, between 931 and 1284, succeeding kings competed in constructing vast numbers of sacred monuments and even a library.[167] Today the area is a dusty plain littered with the crumbling remains of about 13,000 temples and pagodas, built in a more hospitable era.

Archaeologists studying the compositions of forests in New Zealand have found that the South Island enjoyed a warmer climate between 700 A.D. and 1400, about the time when Polynesians were colonizing the South Pacific Islands and the Maoris were settling in New Zealand.[168] Partially confirming that warming are data from Tasmania of tree rings which show a warm period from 940 to 1000 and another from 1100 to 1190.[169]

The Americas

Less is known about civilizations in the Americas during the Little Climate Optimum or even how the prevailing weather changed. Much of the currently arid areas of North America were apparently wetter during this epoch. The Great Plains areas east of the Rocky Mountains, the upper Mississippi Valley and the Southwest received more rainfall between 800 A.D. and 1200 than they do now.[170] Radiocarbon dating of tree rings indicates that warmth extended from New Mexico to northern Canada. In Canada, forests extended about sixty miles north of their current limit.[171]

Starting around 800 to 900 A.D., the indigenous peoples of North America extended their agriculture northward up the Mississippi, Missouri, and Illinois river basins. By 1000 they were farming in southwestern and western Wisconsin and eastern Minnesota.[172] They grew corn in northwestern Iowa prior to 1200 in an area which is now marginal for rainfall.[173] Indian settlements on the northern plains of Iowa were abandoned with colder drier weather that set in after 1150 to 1200. After that time, the natives substituted bison hunting for growing crops. In general the land east of the Rocky Mountains enjoyed wetter conditions from 700 to 1200 and then turned drier as it experienced greater intrusions of colder Arctic weather.

The Anasazi civilization of Mesa Verde flourished during the warm period, but the cooling of the climate at the end of the medieval warmth around 1280 probably led to its disappearance.[174] This climatic shift brought drier conditions to much of the region, leading to a retreat from the territory and forcing the Pueblo Indians to shift their farming to the edge of the Rio Grande River.

Around 900, the Chimu Indians in South America developed an extensive irrigation system on Peru's coast to feed their capital of between 100,000 to 200,000 souls -- a huge number for the era.[175] The Toltec civilization, which occupied much of Mexico, reached its apogee in the thirteenth century.[176] By 1200, the Aztecs had built the pyramid of Quetzalcoatl near modern Mexico City.[177] The Mayas' civilization, however, reached a peak somewhat earlier, before 1000, and declined subsequently for reasons that remain unclear. It is possible that the warming after 1000 led to additional rainfall in the Yucatan, making the jungle too vigorous to restrain and causing a decline in farming, while at the same time improving agricultural conditions in the Mexican highlands and farther north into what is now the southwestern United States.

Thus warmer times brought benefits to most people and most regions, but not all. As is always the case with a climate shift, the changes benefited some while affecting other adversely. Change is disruptive; at the same time it produces new ideas and new ways of coping with the world. Nevertheless, for most of the known world, the Little Climate Optimum of the ninth through the thirteenth centuries brought significant benefits to the local populations. Compared with the subsequent cooling it was nirvana.

The Mini Ice Age

The Little Ice Age is even less well defined than the medieval warm period. Climatologists are generally agreed that, at least for Europe, North America, New Zealand and Greenland, temperatures fell after 1300 to around 1800 or 1850, although with many ups and downs. There was a cold period in the first decade of the fourteenth century, another around 1430 and again in 1560. The end of this period of increasingly harsh temperatures could have been as early as 1700, 1850 or even 1900 for Tasmania. The worst period for most of the world occurred between 1550 and 1700.[178] One reasonable interpretation of the data is that the world has been cooling since around 4500 B.C. with a temporary upswing during the High Middle Ages.

Europe and Asia cooled substantially from around 1300 to 1850, especially after 1400, with temperatures falling some 2deg. to 4deg.F below those of the twentieth century. This indicates that temperatures may have dipped by as much as 9deg.F in the two hundred years from 1200 to 1400, a drop of about the same magnitude as the maximum rise forecast from a doubling of CO2. These frigid times did bring hardships, and as the chart shows world population growth slowed. For much of these centuries, famine and disease stalked Europe and Asia.

Glaciers in North America and northern Europe peaked between the late 1600s and 1730 to 1780. In the Alps these ice sheets reached their maximum between 1600 and 1650. The coldness came later below the equator where the glaciers reached their extreme between 1820 and 1850.[179]

Oxygen isotope ratios from oak trees in Germany document a steady decline in average temperatures from 1350 to about 1800, with the exception of a few small upsurges and one strong temperature spike in the first half of the eighteenth century.[180] Since late in the 19th century they confirm a recovery to much higher levels. Icelandic records of sea ice attest to an increase between 1200 and the middle of the fourteenth century and then, starting in the latter half of the sixteenth century, a marked upswing in ice which appears to have peaked around 1800.[181] As H. H. Lamb points out, "in most parts of the world the extent of snow and ice on land and sea seems to have attained a maximum as great as, or in most cases greater than, at any time since the last major Ice Age."[182]

The Little Ice Age, especially the century and a half between 1550 and 1700 -- the exact timing varied around the globe -- produced low temperatures throughout the year and considerable variation in weather from year to year and from decade to decade. It included some years that were exceptionally warm.[183] The polar cap expanded as did the circumpolar vortex, driving storms and the weather to lower latitudes. Although much of Europe experienced greater wetness than during the earlier warm epoch, this dampness was more the product of less evaporation due to the cold than an excess of precipitation.

The cooling after the High Middle Ages can be seen in the lowering of tree lines in the mountains of Europe, changes in oxygen isotope measurement, and advances of the glaciers and of sea ice. This cooling diminished the abundance and quality of wine production in France, Germany and Luxembourg as depicted in historical documents such as weather diaries and farm records.[184] The ocean, which had reached relatively high levels both in the late Roman period and again during the High Middle Ages, fell to lower elevations in the seventeenth and nineteenth centuries.[185] As a result of an expanded ice cap, the circumpolar vortex, which funnels weather around the globe, moved south and spawned increasingly cold and stormy weather in middle latitudes. With the exception of southern United States and central Asia, both of which enjoyed more rainfall, this brought a worsening of the climate and disasters to people almost everywhere. During the coldest period of the seventeenth century, snow fell in the high mountains of Ethiopia above 10,000 feet which today never see snow. The subtropical monsoon rains decreased and receded farther south causing droughts in East Asia and parts of Africa.[186]

The expansion of the circumpolar vortex produced some of the greatest windstorms ever recorded in Europe. A terrible tempest destroyed the Spanish Armada in 1588. Fierce gales wracked Europe in December 1703 and on Christmas Day 1717.[187] The contrast between the cold northern temperatures which moved south and the warm subtropical Atlantic undoubtedly generated a fierce jet stream. Although we lack any information, this may also, have enhanced tornado activity on the plains of the United States.[188]

The reduced temperatures had the following general effects: (1) Arctic sea ice expanded in the Atlantic eventually cutting off Greenland; (2) glaciers advanced in Iceland, Norway, Greenland, and the Alps; (3) the upper tree line in North America and central Europe lowered; (4) enhanced wetness spawned bogs, marshes, lakes, and floods; (5) rivers and lakes froze more frequently; (6) the number and strength of storms, some of which were extraordinarily destructive, intensified sharply; (7) harvests failed engendering famine and higher prices for basic foods; (8) peasants abandoned farms that no longer enjoyed reliable weather; (9) disease for both animals and humans spread.[189]

As early as 1250, floating ice from the East Greenland ice cap was hindering navigation between Iceland and Greenland.[190] Over the next century and a half the prevalence of icebergs became worse and by 1410 sea travel between the two outposts of Scandinavia ceased. Based on the ratio of isotopes of oxygen in teeth of ancient Norsemen, researchers have estimated that the climate in Greenland cooled by about 3deg. Fahrenheit between 1100 and 1450.[191] For about 350 years, from the third quarter of the fifteenth century to 1822, no ships found their way to Greenland and the local population perished.[192]

The deteriorating climate in Europe was heralded by harvest failure in the last quarter of the thirteenth century. Compounding the insufficiency was a shift of land from farming, which because of the change in climate was more chancy, to enclosure and sheep rearing.[193] Average yields, which were already low by modern standards, worsened after the middle of the thirteenth century.[194] One of the first severe bouts of cold wet weather afflicted Europe from 1310 to 1319, leading to large scale crop failures.[195] Food supplies deteriorated sharply generating famine for much of Europe in 1315-18 and again in 1321.[196] Harvest deficits and hunger preceded the Black Death by 40 years.[197] According to Lamb, for much of the continent, "the poor were reduced to eating dogs, cats and even children."[198] This scanty food output contributed to a decline in population which was aggravated by disease. The history of many villages shows that they were abandoned before the beginning of the plague not afterwards. By 1327, the population in parts of England -- especially those later devastated by the plague -- had fallen by 67 percent.[199] People poorly nourished were quickly carried off by disease. Between 1693 and 1700 in Scotland, seven out of the eight harvests failed and a larger percentage of the population starved than died in the Black Death of 1348-50.[200]

In two terrible years, 1347 and 1348, famine struck northern Italy, followed by the Black Death, which decimated most of those not already carried away by lack of food.[201] Bubonic plague spread across the Alps after 1348, killing in the next two years about one-third of northern Europe's people. Life expectancy fell by ten years in a little over a century: from 48 years in 1280 to 38 years in the years 1376 to 1400.[202] Crops often failed; peasants abandoned many lands that had been cultivated during the earlier warm epoch. Between 1300 and 1600 the growing season shrank by three to five weeks with a catastrophic impact on farming.[203] In Norway and Scotland, the population declined and villagers deserted many locales well before the plague reached those areas.[204] The capitals of both Scotland and Norway moved south before both areas lost their autonomy.

The cooling after 1300 may also have contributed to the bubonic plague, the greatest disaster to ever befall Europe. The disease appears to have originated around 1333 in China, shortly after major rains and floods in 1332, which are reputed to have caused 7 million deaths, while disturbing wildlife and displacing plague-carrying rats.[205] The Black Death then spread to central Asia around 1338-9, which, with the increased coldness, was also drying out. By 1348 rodents carrying fleas infested with bubonic plague had marched or been carried from the Crimea into Europe. Historians have estimated that as many as one-third of all the people in Europe died in the raging epidemic that swept the continent.[206] This outburst of the plague, like a similar one in the sixth century, occurred during a period of increasing coolness, storminess, and wet periods, followed by dry hot ones. The unpleasant weather is likely to have confined people to their homes where they were more likely to be exposed to the fleas that carried the disease. In addition the inclement weather may have induced rats to take shelter in human buildings, exposing their inhabitants to the bacillus.

Not only did the cold facilitate the spread of the plague, but it caused much other human suffering. In July of 1789, just prior to the French Revolution, wet weather and air temperatures between 59 and 85 caused an ergot blight in the rye crop of Brittany and other parts of France. This blight caused hallucinations, paralysis, abortions and convulsions and came after a very cold winter that had created severe food shortages.[207] Earlier in that century wet cold summers had produced two famine years in Europe.

The end of the medieval warmth had devastating effects on populations that lived at the edge of habitable lands. For example, historians have estimated the population of Iceland at the end of the eleventh century at about 77,000, and early in the fourteenth it still numbered over 72,000. By the end of the eighteenth century, after several hundred years of coolness and stormy weather, the number of Icelanders had been cut in half to 38,000.[208]

The poorer climate in Europe after the thirteenth century brought a halt to the economic boom of the High Middle Ages. Innovation slowed sharply.[209] Except for military advances, technological improvements ceased for the next 150 years. Population growth not only ended but, with starvation and the black death, fell. Without the drive of additional numbers of people, colonial enterprise ceased and no new lands were reclaimed nor towns founded. The economic slump of 1337 brought on the collapse of the great Italian bank, Scali, leading to one of the first recorded major financial crises.[210] Construction on churches and cathedrals halted.

The hardships of the fourteenth century induced a search for scapegoats. In 1290 after some years of crop failures, the king of England expelled the Jewish population from the country. The French king followed this example in 1306 and again in 1393.[211] In 1349, the Christians of Brabant massacred the local Jews and expelled the remainder twenty-one years later.

The Mini Ice Age at its coldest devastated the fishing industry. From 1570 to 1640, during the most severe period, Icelandic documents record an exceptionally high number of weeks with coastal sea ice. Except for a few years, fishermen from the Faeroe Islands suffered from a lack of cod from 1615 to 1828 -- cod needs water warmer than 36deg. Fahrenheit to flourish. During the worst periods, 1685 to 1704, fishing off south-west Iceland failed totally.[212] In the very icy year of 1695, Norwegian fishermen found no cod off their coast. Lamb calculates that the sea around the Faeroe Islands was probably 7deg. to 9deg.F colder than it has been over the last century.[213]

The Mini Ice Age brought hard times to Southern Europe as well. Severe winters and wet summers created shortages and famines in the south of France and in Spain. The great variability in the weather made agricultural output quite uncertain and contributed to a farming crisis in the Iberian Peninsula. Although one cannot know for sure that it was the weather, the whole of the Mediterranean littoral declined economically in the seventeenth century.[214]

The cold had devastating effects elsewhere in the world. In China, frosts killed the orange trees in Kiangsi province between 1646 and 1676.[215] Per capita incomes fell as food prices rose. As already mentioned, cooler weather brought an end to the Anastazi Indian pueblo culture, as well as ending native American farming in the upper middle west.

According to Nicolas Cheetham, in the second half of the thirteenth century warfare in Greece and the necessity of keeping a large military establishment under arms reduced its previous prosperity. War does exact a high toll on economies, but it seems extraordinarily coincidental that economic troubles occurred at the time Europe was experiencing a deteriorating climate. In 1268, the King of Naples, in gratitude for military service send wheat, barley and cattle to the Peloponnese.[216] Was this needed because of crop failures solely due to military disruptions? Although not necessarily weather related, in 1275 Geoffroy de Briel, a major figure in medieval Greece, died during a military campaign of dysentery, a disease often exacerbated by cold wet conditions.[217]

Notwithstanding the cooling climate and the ravages of disease after 1300, European civilization recovered with the advent of the Renaissance in the fifteenth century. This burst of cultural activity represented a continuation, an expansion, and a deepening of the artistic and intellectual activity of the High Middle Ages. Ironically, the outpouring of art, science and literature that made up the Renaissance may have been sustained by the plague. The colder climate made agriculture more chancy, reduced the territory available for farming, and cut yields. Yet without the one-third drop in Europe's population caused by the Black Death, food supplies would have been too meager to support a large artistic and cultured class that promoted and supported the arts. The reduced agricultural output, however, was still large enough to support the even more diminished population. In China, which experienced a slower decline in numbers, real wages fell and the people became increasingly impoverished.[218] But in Europe, as a result of such a terrible death rate over a very short period, real incomes for the survivors actually climbed.[219]

From around 1550 to 1700 the globe suffered from the coldest temperatures since the last Ice Age. Lamb estimates that in the 1590s and 1690s the average temperature was 3deg.F below the present. Grain prices increased sharply as crops failed. Famines were common. The Renaissance had ended; Europe was in turmoil. The Continent suffered from cold and rain, which produced poor growing conditions, food shortages, famines and finally riots in the years 1527-29, 1590-97, and the 1640s. The shortages between 1690 to 1700 killed millions and were followed by more famines in 1725 and 1816.[220]

China, Japan, and the Indian subcontinent were also afflicted with severe winters between 1500 and 1850-80. Despite the development of a new type of rice that permitted the cultivation of three crops a year on the same land -- up from two -- the population of China, as well as that of Korea and the Near East, declined for two centuries after 1200, undoubtedly reflecting a deteriorating climate.[221] The abandonment of sea trade by the Chinese most likely resulted from deteriorating weather and less population pressure.

Costs and Benefits of Efforts to Mitigate Warming

If mankind had to choose between a warmer or a cooler climate, humans, most other animals and, after adjustment, most plants would be better off with higher temperatures. Not all animals or plants would prosper under these conditions; many are adapted to the current weather and might have difficulty making the transition. Society might wish to help natural systems and various species adapt to warmer temperatures (or cooler, should that occur). Whether the climate will warm is far from certain; that it will change is unquestionable. The weather has changed in the past and will no doubt continue to vary in the future. Human activity is likely to play only a small and uncertain role in climate change. The burning of fossil fuel may generate an enhanced greenhouse effect or the release into the atmosphere of particulates may cause cooling. It may also be simply hubris to believe that Homo Sapiens can affect temperatures, rainfall and winds.

As noted, not all regions or all peoples benefit from a shift to a warmer climate. Some locales may become too dry or too wet; others may become too warm. Certain areas may be subject to high pressure systems which block storms and rains. Other parts may experience the reverse. On the whole, though, mankind should benefit from an upward tick in the thermometer. Warmer weather means longer growing seasons, more rainfall overall, and fewer and less violent storms. The optimal way to deal with potential climate change is not to strive to prevent it, a useless activity in any case, but to promote growth and prosperity so that people will have the resources to deal with any shift.

It is much easier for a rich country such as the United States to adapt to any long term shift in weather than it is for poor countries, most of which are considerably more dependent on agriculture than the rich industrial nations. Such populations lack the resources to aid their flora and fauna in adapting, and many of their farmers earn too little to survive a shift to new conditions. These agriculturally dependent societies could suffer real hardship if the climate shifts quickly. The best preventive would be a rise in incomes, which would diminish their dependence on agriculture. Higher earnings would provide them with the resources to adjust.

The cost of trimming emissions of CO2 could be quite high. William Cline of the Institute for International Economics -- a proponent of major regulatory initiatives to reduce the use of fossil fuels -- has calculated that the cost of cutting emissions from current levels by one-third by 2040 as 31/2 percent of World Gross Product.[222] Given his assumption that cutbacks of CO2 emissions are done by the least cost methods and his bias, we can be certain that in the real world outlays to slow warming would be considerably higher. In terms of the estimated level of world output in 1992, his estimate would amount to roughly $900 billion annually, an amount that could slow growth and impoverish some who survive on the margin. These resources could be better spent on promoting investment and growth in the poorer countries of the world.

Should warming become apparent at some time in the future and should it create more difficulties than benefits, policy makers would have to consider preventive measures. Based on history, however, global warming is likely to be positive for most of mankind while the additional carbon, rain, and warmth should also promote plant growth that can sustain an expanding world population. Global change is inevitable; warmer is better; richer is healthier.