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Interpreting Global Climate Change: Clues From the Past

Kevin Theissen
Geological and Environmental Sciences
Stanford University
June 2001

My research uses geologic indicators from marine and lake settings to look at past records of climate change from two globally important regions: Antarctica and subtropical South America.

The threat of global warming is a subject of intense debate in scientific and political arenas.  While it is widely accepted that this phenomenon is occurring, there are still uncertainties as to how it will impact humanity. One way to better understand ongoing climate change is to examine the geologic evidence of past changes. My research contributes to a growing effort to improve our understanding of the timing and impact of past and present climatic change events and to provide insight into important questions about global warming and other climate variability we might expect in the future.

There is a growing body of scientific evidence of global warming and environmental change in our world today.   Significant questions remain about these changes, the most important being how much temperatures will change on a local, regional, and global basis, how much global sea levels will rise, and how rapidly the changes will take place. A warmer earth promises rising sea levels, more frequent and destructive storm events, and widespread drought.  These problems present a significant threat to humans and many other forms of life on the planet, and for this reason scientists in my field of geology, called paleoclimatology, are using past records to gain a better understanding of regional climate change and the global climate system.

My research involves the recovery and interpretation of climate records from two distinct regions.  The first is a detailed record of climate and environmental change from geologically recent sediments taken from Lake Titicaca, South America. Lake Titicaca is located in a region known as the Altiplano, between the eastern and western ranges of the Andes mountain chain in Bolivia and Peru. Sediments stored in the lake basin hold a record extending back at least several hundred thousand years.  Recent paleoclimate research in this area is concerned with the time since the end of the last glacial period, around 12,000 years ago. During this time, the lake underwent a number of transitions from higher to lower lake levels and back again, which indicate corresponding changes in climate from wetter to drier conditions.

In my research group, we obtain records of these fluctuations and other environmental information by measuring changes in the ratios of stable isotopes of carbon, oxygen, and nitrogen from organic and inorganic components of the sediment. My focus is on sediments which were deposited on the lake floor 4,000-7,000 years ago when the lake was much lower than it presently is. Much of the sediment record from this time period was deposited in packages called varves. Each varve represents one year of sediment accumulation, thus providing a detailed account of changes in lake conditions.  My objective is to reconstruct this detailed record and to examine changes in the lake that took place on a decade to century timescale over this time period.  I will then attempt to link these changes to regional and global climate processes.  This is important because while we have a good sense of long-term (millennial and longer) changes, there is less information available about change over shorter timescales, driven by events such as the El Niņo Southern Oscillation, which can be highly significant as well.

My results suggest that there were several rapid and sometimes large lake level changes during this period and that the ecology of the lake and regional climate were significantly different.  The implications from this work are that the Altiplano region is subject to rapid and erratic climatic change, which can result in long-term drought over centuries or millennia.  Such events have enormous local and regional impact on human populations and are believed to have led to the fall of civilizations such as the Mayan and Tiwanaku cultures in the past.

The second project is a study of climate records derived from marine sediments from Prydz Bay in eastern Antarctica.  Antarctica is an area of increasing study because of the potential effects of climate change often most apparent in polar regions. The news of global warming has resulted in a growing concern about the stability of the polar ice sheets, which are comprised of large volumes of frozen seawater. If these ice sheets were to melt, the oceans would rise globally by tens to hundreds of meters, inundating coastlines and causing worldwide catastrophes. Paleoclimatologists and other geologists are able to study the past extent of ice sheets by using a number of geologic indicators that are left in the sediment accumulating on the sea floor. I am investigating the Prydz Bay record for clues about the magnitude and timing of past climate change. In order to do this, I am using stable isotope ratios in tiny marine fossils to reconstruct the climate history of the Prydz Bay region over the past 2-3 million years. The isotopic record provides information about ocean temperature, ice volume, salinity, and other important characteristics.  Using this information, I can determine relative changes in the size and stability of the East Antarctic Ice Sheet, which are controlled by climate.

The results I've gathered so far reveal a history of regular, periodic changes from glacial to interglacial conditions which occurred on 20,000-, 40,000-, and 100,000-year timescales. These transitions involved significant changes in temperature, sea level, ocean/atmospheric chemistry, and productivity. I will compare these results with those from paleoclimate studies in other world regions to determine which changes have local, regional, or global impact.

Though the two projects have important differences, both involve the use of past records to better predict future climate change.  Regions as distinct as Antarctica and the South American subtropics are connected in the global climate system, and changes that occur in one region influence the rest of the system.  For this reason, past climate data are now routinely used in predictive global climate models. My research will improve our knowledge of the stability of the Antarctic Ice Sheet and give us a sense of the magnitude and speed of past subtropical climate change. The results of my work in both regions will aid in the larger effort to better characterize the global climate system through modeling, to identify climate links between regions, and to better address the potential impact of global warming.