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The Good, The Bad, and The Ugly: Climate Change

Nancy Grumet
Department of Geological and Environmental Sciences
Stanford University
May 2001

"Ski season ended early this year!" "Why are the leaves changing so late?" "I haven't seen rain like this in decades!" How many times have we heard people speculate on the dramatic change of climate? Every time the ski season ends early or our tomato plants grow poorly, we wonder why climate is changing. We may, however, simply be accustomed to a certain type of climate because that's all we've known for the last 20 or 50 years. In the scope of the earth's history, though, this is a mere blink. One is tempted to ask how important these environmental changes are for predicting climate change in the next year, decade, century, 100,000 years or 1 million years? Therefore, the scientists who study climate change must decide how significant these short-term changes are in comparison to natural climate change (pre-industrial, pre-1900's).

To resolve the current scientific and political debates over the importance of greenhouse gas emissions and global warming, we must examine whether recent climatic changes are short-term "natural" variations or whether these variations are man-made and would not have occurred without human interference. How do we know how climate has varied before human interference? "Natural recorders" (i.e., ice cores, tree rings, and coral records) of climate variability allow us to reconstruct records of past climate (paleoclimate) change beyond the current instrumental recordings (i.e., weather stations and computers) and recent historical periods (i.e., shipping log books and hunting and trapping log books).

Unlike instrumental records that tell us only about the most recent century, proxy records (natural archives of climate change) enable us to place recent climatic change in the context of the last several hundred to thousand years. The data and information we gain from these natural archives allow us to "tune" and test the accuracy of our climate models. Before a model result can be taken seriously, the model must reasonably demonstrate that it can simulate current and past climate change. Eventually, these combined efforts will help us identify the range and pattern of natural climate variability and determine whether observed changes in climate are within these limits of natural variation or whether they are human-induced changes.

My research involves the reconstruction of ocean sea-surface conditions in the Indian Ocean, a region with sparse data and of limited understanding. By looking at the chemistry in coral samples, I can analyze how these coral animals have responded to changes in sea-surface temperature and ocean circulation. As corals grow, they excrete a skeleton made of calcium carbonate, called aragonite, the chemistry content of which reflects the chemistry of the surrounding water. In turn, the chemistry of the water changes when there are local changes in rainfall, river input, evaporation, and temperature. Most coral species grow several millimeters to several centimeters per year and produce annual growth bands, much like tree rings, which can record monthly environmental changes. Fossil corals from the Holocene Epoch (~10,000 years before present) tell us of the climate of that era. The age of the fossil corals are determined through precise geochemical measurements and radiometric dating. Corals have been collected from a variety of sites, including the western and eastern equatorial Pacific, the Indian Ocean, the Coral Sea and the equatorial Atlantic. These studies have allowed us to gain a better understanding of global sea-surface conditions.

In order to reconstruct paleo sea-surface conditions, I measure, calibrate and interpret oxygen isotope ratios (18O/16O) of coral aragonite (CaCO3). The 18O/16O ratio varies as a function of temperature (and salinity). In general, higher ratios are indicative of colder temperatures, and lower ratios are indicative of warmer conditions. In addition to oxygen isotope values, I measure, calibrate and interpret trace element (Sr, Ca, Ba, Mg) and radiocarbon (D14C) concentrations. Trace elements offer a wider range of climatic interpretation by recording site-specific features, such as upwelling of cold deep water to the surface, runoff of freshwater from rivers, and mixing of different water masses. Radiocarbon (14C) measurements have been used to reconstruct ocean circulation. Nuclear bomb testing in the 1950-60's dramatically increased the surface ocean radiocarbon concentration. I am able to document when deep water is brought to the surface because the 14C content of the coral is decreased.

Climate variability in the Indian Ocean is very complex due to the seasonally reversing monsoon system and the ability to predict climate change is difficult. The benefits of improved predictability are enormous. In those countries affected by the monsoon system, the failure or delay of the monsoon can make the difference between famine and life due to devastating floods or drought. Furthermore, economies around the world are now so closely linked that the impact of a failed monsoon may be felt globally. Hence, there are both financial and agricultural benefits to improving our understanding and predictability of climate variability in the Indian Ocean.

My thesis is that climate change in the Indian Ocean region behaves independently of that in the Pacific Ocean. As of yet, no records have been published from corals in the eastern basin of the Indian Ocean. Situated 140 km off the coast of West Sumatra, the Mentawais are a group of islands bordering the equator in Indonesia. This region represents an optimal site to monitor SST reversals that signal the onset of climate change in the Indian Ocean. Monthly resolved time series of isotopic values (d180, d13C), trace element concentrations (Sr, Ca, Ba, Mg), and radiocarbon measurements (D14C) collected from corals in the Mentawis Islands will be used to explore several hypotheses about internal variability in the Indian Ocean and periodic variability in the monsoon and SST. Ultimately, we can improve our predictive capabilities and potentially save millions of lives by integrating results from my research into regional climate models.