INTRODUCTION:

Dr. Thomas Lovejoy
Chief Biodiversity Advisor to the President of the World Bank, The World Bank, Washington, DC

SPEAKERS:

Dr. Alan C. Mix
College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR

Dr. Lonnie G. Thompson
Byrd Polar Research Center, Ohio State University, Columbus, OH

Unprecedented global changes in the 20th Century have heightened awareness of human vulnerability to potential climate changes in the 21st Century. The influence of the tropics on the Earth's climate, as well as the sensitivity of the tropics to a global climate warming (and variations in the occurrence and intensity of El Niño/La Niña events and monsoons), are issues of growing concern. Because the tropics are the primary heat reservoir for the Earth's climate, events in the tropics are also likely to influence the entire globe. Furthermore, half of the Earth's surface and three-quarters of the Earth's population are situated between 30°N and 30°S.

On time scales of years to decades, intermittent warming and cooling of the tropics resulting from El Niño (warm) and La Niña (cold) events yield global impacts. During the last ice age (about 20,000 years ago), tropical cooling patterns resembled La Niña events, suggesting that La Niña-like conditions may have existed at various times in the recent past, as long-term climate regimes, as opposed to the short-lived events one sees today. Deep-Sea sediment records also reveal that changes in the tropics often preceded, and thus, may have helped bring about, climate changes in high northern latitudes. Shifts in tropical temperatures modify large-scale winds that transport heat and water vapor between ocean basins, eventually leading to massive reorganizations of ocean circulation and salinity. Ice cores from tropical South America also indicate that long-term changes in the frequency of El Niño/La Niña events have occurred in the past, suggesting that the climate system can possibly become locked into an El Niño-like or La Niña-like state for decades, centuries, or even millennia. Consequently, such findings carry implications and concerns about a long-term globally-averaged climate warming and its resultant impact on the tropics and elsewhere.

Until quite recently, many scientists considered long-term climate changes, such as the waxing and waning of ice ages and intermittent warm periods, to be primarily a polar-driven phenomenon that had very little impact on the climate of the tropics. However, recent evidence of past tropical climate conditions, based on high-resolution deep-sea sediment cores and ice cores from tropical mountain glaciers, reveal extensive cooling in the tropics during the last ice age, on the order of 4-5°C in the oceanic upwelling areas of the eastern equatorial Pacific and the Atlantic Oceans. The pattern of these changes is broadly similar to, but of greater magnitude and extent, a modern La Niña (cooling) climate event.

From a modeling perspective, Atmospheric General Circulation Model (AGCM) simulations of the ice-age tropics suggest that cooler sea surface temperatures produce anomalous high-pressure cells around the equator, resulting in enhanced westward-flowing winds across northern South America, from the Atlantic to the Pacific. These enhanced winds lead to further cooling in the eastern Tropical Pacific by bringing cooler, deeper ocean water to the surface due to increased upwelling. The net effect is colder La Niña-like climate regimes that either occur more frequently or are sustained over longer time scales.

Rising equatorial air masses in the ice-age simulations are suppressed over South America, while descending air masses are enhanced over the eastern Pacific and Atlantic, leading to changes in the distribution of atmospheric moisture and water vapor, resulting in aridity over the Amazon Basin. Ice-age drying also extended eastward over Central Africa to the Indian Ocean. The central Andes and northern Mexico received more moisture, resulting in a rise in lake levels and net growth of tropical mountain glaciers, some of which extended nearly 800 meters below their present altitudes. Thus, it appears that a cooling of the tropical oceans and advances in tropical glaciers are linked.

Stronger winds also increase the westward transport of water vapor from the Atlantic to the Pacific Oceans. Over time, such changes in the transport of fresh-water from one ocean to another carry the potential to either dilute or concentrate the salt in each ocean which, in turn, affects deep ocean circulation and climate at higher latitudes. Furthermore, tropical climate changes, reflected in Amazon River runoff, preceded changes in the high northern latitudes. Thus, large-scale influences of tropical aridity may have been part of the cause of the ice ages, rather than having been solely a consequence of changes initiated at high latitudes. Connections between the atmosphere and oceans in the low latitudes may carry tropical influences to the poles via oceanic circulation.

In tropical South America, three ice core records from high altitude sites (Huascaràn and Quelccaya in Peru, and Sajama in Bolivia) reveal climate changes on millennial, decadal and interannual time scales. Along the western edge of South America, the climatic consequences of temperature variations in the Pacific are different north and south of the equator. In the south, where Quelccaya and Sajama are located, drought conditions occurred during El Niño events (such as that of 1988), and wet conditions occurred during La Niña events. The opposite pattern occurs to the North where Huascaràn is located. For example, in Peru, the 1998 El Niño brought wet conditions to the north, with lakes forming in the coastal desert, and drought conditions to the south, as shown by very low water levels in Lake Titicaca. Evidence from these three sites suggests the former existence of patterns of climate that are not unlike the El Niño/La Niña states (typically lasting 2-5 years in duration) that are observed in the modern climate record, although in many instances these historic look-alikes were more long-lived. In addition, archeological sites in Peru and elsewhere indicate that these climate changes impacted regional civilizations and cultures.

A 1500-year ice core record containing annual records of climate (precipitation and temperature) from Quelccaya indicates that drought years have been associated historically, with El Niño-like conditions. From a decadal perspective, this 1500-year record of precipitation, when compared with prehistoric Peruvian archeological records, shows that when it was wet in the south, highland cultures flourished. During extended dry periods in the south (similar to today's El Niño states, but operating on decadal to centennial time scales) the highland cultures disappeared while the coastal cultures of northern Peru and southern Ecuador made their first appearance. These climate patterns suggest the former existence of El Niño/La Niña-like states lasting 200 to 300 years in duration. While cultures in the tropics seemed quite capable of adapting to the short-term (two-five years in duration) El Niño climate states, they were often unable to cope with the longer-term, El Niño-like climate states that persisted from decades to centuries to millennia.

The Huascaràn and Sajama records extend back in time over 20,000 years, and consistently show that over millennial time scales, high precipitation periods on Huascaràn (at 9°S) occurred concurrently with dry periods on Sajama (18°S). During the last 11,000 years, (and especially from 11,000 to 6,000 years before present) the north has experienced a warm, relatively humid climate while the south has been dry. Conversely, during the last ice age it was very dry around Huascaràn while it was very wet in the region around Sajama. These historic climate records suggests a "see-saw" pattern similar to that of the El Niño/La Niña climate states as observed in modern times but, unlike their modern equivalents, these historic El Niño/La Niña-like states appear to have operated on time scales of centuries to thousands of years. These long duration El Niño/La Niña-like events might well be indications that the climate system was occasionally locked into El Niño or La Niña-like states for extended periods of time, having implications regarding present and future climate trends.

Presently, evidence of a significant climate warming in the tropics during the second half of the 20th century, is rapidly accumulating. This warming is causing the rapid retreat and, in some cases, the disappearance of ice caps and glaciers at high elevations in the tropics and subtropics. These ice masses are particularly sensitive to small changes in localized temperatures as they are very close to their melting points. Retreat (melting) of the Qori Kalis Glacier, a valley glacier on the western side of the Quelccaya ice cap, has accelerated over time. The rate of glacial retreat from 1995 to 1998 (49 meters per year) is almost twice what it was during the period 1993-1995, and ten times greater than it was between 1963 and 1978. At the current rate of retreat, the Quelccaya ice cap will disappear entirely in less than twenty years. Of the six tropical glaciers that existed in Venezuela in 1972, only two remain, but are also likely to disappear within twenty years. Moreover, 75% of the glacial ice that existed on Mt. Kiliminjaro in 1912, and 40% of the glacial ice that existed on Mt. Kenya in 1963, have now melted.

Contrary to a previously widely held scientific notion which assumed that the tropics were largely insensitive to global climate changes, observational evidence of recent as well as former abrupt climate changes suggests that the tropics are extremely sensitive to changes in global and regional climates. As the Earth's climate is thermodynamically non-linear, water vapor feedbacks in the tropics, associated with a global warming, may, in fact, be partially responsible for the present and possible future amplification of climatic changes in the tropics. These results have important societal implications given the present observed and projected future climate trends, and the rates at which these changes are occurring.

Dr. Alan C. Mix is a Professor in the College of Oceanic and Atmospheric Sciences, Oregon State University (OSU) where he teaches graduate courses in marine geology and paleoceanography. He also directs the OSU Stable Isotope Facility and the Northwest Consortium for Oceanographic Research (NORCOR) Marine Geology Repository, one of the nation's largest sediment core laboratories. Dr. Mix's research interests lie in paleoclimatological studies using marine geochemistry and paleontology.

Dr. Mix has participated in numerous international research programs, including over 20 sea-going expeditions, and has served as an advisor to the Ocean Drilling Program and other advisory panels. He presently chairs the international EPILOG program (Environmental Processes of the Ice Ages, Land, Ocean, and Glaciers), and is associate editor of the journals Geology, and Quaternary Science Reviews. He has authored 104 peer-reviewed scientific papers and technical reports, and has edited one book. Dr. Mix received a B.S. degree in Geology from the University of Washington in 1979, and a Ph.D. degree in Geology from Columbia University in 1986.

Dr. Lonnie G. Thompson is a Professor in the Department of Geological Sciences, and a Research Scientist, at the Byrd Polar Research Center at the Ohio State University, where he teaches courses in environmental geology, paleoclimatology and Earth System Sciences. His research interests are focused on the history and operation of the El Niño and monsoonal systems that dominate the climate of the tropical Pacific, and the effect they have on global-scale oceanic and atmospheric circulation patterns. Dr. Thompson's research has helped draw attention to the existence and importance of tropical and subtropical ice fields, and the wealth of climate information housed within them. He has led 37 expeditions to polar and high-altitude regions. His work on tropical ice cores outside the polar regions has also resulted in the development of solar-powered drills for acquisition of climatic histories from the South American Andes to the ice fields on the Tibetan Plateau. Much of Dr. Thompson's work appears in 138 peer-reviewed scientific articles.

Dr. Thompson has served on a number of committees of the National Academy of Sciences, as well as having served on planning panels for the National Science Foundation and the National Oceanic and Atmospheric Administration. He received his B.S. degree in Geology from Marshall University in 1971, and his M.S. and Ph.D. degrees in Geology in 1973 and 1976, respectively, from The Ohio State University.