News Archive

posted 07/11/02

Columbia University Researchers Reveal Surprising El Niņo-Like Conditions During Last Ice Age
Research Indicates Unexpected Role of Tropical Pacific Ocean in Global Climate Change

El Niño has always been associated with warming of tropical Pacific surface waters and global temperatures. However, new research publishing in the journal Science shows that conditions resembling El Niño were the norm during the last ice age, 18,000 plus years ago, when global temperatures were dramatically cooler than they are today.

Athanasios Koutavas, a Ph.D. student at Columbia University's Lamont-Doherty Earth Observatory, working with Professor Jean Lynch-Stieglitz and Dr. Thomas Marchitto, led a study to examine the climate over the last 30,000 years in the eastern equatorial Pacific Ocean–the region most sensitive to the periodic onslaught of El Niño, now showing fresh signs of an impending return.

Scanning electron micrograph of a fossil foraminifer shell (Globigerinoides sacculifer) preserved in 10,000-year old sediments from the eastern equatorial Pacific Ocean. Photo: Dee Breger

"The tropical Pacific is the primary mover of modern climate variability," said Koutavas. "The question we set out to answer with this study was what role did it play in the sudden and dramatic glacial terminations that have occurred every 100,000 years over the last million years, the most recent of which marked the end of the last ice age and the beginning of the present interglacial age known as the Holocene. What we discovered goes against current thinking in regard to long-term tropical temperature patterns and their effect on global climate. Using El-Niño and La Niña as examples, we expected that the ice-age tropical Pacific would resemble a La Niña, which normally promotes global cooling. Yet the dominant pattern we observed is that of El Niño, despite the presence of cooler ocean temperatures and a glaciated world."

Temperatures in the ancient ocean were reconstructed through the magnesium/calcium ratios in the remains of organisms found in seafloor sediments. These organisms incorporated quantities of magnesium in relation to the water temperature. The resulting data enabled the researchers to consider ice-age temperature contrasts in the equatorial Pacific Ocean, which spans 45% of the earth's equatorial circumference. The temperature contrast between the warm waters of the western and the cool waters of the eastern Pacific Ocean is a defining characteristic of the tropical Pacific ocean-atmosphere system, changing appreciably during El Niño and La Niña episodes. During El Niño the contrast is unusually small whereas during La Niña, it is unusually large.

Koutavas and his colleagues found that during the last ice age, the east to west temperature contrast was markedly reduced, similar to what happens during El Niño today. Oppositely, they found that the contrast was largest during the part of the interglacial period known as the mid-Holocene, 5 to 8 thousand years ago. This supports an emerging hypothesis that the mid-Holocene, which was on average 4 to 8 degrees warmer than its preceding ice age, resembled a permanent La Niña.

Interestingly, during both the last ice age and the mid-Holocene, the tropical Pacific was on average cooler than the present, even as the former was El Niño-like and the latter La Niña-like. This suggests that whatever role the tropical Pacific plays in global glacial to interglacial transitions, it is more likely to be related to a change in the dominant sea surface temperature pattern rather than the absolute temperature change, although the latter undoubtedly plays a role as well.

Changes in the prevailing temperature pattern in the tropical Pacific are thought to be related to changes in the Earth's orbit caused by the gravitational tug of the planets, which alter the seasonal heating by the sun. Because the seasonal solar heating cycle in the tropics has been steadily increasing over the last 6,000 years and will continue to do so until it maxes out 6,000 years into the future, we could expect more El Niño-like conditions in future millennia.

"Whether this will help initiate or inhibit the next ice age remains unknown. This determination requires great improvements in our understanding of long-term tropical climate processes, which undoubtedly has much to gain from continuous study of the paleoclimate record," said Koutavas.

A. Koutavas is a Ph. D. student with the Lamont-Doherty Earth Observatory, a research unit of the Earth Institute of Columbia University.

The Columbia Earth Institute is a leader in Earth systems science teaching, research and the application of Earth and social science for human needs. The Institute is the outcome of Columbia University's commitment to enhance understanding of global sustainability through the collaborative work of physical, biological, and social scientists in cooperation with an involved citizenry. One of its research units, The Lamont-Doherty Earth Observatory is among the world's leading research centers examining the planet from its core to its atmosphere, across every continent and every ocean. From global climate change to earthquakes, volcanoes, environmental hazards and beyond, Observatory scientists continue to provide the basic knowledge of Earth systems that must inform the difficult decisions that will determine the future health and habitability of our planet.

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