February 10, 2003
Ancient climate may augur future effects of global warming
WEST LAFAYETTE, Ind. Ancient lake sediments and modern computers both indicate that El Niño might react differently to global warming than current theory claims, according to a Purdue research report.
Purdue University's Matt Huber has simulated the "hothouse" climate of the distant past with a computer model to study the reaction of the tropical Pacific Ocean, a key player in removing heat from the atmosphere. While it cannot absorb an unlimited amount of atmospheric heat, Huber has found that even when the climate warms, the tropical Pacific Ocean maintains its ability to remove heat periodically the permanent loss of which could encourage runaway global warming. Huber has found historical evidence for his theory in 45 million-year-old lake sediments, which may indicate that the relationship between global warming and El Niño needs to be re-examined.
"The tropical Pacific's ability to cool the atmosphere may be less susceptible to global warming's effects than we believed," said Huber, an assistant professor of earth and atmospheric sciences in Purdue's School of Science. "We should still be greatly concerned about global warming, but it appears that one mechanism involved in climate change operates differently than we have imagined."
The research appears in the Feb. 7 issue of Science.
In the decades since theories of global warming came to prominence, there has been intense scientific debate over how an influx of greenhouse gases into the atmosphere would affect the Earth's climate, particularly El Niño-La Niña oscillations. El Niño refers to a warming of the surface layers of the eastern Pacific Ocean, which occurs in those years when the prevailing westerly winds in the South Pacific die down, allowing the warm waters from the western Pacific to slosh eastward. Conversely, in a La Niña year, the winds pile up warm water in the western Pacific and drag cooler water up from the depths in the east. For the past several millennia the Pacific has alternated between these two states in an irregular but basically stable oscillation.
"A question climate scientists have debated is whether future global warming might make the oscillation stop," Huber said. "The worry is that Earth would suffer a runaway greenhouse effect if that happened."
Ordinarily the cool surface layer of the eastern Pacific absorbs heat from the tropical atmosphere and carries it far away via ocean currents that flow hundreds of meters below the surface. But in an El Niño year, both the shallows and depths grow so warm that the atmospheric heat has nowhere to go, causing a warming of the tropics and strange weather patterns worldwide.
"If you compare the eastern Pacific to a water-cooled radiator, then the ocean currents are the coolant that absorbs atmospheric heat," Huber said. "El Niño heats the radiator to the point where it can't do its job. Nowadays, El Niño events are too brief to have lasting effect, but some have theorized that if the oscillation stops the Earth will suffer a 'continuous El Niño-like state' that would warm the planet very quickly."
Huber's desire to ground such theories with historical evidence led him to examine a period of the distant past when the Earth's climate was considerably warmer the Eocene epoch. During the Eocene, nearly 50 million years ago, palm trees grew in the north of England and alligators thrived far above the Arctic Circle on Canada's Ellesmere Island.
"We figured that if a continuous El Niño state had ever existed, it would have been during the Eocene," Huber said. "So we decided to use a computer model of the Eocene's climate to see what the eastern Pacific Ocean would do."
Huber and Rodrigo Caballero, both working at the time at Denmark's Neils Bohr Institute, spent several years simulating the Eocene atmosphere and oceans with computers at the National Center for Atmospheric Research in Boulder, Colorado. Their results, which surprised even them, indicated that the tropical oceans were more resilient at absorbing heat than current theory states.
"It seems that when the global climate was considerably warmer, the tropical eastern Pacific was still relatively cool, even though most theories suggest it would have warmed as well," Huber said. "It turns out these theories were not wrong, merely oversimplified. Instead of a two-layer ocean, with shallows that absorb heat and depths that carry it away, there was a third layer wedged between them. It was this third layer that was the key to it all."
Huber theorizes this third layer of water remained cool even when the temperature increased above and below it. This wedge, which in Huber's computer model extends across thousands of miles of ocean, would enable the radiator to keep operating despite a high level of atmospheric greenhouse gases.
"The wedge formed a cool barrier between the warmer shallows and depths," Huber said. "It was a place for the heat to go from above, and, for complex reasons, it did not absorb heat from the depths. The computer model told us that for this reason the Eocene tropics were not much different than they are today, even though the Earth's poles were much warmer."
All these results could be written off as merely so much computer gaming, but Huber found that other scientists had turned up evidence for ancient El Niño events in the sediments of ancient lakebeds in present-day Wyoming and Germany.
"These lakes were far from the Pacific Ocean, but they still felt the effects of El Niño," Huber said. "Sediment records show that El Niño events occurred in the Eocene with the exact same frequency as the computer model shows. In short, there was no continuous El Niño state, even though the Earth's climate was considerably warmer."
While such news might imply that we can be less concerned about global warming, Huber is careful to point out that his research does not contradict the climatic catastrophes that global warming will supposedly bring.
"Just because the tropics appear to be more resilient than we had thought does not mean that they will remain comfortable regions even if the globe warms up significantly," he said. "Global warming could make their average temperatures rise about 10 degrees Fahrenheit. It may also bring the more extreme storms and disastrous rises in ocean water levels that scientists have predicted for decades. It's important to remember that we are clarifying a single important aspect of the global heat engine, but not reimagining the net result of global warming on our climate as a whole."
Huber said he would like the research to inspire further investigation of climate history, which will be necessary to bolster his theory.
"The major weakness of this study is that we have taken data from lakebeds in Wyoming and Germany, but not from the tropics themselves," he said. "While these ancient lakes were affected by El Niño in their day, it would be useful to have tropical records of El Niño from 50 million years ago as well."
Funding for this research was provided in part by a grant from the National Science Foundation and involved support from, and collaboration with, Lisa Sloan of the University of California Santa Cruz, Bette Otto-Bliesner at the National Center for Atmospheric Research and the Danish Center for Earth System Science.
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A publication-quality graphic is available at ftp://ftp.purdue.edu/pub/uns/huber.elnino.jpeg.
Eocene El Niño: Evidence for Robust Tropical Dynamics in the "Hothouse"
By Matthew Huber and Rodrigo Caballero.
Much uncertainty surrounds the interactions between the El Niño Southern Oscillation (ENSO) and long-term global change. Past periods of extreme global warmth, exemplified by the Eocene (55 million to 35 million years ago), provide a good testing ground for theories for this interaction. Here, we compare Eocene coupled climate model simulations with annually resolved variability records preserved in lake sediments. The simulations show Pacific deep-ocean and high-latitude surface warming of ~10° C but little change in the tropical thermocline structure, atmosphere-ocean dynamics, and ENSO, in agreement with proxies. This result contrasts with theories linking past and future "hothouse" climates with a shift toward a permanent El Niño-like state.