Science, June 9, 2006
Looking Way Back for the World's Climate Future
Richard A. Kerr
Researchers worry that if they cannot recall the distant climatic past, the world may be condemned to repeat it. And repeating the warmth of the early Pliocene epoch of 3 million to 4 million years ago would be a shocker. With no more carbon dioxide warming the greenhouse than today, the globe was a good 3°C warmer, and sea level was a whopping 25 meters higher. But how could such a modest stock of greenhouse gas fuel such warming? Unfortunately, no one knows.
On page 1485 of this issue, a group of climate researchers takes a look back at the Pliocene, pulls together models of oceanic and atmospheric behavior under those conditions, and concludes that humans may already have put the world on a path back to that epoch. "It's a very interesting period to study, a great scientific puzzle," says paleoceanographer David Lea of the University of California (UC), Santa Barbara. "I like the way they're thinking." But better paleo-data and more realistic modeling will be needed before anyone knows for sure.
The key to understanding Pliocene and possibly future climate, say climate dynamicist Alexey Fedorov of Yale University and his colleagues, could be the climate changes occurring at mid- to high northern latitudes. Those changes might constitute a climatic switch: Throw it one way, and trigger a permanent El Niño in the Pacific Ocean capable of warming the whole world. Throw it the other way, and El Niño and La Niña alternate in a cooler world as they do today.
In an earlier study of Pliocene climate, paleoceanographer Christina Ravelo of UC Santa Cruz, a co-author of this paper, and her colleagues found continuously warm water from one end of the tropical Pacific to the other: the hallmark of an El Niño. Then about 3 million years ago, the eastern Pacific began cooling, according to their analysis. That set up the warm-in-the-west, cool-in-the-east arrangement that typifies modern conditions. Another analysis of the same deep-sea sediment cores as Ravelo used came up with the opposite Pliocene arrangement: permanent cold in the east (Science, 29 July 2005, p. 687). But Ravelo recently analyzed another kind of paleotemperature record across the Pacific and again found a permanent Pliocene El Niño, which persuades Lea that the weight of evidence now favors a Pliocene El Niño over La Niña.
If El Niño ruled the Pliocene, what threw the climatic switch to end its reign some 3 million years ago? To answer that and, conversely, to learn what might switch climate back to Pliocene conditions, Fedorov and his colleagues draw on several modeling studies they have published in recent years. In their scenario, the long-term cooling of the past 50 million years and accompanying drying at high latitudes in the North Atlantic would have cooled surface waters there and made them saltier. Making waters denser would have swelled the river of cold water that sinks into the depths there. That in turn would have increased the volume of cold, deep seawater. Almost all the ocean's water is near freezing, even beneath the tropics; during a permanent El Niño, cold water does not rise to the surface in the eastern Pacific.
But the overlying warm layer would have thinned as the underlying cold water expanded. In the scenario's eastern tropical Pacific, it eventually thinned enough for winds to raise cold water to the surface and break El Niño's grip on the Pacific. That, in turn, would have sharply increased the amount of low-lying clouds reflecting solar heat into space and decreased the amount of water vapor trapping heat in the atmosphere. The breakthrough of tropical cold waters would have thus accelerated global cooling roughly 3 million years ago, when ice first began growing in the north. Today, the strengthening greenhouse seems to be pushing the other way on the switch, warming high latitudes and freshening northern waters with melting ice and more rain.
Nice story, but is it true? "They might be right," says El Niño modeler Amy Clement of the University of Miami in Florida, but so far the modeling has been piecemeal. "You have all the pieces of the puzzle," she says, such as the oceans and atmosphere. But when "you put them together, the result is a lot more complicated than you expect."
Researchers agree that it's urgent to sort through the complications. If there is a climatic switch as described by Fedorov and his colleagues, humans are pushing it harder and harder toward Pliocene conditions. Carbon dioxide emissions are already raising atmospheric levels into the top of the estimated range during the Pliocene, and high northern latitudes are getting warmer and wetter. That alone, say Fedorov and his colleagues, could possibly push Earth back to a permanent, globe-warming El Niño within decades to centuries. In their scenario, all it would take would be a warm surface layer in the eastern Pacific just a few tens of meters thicker than today, and the Pliocene would be back.