In Hot Water: Ice Age Defrosted by Warming Ocean, Not Rise in CO₂

Earth's climate can be sensitive, changing after a variety of events. A volcanic eruption or meteorite impact, for instance, can send enough particles into the air to block the sun and cool the climate. A thickening blanket of greenhouse gases can trap heat. And, more commonly, according to some scientists, slight changes in Earth's orientation toward the sun can cause it to cool or warm in so-called Milankovitch cycles (named after the Serbian engineer who first described them). Now, new evidence from a marine sediment core from the deep Pacific points to warmer ocean waters around Antarctica (in sync with the Milankovitch cycle)—not greenhouse gases—as the culprit behind the thawing of the last ice age.

Ice cores drawn from Antarctica and Greenland have shown that carbon dioxide (CO₂) levels in the atmosphere began to rise at roughly the same time as the vast ice sheets began to melt. But it remained unclear exactly which came first: melting ice and warming seas released more CO₂ or more CO₂ led to melting ice and warming seas.

By studying sediment cores from the deep Pacific near the Philippines, paleoclimatologist Lowell Stott of the University of Southern California in Los Angeles and his colleagues revealed that the temperatures of the deepest seas rose by around 2 degrees Celsius (3.6 degrees Fahrenheit) at least 1,000 years before sea-surface temperatures. "Even accounting for the uncertainties of the age of CO₂, the deep sea warmed substantially before the CO₂ began to rise," Stott says. "The deep Pacific is such an enormously large volume of water that [this warming] reflects the input of a tremendous amount of energy into the global system."

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Stott and his colleagues used the isotopes of oxygen contained within the remnants of microscopic surface and deep-sea creatures to establish temperatures; they then used a radioactive isotope of carbon to date their age. Combining the two techniques showed that deep-sea creatures dealt with a warmer climate long before their surface brethren did, they report in the online edition of Science.

Because such deep seawater circulates from the coast of Antarctica, this deep-water warming implies that the Southern Ocean drove the last major climate change. Stott notes that the periodic wobble in the Earth's rotational axis described by the Milankovitch cycles led to more sunshine falling on the Antarctic at the same time—a likely cause of the warming waters. "The amount of solar energy increased at the same time as this deep-sea warming," he says. "Sea ice around the Southern Ocean was withdrawing."

According to the marine core sample, a full millennium passed—enough time for both the deep and surface waters to entirely switch places—before sea-surface temperatures and global atmospheric levels of CO₂ began to rise. The greenhouse gas then further warmed the changing climate, Stott says.

This year, the sea ice around Antarctica grew to its largest extent since satellite observation began in 1979—whereas the Arctic arrived at record minimum—meaning present climate change is a far different scenario. In fact, the Milankovitch cycles would predict gradual global cooling. Man-made greenhouse gases, primarily CO₂, are unequivocally driving present-day warming, according to the Intergovernmental Panel on Climate Change. "This kind of study discusses the natural cycle and could help define the likely positive feedbacks we can expect in the long-term future, [for example] as temperatures warm, the ocean will want to give up more CO₂, or rather absorb less," says climatologist Gavin Schmidt of NASA's Goddard Institute of Space Studies. "But it has no direct impact on attribution of 20th century warming."*

But this research does argue for a different path to global warming in the past, which means that estimates of the planet's sensitivity to various levels of CO₂ based on measurements from the Ice Age may be flawed, Stott argues. "We are a long way from refining the climate sensitivity to a doubling of CO₂ [that] is going to take place in the next 100 years," he notes.

Stott plans to investigate how ocean warming led to a CO₂ rise in the past, research that could also have implications for present climate change. But one impact of the new finding is already clear, Stott says: "a regional change in climate can propagate into a global response."

With Arctic ice retreating more and more as local summers heat up, exposing ever more cold northern waters to warming sunshine—along with a host of other regional changes—it remains to be seen exactly how sensitive global climate really is. "We just don't know very well," Stott acknowledges, "how the climate itself, which is much more than temperature, is going to behave."

*Article updated on 9/28/07.