Did Icebergs Warm the World?

by Phil Berardelli, ScienceNOW Daily News, 21 November 2008

Rube Goldberg is alive and well in the climate record. In an effort to explain several spikes in global carbon dioxide (CO2) levels during the last ice age, researchers have come up with the following scenario: Fresh water flooded the North Atlantic Ocean, which slowed ocean circulation, which impeded the transport of nutrients to the ocean's food web, which starved CO2-consuming organisms that form the basis of that web, which resulted in CO2 buildup in the atmosphere, which eventually heated Greenland and possibly much of the world. The biggest surprise is the probable source of the fresh water that got the whole thing started: rogue icebergs.

Setting Earth's thermostat isn't easy. It's determined by a combination of many interconnected factors, local and global, each of which can impact any or all of the others. One such factor is the huge underwater current known as the North Atlantic Deep Water--or the Conveyor Belt--which begins off Greenland and encircles the globe. The current has long been suspected of helping to drive climate change, though exactly how has remained unknown. Climate scientists have also been curious about three seemingly unrelated but simultaneous developments that occurred on and off during the last half of the most recent ice age, between 65,000 and 13,000 years ago: a drop in salinity in the North Atlantic, a distinct chemical change in ocean sedimentary layers around the world, and sudden cooling and even more sudden heating in Greenland.

Climate scientists Andreas Schmittner of Oregon State University, Corvallis, and Eric Galbraith of Princeton University developed a computer model to see if they could connect ancient climate upheavals to ocean circulation. They designed the model to mimic present-day conditions, but they dropped temperatures to ice-age levels. Then, they flooded their virtual North Atlantic with fresh water and watched what happened. "We hit our climate machine with a big hammer," Galbraith says.

As the researchers report this week in Nature, the action created the climate cascade described above, building up CO2 over a millennium and warming Greenland by as much as 16°C. Then, after a few more millennia, the process repeated, just like in the geological records.

The model shows that disruptions in the North Atlantic Deep Water are "tightly linked to CO2" in the atmosphere, Galbraith says. "This had been a vexingly difficult relationship to understand," he adds, "but these simulations show a remarkably simple way to connect them." And where did all the freshwater come from? Galbraith says the most likely source was periodic melting of migrating icebergs from northern Canada and Eurasia, which broke off from the massive polar ice sheets.

"People have been saying for years that we don't understand what makes the level of CO2 in the atmosphere go up and down during the ice ages," says oceanographer J. R. Toggweiler of the Princeton-based National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory. But the paper explains the phenomenon "very nicely by a simple change in the oceanic circulation," says Toggweiler, who was not involved in the study. In terms of applications, climate scientist Claire Parkinson of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says that understanding the ocean's past cycles can help researchers predict "how much the ocean can continue to be a sink for carbon dioxide and other greenhouse gases."

Link to article: http://sciencenow.sciencemag.org/cgi/content/full/2008/1121/3