Data reveal poles are at risk
A UH scientist helps spot the driving force behind the ice age's end 17,000 years ago
As government leaders weigh approaches to global warming, new research points to the vulnerability of polar regions to increased heat.
While atmospheric carbon dioxide was "a contributor" to the end of the last ice age 17,000 years ago, scientists have found that the process likely began when more solar heat melted ice around Antarctica.
Axel Timmermann, associate professor of oceanography and research team leader at the UH-Manoa International Pacific Research Center, said sea ice began retreating in the Southern Ocean as springtime heating increased.
Based on a study of ice cores, he said, "We see that it is starting to retreat around 19,000 or 18,000 years ago with solar insulation increasing."
That in turn might have "opened the door for CO2 stored in the ocean to come out," Timmermann said in an interview.
It has been commonly thought that greater carbon dioxide levels in the atmosphere caused warming in the tropics that ended the last ice age, he said.
"But part of the (scientific) community recognizes more is needed than just CO2. In particular, if it were CO2, where did the rise come from? That was always a difficult issue."
Current atmospheric carbon dioxide is the product of human activities like the burning of fossil fuels or forests, which creates a blanket effect, trapping solar rays. Among the more glaring effects in recent years has been diminishing ice in the Arctic in summer.
"In the general public and in Al Gore's movie, you see an interesting correlation between CO2 and temperature for the last 600,000 years, and Gore is alluding that this is simply a greenhouse effect," Timmermann said. "That is not the most consistent explanation. Greenhouse effect is part of the correlation but not the only reason."
The former vice president won an Academy Award for his documentary, "An Inconvenient Truth," and has been nominated for the Nobel Peace Price for his work on global warming, with the winner to be announced tomorrow (10 tonight, Hawaii time).
Lowell Stott of the University of Southern California led the sea ice study with Timmermann and Robert Thunell of the University of South Carolina. The three are paleoceanographers, studying climate changes over hundreds of thousands of years.
Sea salt in the cores recorded the evolution of the sea ice, Timmermann said.
According to their findings, reported recently in the journal Science, the cause for initial warming was increased solar insulation driven by Earth's orbit around the sun between 19,000 and 17,000 years ago. Earth's path around the sun changes in 23,000-, 41,000- and 100,000-year cycles, called the Milankovitch Cycles, which change the light and heat that Earth receives from the sun.
"We can basically say 2 degrees' warming in the Southern Hemisphere can be attributed to this," Timmermann said, adding that that happened before any changes in atmospheric carbon dioxide and before any substantial warming in the tropics.
Furthermore, ice reflects solar radiation back into space in what is called the albedo effect, but once it melts it exposes more ocean, which absorbs more heat -- an accelerating process, the scientists said.
The researchers also studied a sediment core from the tropical Pacific containing some tiny marine creatures that had lived on the ocean bottom and some that had lived at the surface.
Those that lived on the ocean floor had been in water that formed 1,000 years earlier in the Antarctic Southern Ocean, the scientists said.
Comparing the isotope records of the two types of fossils, the scientists discovered the Southern Ocean had warmed more than 1,000 years before the tropical Pacific and before the ice core records showed any significant rise in atmospheric carbon dioxide.