New study offers climate clues from most recent interglacial warm period
By Bob Berwyn
The last time the Earth was as warm as today was about 128,000 years ago — and Antarctic sea ice extent was 65 percent smaller than it is now, according to British scientists who tracked past climate change in the region by studying ice core samples from that era.
That means Antarctic sea ice is on course to shrink dramatically in the decades and centuries ahead, said British Antarctic Survey scientist Max Holloway, who with a team of researchers analyzed oxygen isotopes in ice and air bubbles trapped for 128,000 years in the East Antarctic Ice Sheet.
Sea ice around Antarctica so far has not been greatly affected by global warming, but in the decades ahead, it will probably start melting just as fast as in the Arctic under a thickening layer of greenhouse gases.
The new study focused on the most recent warm interglacial period when global temperatures were similar to today’s. The researchers said they were expecting to see a link between that era’s warmer temperatures and a collapse of the West Antarctic Ice Sheet — vast land-based glaciers and ice fields that would raise sea level significantly if they melt.
“It’s a nice example from the past when Antarctic sea ice experienced a major reduction. It means the long-term trend is going to be negative.” The findings help confirm current climate models that project a reduction in Antarctic sea ice of up to about 60 percent by 2200, Holloway said.
The research team found strong evidence that Antarctic sea ice, which floats atop the ocean, dwindled thousands of years before the ice sheet melted. Precisely timing the sequence of those events is important because it helps show how Antarctica will respond to the global warming caused by the buildup of carbon dioxide and other heat-trapping gases.
Melting sea ice won’t raise sea level by much. But if it shrinks by two-thirds around Antarctica — similar to the meltdown in the Arctic — it would speed up the pace of warming in the Southern Hemisphere. Ocean water is much darker than reflective white sea ice and absorbs the sun’s heat efficiently. In the Arctic, the melting of sea ice has amplified global warming to twice the pace of the global average.
“In a way, this paper has good and bad news,” said Cambridge University climate scientist Eric Wolff, who was not involved in the study. “The last interglacial is a time that was a little warmer than today in the polar regions. Sea ice was sensitive to this, which is a worry. But this study suggests that the West Antarctic Ice Sheet survived at least the initial warmth.” That means big and immediate cuts in greenhouse gas emissions might avert a complete meltdown of the West Antarctic Ice Sheet, which could raise sea level by 11 feet, he said.
The results imply that the West Antarctic Ice Sheet did not disappear under the moderate warming of the last interglacial, “meaning that it is not as sensitive as the worst cases might suggest,” Wolff said. “However we should be cautious because this is only one model; and the good news is mitigated by the fact that other work shows that sea level did rise in the last interglacial, so if it wasn’t from West Antarctica it must have been from somewhere else.”
The sea ice helps buffer the Antarctic continent from warming ocean waters. If the sea ice dwindles, warmer air and water would intrude over and under the West Antarctic ice sheet, it could “pre-condition the ice sheet for collapse,” according to Holloway.
To reach their findings, the scientists compared the ratio of heavy and light oxygen isotopes in the ice core layers, which represent annual cycles of precipitation.
“That ratio responds to changes in temperature, arctic circulation and sea ice extent,” Holloway said, “showing the distance from where ocean water evaporates to where it falls as precipitation over the ice sheet, which then shows up in the ice core.”
As the water vapor travels, it cools, condenses and falls as rain or snow, and the heavier isotopes “rain out” first. When the researchers found a spike in heavier isotopes in the ice core record, it corresponded with years when sea ice extent around Antarctica was low.
Arctic sea ice has been declining by about 12 percent each decade, measured during the summer melt season. But the sea ice around Antarctica has grown slightly in recent decades. The new study is the latest in an ongoing effort to understand the different responses at either end of the Earth.
British Antarctic Survey research group leader Louise Sime said the rapid retreat of Arctic sea ice is critically important to Arctic ecosystems and global climate. Pinpointing a huge sea ice retreat around Antarctica during a similar climate era shows will help researchers determine whether “sea ice in the Southern Hemisphere will retreat in a future high-CO2 world,” she said.
Marine sediments show that when the Antarctic sea ice retreated 128,000 years ago, the ocean was about 2 degrees Celsius warmer than today.
“We expect that eventually the ocean around Antarctica will warm if the CO2 concentration in the atmosphere continues to rise. So, if we can further define just how much warmth there was 130,000 years ago then this helps us understand what the sensitivity of sea ice in the Antarctic to warming is,” Wolff said.