The last ice age in the Northern Hemisphere ended about 20,000 years ago, when changes in Earth’s orbit increased the amount of sunlight there. Until now, most evidence has indicated that the same ice age lasted 2,000 years longer in the Southern Hemisphere and that the onset of deglaciation (ice melting) in the south was a response to warming in the north.
But new research suggests that gradual warming in the Antarctic may have started two to four thousand years earlier than previously believed, suggesting that the deglaciation in the South Pole was independent of the warming that occurred in the north. This is based on evidence from a new ice core that is over 2 miles deep and goes back 68,000 years, taken from the West Antarctic Ice Sheet (WAIS) Divide. It is the first ice core to come from this area since 1968, when the first core ever drilled in Antarctica was taken from this region. This deglaciation is the last big climate change that scientists are able to go back and investigate, so understanding what caused warming in the Antarctic during this time is valuable information on how our climate system works.
The finding is described in a new paper from members of the West Antarctic Ice Sheet Divide project, a U.S.-led team of more than 40 researchers, including Scripps Institution of Oceanography geoscientist Jeff Severinghaus, and Anais Orsai, one of his doctoral students. The paper describes the possible changes in Earth’s orbit that caused an increase in the sun’s intensity in the north resulted in changes in sunlight distribution in the Antarctic. This warmed not only the West Antarctic ice sheet, but also the Southern Ocean surrounding Antarctica.The location of the ice core is significant, because most of the ice cores previously studied have come from East Antarctica, the highest and coldest part of the continent, which is separated from West Antarctica by a high mountain range. The scientists chose to drill an ice core from West Antarctica where it snows heavily, creating a high rate of ice accumulation—about 22 centimeters (8.7 inches) per year. That makes it easier to tell the years apart in the ice core and create a high-resolution record of past atmospheric CO2 concentrations.
“We wanted to work there because it snows a lot, but it was also logistically difficult to work there, because it snows a lot,” Severinghaus said.
WAIS Divide took 15 years to plan and carry out, largely due to the logistical difficulties of setting up the project in such a remote and harsh location. The drilling site is about a seven-hour flight northwest from McMurdo station, the main Antarctic research station, and special fueling stations were built along the route for planes to fuel up on their way to the WAIS Divide station. A shelter for the drilling equipment was built in the form of a 30-foot tall steel structure with a curved roof, and the structure was soon covered by snow, which gets deeper every year. Scientists now have to dig a trenched ramp through the snow to access the building. The scientists live in tents on the ice.
This paper’s findings come from only the most recent 30,000 years that the ice core covers, as the final 30,000 to 68,000 years of data are still being analyzed. The information from this ice core will provide a wealth of information on the climate and CO2 dynamics of the past glacial cycle.
In addition to occupying a leadership role in the WAIS Divide project, Severinghaus’s lab is analyzing data from the gases in the ice core.
The team is looking at noble gases and nitrogen trapped in the bubbles of the ice core, and using the ratios of these gases to date the ice, Severinghaus said. For the first 30,000 years they were able to count the ice layers to identify each year, but for the older ice the layers were too compressed to be dated this way.
Today, the Antarctic climate is warming once again. But this modern-day warming, caused by increasing atmospheric carbon dioxide, is not strongly affected by orbital changes. The climate change that ended the last ice age occurred over thousands of years, and now the climate is changing and ice is melting in just decades. “What we hope to learn from this ice core is a better understanding of the climate change that occurred in the last glacial cycle, on a time scale that is relevant to humans,”said Severinghaus.
– Mallory Pickett is a master’s student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography, UC San Diego