The planet is warming up, ice is melting, and the sea is rising. But to what extent? How exactly is our planet going to change, and what does this mean for us?
The answers to these questions are hard to come by. Climate systems are incredibly complex, and modeling how they will respond to global warming is fraught with uncertainty. However, modeling is not the only way to get a glimpse of our future. We can also look to the past, which is what a group of scientists including Scripps Institution of Oceanography, UC San Diego geoscientist Jeff Severinghaus have done, by drilling a 2.5-kilometer (1.6-mile) ice core from some of the oldest ice in Greenland. By analyzing ice from a recent period of global warming, these scientists found that Greenland may not be as vulnerable to increasing temperatures as had been feared. The results of the North Greenland Eemian Ice Drilling (NEEM) project, which was led by Dorothe Dahl-Jensen of the Centre for Ice and Climate at the University of Copenhagen’s Niels Bohr Institute, appear in the Jan. 24 issue of Nature.
This “recent” global warming began about 130,000 years ago and lasted for 16,000 years during is called the Eemian interglacial period. The warm period was caused by variations in Earth’s orbit and axis of rotation.
“This is an interesting time period because it was so similar to the present. It was only about 100,000 years ago and the global average temperature was about three degrees warmer than today, which we are going to experience in the next 100 years,” said Severinghaus.
Severinghaus played several roles in the NEEM project, but his principal contribution was to determine the extent of surface melting of the ice sheet using the ratios of certain gases trapped within bubbles in the ice.
“There is abundant evidence for sea level rise of about six to nine meters (20-30 feet) [during this time period],” Severinghaus said, “and the question was where do we get that [water] from? We had thought maybe half came from Greenland and half came from Antarctica.”
But the evidence in the ice core showed that this was not the case. In fact, the NEEM data show that the Greenland ice sheet contributed at most two meters (6.6 feet) of sea level rise. This is good news for Greenland watchers, who have expressed concern that its large ice sheets could trigger catastrophic sea-level rise. However, this strongly implies that Antarctica must have contributed the remaining 4 to 7 meters (13-23 feet) of Eemian sea-level rise.
“This means Antarctica must have suffered some pretty catastrophic changes,” Severinghaus said. “If the sea level rose by eight meters, six of those came from Antarctica. That means the whole Western ice sheet was gone. This is something that people have worried and wondered about for a long time, and now we have some very strong evidence.”
So far, attempts to find direct evidence of Eemian melting in Antarctica have been unsuccessful. The deep, old ice in this area has been destroyed by volcanic heat, so there is none left from the Eemian period. Marine sediment cores gathered near the western ice sheet provide some indirect evidence for the melting, as they show increased abundance of marine life during the Eemian. This suggests a period of little to no ice in the area. But the NEEM core is the strongest evidence to date for this significant loss of Antarctic ice during the Eemian.
The findings of Severinghaus and his colleagues are good news for the Greenland ice sheet but suggest a dire future for Antarctica. In the meantime, the threat of rising oceans remains a real threat for the rest of the world. Since 1992, the Antarctic and Greenland ice sheets have lost enough ice—over 344 billion tons—to contribute 0.6 millimeters (0.02 inches) to the total observed 3-millimeter (0.1-inch) sea-level rise per year. The rest comes from thermal expansion as the ocean heats up, as well as contributions from melting temperate glaciers. Over this same time period, the global average temperature has heated up by only 0.5 degrees Celsius. The Eemian global average temperature increase of three degrees caused a sea-level rise between 6 and 9 meters (20-30 feet), suggesting that whether the water comes from the North or South Pole, the consequences are sure to be profound.
– Mallory Pickett is a first-year masters student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography, UC San Diego