New study accounts for rebound of land masses after recent ice ages
By Summit Voice
SUMMIT COUNTY — Rising sea levels could swamp some of the world’s most populated coastal areas during the coming centuries, but scientists are still not sure exactly how high the waters will reach.
Projections for the next 90 years or so range from just a few inches to several feet or more, and pinpointing this significant global warming impact is one of the big challenges for climate scientists.
In a recent study, scientists with Columbia University’s Lamont-Doherty Earth Observatory estimated that sea level rose 20 to 43 feet during a warm interglacial period about 400,000 years ago — up to about 30 percent less than previous estimates.
They reached their conclusion after studying exposed shorelines on the sub-tropical islands of Bermuda and the Bahamas. After correcting for what they say were the sinking of the islands at that time, The results suggest that Greenland and West Antarctica ice sheets collapsed at that time, but not the even bigger East Antarctic Ice Sheet.
“Our study provides a simple explanation for these high beach deposits,” said Columbia University climate researcher Maureen Raymo, lead author of the study.
By most measurements, average global sea level has risen eight inches since the 1880s. It is currently rising an inch per decade, driven by thermal expansion of seawater and melting of glaciers and ice sheets, including the still mostly intact ice sheets of Greenland and West Antarctica.
In its most recent report, the Intergovernmental Panel on Climate Change estimated that the seas could rise up to two feet by 2100; but that number could go higher depending on the amount of polar ice melt, and quantity of greenhouse gas emissions by humans. The United Nations estimates that five feet of sea-level rise would be enough to swamp 17 million people in low-lying Bangladesh alone.
Previous studies of the sea cliffs on Bermuda and the Bahamas estimated that, during the period 400,000 years ago, the seas rose nearly 70 feet, leading to the hypothesis that the East Antarctic ice sheet must have partly melted to produce such a rise.
The Columbia study reaches a different conclusion by factoring in the loading and unloading of ice from North America during the ice ages preceding the sea-level rise. As the ice sheets grew, their weight pushed down the land beneath them, while causing land at the edges of the continent—including Bermuda and the Bahamas–to bulge up, said Raymo. When the ice pulled back, the continent rebounded, and the islands sank.
“Bermuda and the Bahamas are not a pristine measure of the volumes of ice that melted in the past, because they’re contaminated by effects left over from the ice ages,” said study coauthor Jerry Mitrovica, a geophysicist at Harvard University.
The new study infers that the huge Greenland and West Antarctic ice sheets indeed collapsed at the time, but that loss from the even vaster East Antarctic Ice Sheet was negligible.
Today, both Greenland and West Antarctica are losing mass in a warming world, but signals from East Antarctica—about eight times bigger than the other two combined–are less clear. Raymo said the study helps show that “catastrophic collapse” of the East Antarctic ice is probably not a threat today. “However, we do need to worry about Greenland and West Antarctica,” she said.
The study’s revised estimate of 20 to 43 feet makes sense, said sea-level rise expert Mark Siddall, a climate scientist at the University of Bristol who was not involved in the study. But, he added, it would probably take hundreds to thousands of years for such a rise to occur again. “We’re moving from a place of disagreement about sea level estimates from this past period to one consistent theory that reconciles data from diverse geographic areas,” he said.