Ice sheets across both the Arctic and Antarctic could melt more quickly than expected this century, according to two studies that blend computer modeling with paleoclimate records. The studies, led by scientists at the National Center for Atmospheric Research (NCAR) and the University of Arizona, show that Arctic summers by 2100 may be as warm as they were nearly 130,000 years ago, when sea levels eventually rose up to 20 feet (6 meters) higher than today.

Bette Otto-Bliesner (NCAR) and Jonathan Overpeck (University of Arizona) report on their new work in two papers appearing in the March 24 issue of Science. The research was funded by the National Science Foundation, NCAR's primary sponsor. The study also involved researchers from the universities of Calgary and Colorado, the U.S. Geological Survey, and The Pennsylvania State University.

Otto-Bliesner and Overpeck base their findings on data from ancient coral reefs, ice cores, and other natural climate records, as well as output from the NCAR-based Community Climate System Model (CCSM), a powerful tool for simulating past, present, and future climates.

"Although the focus of our work is polar, the implications are global," says Otto-Bliesner. "These ice sheets have melted before and sea levels rose. The warmth needed isn't that much above present conditions."

The two studies show that greenhouse gas increases over the next century could warm the Arctic by 5-8 degrees Fahrenheit (3-5 degrees Celsius) in summertime. This is roughly as warm as it was 130,000 years ago, between the most recent ice age and the previous one. The warm Arctic summers during the last interglacial period were caused by changes in Earth's tilt and orbit. The CCSM accurately captured that warming, which is mirrored in data from paleoclimate records.

Although simulation results depend on the assumptions and conditions within different models, estimates of warming from the CCSM are within the range projected by other climate models, according to the authors.

"Getting the past climate change correct in these models gives us more confidence in their ability to predict future climate change," says Otto-Bliesner.

The CCSM suggests that during the interglacial period, meltwater from Greenland and other Arctic sources raised sea level by as much as 11 feet (3.5 meters), says Otto-Bliesner. However, coral records indicate that the sea level actually rose 13 to 20 feet (4-6 meters) or more. Overpeck concludes that Antarctic melting must have produced the remainder of the sea-level rise.

These studies are the first to link Arctic and Antarctic melting in the last interglacial period. Marine diatoms and beryllium isotopes found beneath the West Antarctic Ice Sheet indicate that parts of the ice disappeared at some point over the last several hundred thousand years.

Overpeck theorizes that the rise in sea levels produced by Arctic warming and melting could have helped destabilize ice shelves at the edge of the Antarctic ice sheet and led to their collapse. If such a process occurred today, it would be accelerated by global-scale greenhouse-induced warming year round, Overpeck says. In the Arctic, melting would likely be hastened by pollution that darkens snow and enables it to absorb more sunlight.

In the last few years sea level has begun rising more rapidly, now at a rate of about an inch per decade, says Overpeck. Recent studies have also found accelerated rates of glacial retreat along the margins of both the Greenland and West Antarctic ice sheets.

NCAR's primary sponsor is the National Science Foundation. Opinions, findings, conclusions or recommendations contained herein are not necessarily those of NSF.