May 8, 2013 at 1:20 PM ET
For the past ten years, skyscraper-sized icebergs have cracked off glaciers in Greenland and tumbled into the sea at an ever-quickening rate in response to global warming, raising concerns about runaway ice loss and rising seas. The good news? The rate of acceleration will slow, according to a new study.
The slowdown is related to the physics and geography that govern glacier movement, not a forecast that the rise in global temperatures will halt anytime soon. Indeed, the ice sheets will continue to melt and push up sea levels around the world, just not as quickly as feared, the study's lead author said.
The earlier work extrapolated the rate of acceleration seen since the late 1990s out to 2100, explained Faezeh Nick, a glaciologist at The University Center in Svalbard, Norway. But scientists now know that glaciers respond to warming in complex ways, especially those that end at the sea.
These moving tongues of ice, known as outlet glaciers, accelerate in bursts — dumping tons of ice into the ocean — but then the pace slackens. "It doesn't go lower than it was before, but it doesn't stay at the top" rate, Nick told NBC News.
Her work hinges on individual models of the four largest outlet glaciers in Greenland and their individual response to atmospheric and ocean warming. The models are based on more than a decade's worth of scientific observations.
For example, two of the glaciers are particularly sensitive to the production of surface melt water, which falls down crevasses and fractures off icebergs. Submarine melting of ice by warmer seas is big factor in another glacier, which thins the ice and releases a natural brake, allowing more icebergs to calve.
Once the models were tuned to accurately represent the observed historic trends of these outlet glaciers, Nick used the models to forecast future ice loss using two different scenarios of warming in the coming centuries.
Under a mid-range warming of 5.04 degree Fahrenheit (2.8 degrees Celsius) by 2100, the outlet glaciers contribute 0.3 to 0.5 inches (8.5 to 13.1 millimeters) to sea level rise, which is equivalent to 30 to 47 gigatons of water per year, according to the paper published today in the journal Nature.
For perspective, Nick said that Lake Geneva, among the largest lakes in Western Europe, holds about 90 gigatons of water. "You are losing half of that lake to the ocean every year just from these four glaciers," she said.
The four glaciers account for 22 percent of Greenland's total contribution to sea level rise. If the findings are extrapolated to include iceberg production and surface melt from all of the island's glaciers, the researchers estimate a total contribution of sea level rise from Greenland of between 3 and 7 inches (65 and 183 millimeters) by 2100.
Under the warmer scenario modeled — an 8 degree Fahrenheit (4.5 degree Celsius) rise by 2100 — the team found the sea level rise would be 50 percent higher.
The modeling is "good science" and consistent with other recent findings, glaciologist Richard Alley at Pennsylvania State University, told NBC News in an email.
He explained that an icesheet can shrink by waiting for the environment to bring warmth to it or send ice off to find warmth. "Iceberg calving is the fastest way to send the ice off to melt elsewhere," he said.
The geography of the narrow fjords where the outlet glaciers terminate is such that as the glaciers retreat, friction on the glaciers will increase, thus slowing down their seaward march and, eventually, eroding their ability to dump big icebergs.
"Then, the ice must melt in place, from heat in the air, rather than melting elsewhere from water. … Note that this does not mean that we're safe because Greenland is no worry," Alley added. "Too warm, and the ice will melt, and surface melting has been accelerating overall, contributing to sea level rise."
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.