Arctic Ice Melt Adding More Heat to the Atmosphere Than Previously Thought
Sea ice north of Greenland.Credit: Courtesy Andy Mahoney, NSIDC.
For the past 30 years, satellites have been tracking a stunning decline in the sea ice and snow cover blanketing the Arctic. Now, new research has found that the thinning and melting sea ice is having a much more significant impact on global climate change than was previously thought.
The sea ice and snow on the planet, collectively known as the cryosphere, are responsible for reflecting back to space some of the sunlight beaming down on the planet. Computer models have calculated how much the reflectivity of the Northern Hemisphere is changing as snow and ice coverage in the Arctic declines due to global climate change and natural climate variability. Such modeling studies help researchers understand how global temperatures may respond to these changes in the cryosphere.
But this new study, which is based on observations of changing levels of snow and sea ice and trends in the reflectance of snow-covered regions over the past 30 years, suggests the overall loss of reflectivity is more than double than what models have predicted. This has important implications for climate change projections.
“Basically, we have an estimate that the solar cooling effect coming from the Northern Hemisphere cryosphere has decreased significantly over the past 30 years,” says climatologist Mark Flanner from the University of Michigan, who worked on this new study. Given the amount of warming, he says, the reduced reflection from sea ice and snow is greater than what is simulated over the same time span by current climate models. Flanner and his coworkers’ findings were published this week in the journal Nature Geoscience.
Sea ice extent in August, 2010, compared to the 1979-2000 average. Credit: NSIDC.
In their new study, Flanner and his colleagues from Oregon State University, the National Center for Atmospheric Research, the U.S. Army Corps of Engineers Research and Development Center, and the University of Colorado, Boulder, found that ice and snow in the Northern Hemisphere are currently reflecting about 3.3 watts of solar energy per square meter, which is a decrease of about 0.45 watts per square meter from 30 years ago, due to the loss of both ice and snow cover. And for every degree Celsius of warming, Flanner calculates that the feedback is a reflectivity decrease of about 0.6 watts per square meter, which is double what computer models have previously predicted.
Scientists have long understood that as snow and ice melt, the planet's ability to reflect sunlight begins to change. For example, when sea ice melts, it exposes the darker ocean waters underneath to sunlight. The water absorbs much more solar radiation compared to when it was covered by the brightly colored sea ice, thereby raising water and air temperatures. These warmer temperatures then cause more ice loss, in turn leading to more melting." The change of reflectance itself is called the “albedo feedback.”
Computer models have traditionally simulated the changes in the reflectance of the Northern Hemisphere over the course of century-long time spans, but this new study, which relied upon satellite records of snow and ice cover in the Northern Hemisphere, only measures changes during the past three decades. Therefore, Flanner says, it isn’t possible to say how strongly the measured feedback is influenced by natural climate variability compared to manmade climate change, but it does indicate that the snow and ice's reflectance is responding senstively to warming temperatures.
The average changes in reflectance that Flanner and his colleagues measured are based on data gathered throughout the year, but Flanner says that the loss of spring and summer sea ice had a disproportionately large impact on the reflectivity decline. In the Northern Hemisphere, the late spring and summer months see many more hours of sunlight than the fall and winter, so the summer sea ice extent, which has been decreasing dramatically in recent years, plays a big role in how much the cryosphere cools the hemisphere, and the planet.
“We’ve seen an increase in fall snow cover in some places, but if you lose a square meter of snow in May and you gain a square meter in November, it’s not a wash,” he explains.
Scientists are investigating an atmospheric circulation pattern that brings milder-than-average air to parts of the Arctic, while cold air is routed south into North America and Europe during the winter.
Recent research has linked declining Arctic sea ice cover to changes in wintertime atmospheric circulation patterns across the Northern Hemisphere. With the reduced sea ice coverage in the later summer and fall during recent years, both air and water temperatures in much of the Arctic have been unusually warm. Some researchers think this may be redirecting the flow of Arctic air down towards North America and Europe. This “Arctic paradox,” where warming in the Arctic may be accompanied by cooler than average winters on the continents, may become more common if the Arctic continues to absorb more heat.
In their study, Flanner and his collaborators did not investigate what has caused the changes in snow cover and sea ice, although other studies have identified greenhouse gases from human activities as a key contributing factor. “We’re not saying what it is that has caused the crysosphere reflectance to change, whether it is carbon dioxide or soot or variability like the North Atlantic Oscillation,” says Flanner, explaining that this study only focused on measuring the effects of less snow and ice coverage.
“But regardless of what has been causing the melting, the change in its impact on warming has been substantial.”
Climate scientist Mike Winton, from NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, NJ, has also been studying how the coverage of Northern Hemisphere sea ice has changed as global temperatures rise, and thinks this new study is a valuable addition to the understanding of how snow and ice in the Arctic are changing the planet's heat budget.
“[Flanner’s] new study is really useful,” he says. “It tries to put everything together and it will help to get better models in the future.” As a community of climate scientists, Winton says researchers have known that the computer models were coming up short in tracking the shrinking sea ice coverage. Most models did not predict the rapid sea ice decline that has occurred in recent years. Now, Winton says that studies like Flanner’s are helping tackle the problem in a different way.
“The changing albedo feedbacks are a way of explaining the difference, and now we’re digging deeper into that,” says Winton. “It’s going to be a topic that is with us for some time.”