Arctic Ice Darkening Faster Than Previously Thought

Sea ice in the Arctic Ocean reflects sunlight, and therefore has a cooling effect on the Earth. However, the recent retreat of sea ice has been underestimated, according to a new study, and the Earth's albedo, or reflectivity, is decreasing at a faster rate than previously thought.

The study was conducted by researchers at Scripps Institution of Oceanography at the University of California, San Diego, using data from the Cloud's and Earth's Radiant Energy System (CERES) aboard several NASA satellites.

As ice melts its surface area diminishes and is replaced by the relatively dark ocean surface. This dark ocean surface reflects less sunlight back into space, which causes the Earth to absorb an increasing amount of non-reflected solar energy.

Since the 1970s, the Arctic has warmed up by 3.6 F (2 C). During each summer, the "minimum Artic sea ice extent" has dropped, resulting in a 40 percent decrease since the 1970s. The drop of albedo in the artic region can be measured by the CERES instruments used in the study.

Scripps graduate student Kristina Pistone and climate scientists Ian Eisenman and Veerabhadran Ramanathan used data from the CERES instruments to calculate the percent drop in albedo. A perfectly black surface has an albedo of zero percent while a perfectly white surface has an albedo of 100 percent. Fresh snow typically has an albedo of 80 to 90 percent, while the ocean surface has an albedo of less than 20 percent, according to a press release announcing the findings.

The researchers found that the albedo of the Artic region dropped from 52 to 48 percent, between 1979 and 2011. This represents a darkening that is twice as drastic as has been found in prior studies.

The researchers directly correlated albedo measurements made by NASA's CERES instrument data with observations of sea ice extent made by the Special Sensor Microwave Imager radiometer aboard Defense Meteorological Satellite Program satellites.

"It's fairly intuitive to expect that replacing white, reflective sea ice with a dark ocean surface would increase the amount of solar heating," said Pistone. "We used actual satellite measurements of both albedo and sea ice in the region to verify this and to quantify how much extra heat the region has absorbed due to the ice loss. It was quite encouraging to see how well the two datasets - which come from two independent satellite instruments - agreed with each other."

Their study appears in the journal of Proceedings of the National Academy of Sciences.