The retreat of sea ice in the Arctic Ocean is diminishing Earth’s albedo, or reflectivity, by an amount considerably larger than previously estimated, according to researchers at Scripps Institution of Oceanography, UC San Diego.
As the sea ice melts, its white reflective surface is replaced by a relatively dark ocean surface. This diminishes the amount of sunlight being reflected back to space, causing the Earth to absorb an increasing amount of solar energy.
The Arctic has warmed by 2° C (3.6° F) since the 1970s. The summer minimum Arctic sea ice extent has decreased by 40 percent during the same time period. These factors have decreased the region’s albedo.
Scripps graduate student Kristina Pistone and climate scientists Ian Eisenman and Veerabhadran Ramanathan used satellite measurements to calculate changes in the albedo of the Arctic region associated with the changing sea ice cover. Albedo is measured as a percentage. A perfectly black surface has an albedo of zero percent and a perfectly white surface has an albedo of 100 percent. The albedo of fresh snow is typically between 80 and 90 percent whereas the albedo of the ocean surface is less than 20 percent. Clouds and other factors also influence the albedo of the Earth.
The researchers calculated that the albedo of the Arctic region fell from 52 percent to 48 percent between 1979 and 2011.
“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 Kristina 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.”
The National Science Foundation-funded study appears in the journal Proceedings of the National Academy of Sciences 45 years after atmospheric scientists Mikhail Budyko and William Sellers hypothesized that the Arctic would amplify global warming as sea ice melted.