NASA Satellites See Arctic Surface Darkening Faster

The Arctic has warmed by 3.6 F (2 C) 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, or the fraction of incoming light that Earth reflects back into space – a change that the CERES instruments are able to measure.

Scripps graduate student Kristina Pistone and climate scientists Ian Eisenman and Veerabhadran Ramanathan used satellite measurements to calculate Arctic albedo changes 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, like aerosols and black carbon, also influence the albedo of Earth.

The researchers calculated that the overall albedo of the Arctic region fell from 52 percent to 48 percent between 1979 and 2011. The magnitude of surface darkening is twice as large as that found in previous studies. They also compared their results to model simulations to assess the capability of computer models to portray and forecast albedo changes.

Previous studies have used a combination of computer models and observations in their calculations to estimate how much extra energy has been absorbed by the oceans. In contrast, the Scripps team opted to directly correlate albedo measurements made by NASA's CERES instrument data with observations of sea ice extent made by the Special Sensor Microwave Imager (SSM/I) radiometers aboard Defense Meteorological Satellite Program satellites. This approach avoided the possibility of systematic issues in computer models.

"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."

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.