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 a new study that uses data from instruments that fly aboard several NASA satellites.
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.
This image shows a visualization of Arctic sea ice cover on Sept. 12, 2013, with a yellow line showing the 30-year average minimum extent. A new study shows that the magnitude of surface darkening in the Arctic (due to the retreat of sea ice) is twice as large as that found in previous studies. Image Credit: NASA Goddard's Scientific Visualization Studio/Cindy Starr
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.
The study, conducted by researchers at Scripps Institution of Oceanography, at the University of California, San Diego, uses data from the Clouds and Earth's Radiant Energy System, or CERES, instrument. There are CERES instruments aboard NASA's Tropical Rainfall Measurement Mission, or TRMM, satellite, Terra, Aqua and NASA-NOAA's Suomi National Polar-orbiting Partnership (Suomi NPP) satellites. The first CERES instrument was launched in December of 1997 aboard TRMM.
According to NASA, 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.
"Scientists have talked about Arctic melting and albedo decrease for nearly 50 years," said Ramanathan, a distinguished professor of climate and atmospheric sciences at Scripps who has previously conducted similar research on the global dimming effects of aerosols. "This is the first time this darkening effect has been documented on the scale of the entire Arctic."
Eisenman, an assistant professor of climate dynamics, said that the results of the study show that the heating resulting from albedo changes caused by Arctic sea ice retreat is "quite large." Averaged over the entire globe, it's one-fourth as large as the heating caused by increasing atmospheric CO2 concentrations during the same period.
This video uses the historic "Daisyworld" model to illustrate Earth science concepts, such as albedo and feedback loops.