CERES Satellite Data Paint A Grim Picture Of Arctic Surface Darkening
February 19, 2014

CERES Satellite Data Paint A Grim Picture Of Arctic Surface Darkening

Lawrence LeBlond for redOrbit.com - Your Universe Online

Earth’s reflectivity (albedo) is on a much faster decline than previously estimated due to the retreat of sea ice in the Arctic Ocean. A key instrument incorporated on several NASA satellites has revealed significant data on this profoundly concerning issue.

A team of researchers from University of California, San Diego’s Scripps Institution of Oceanography used data from the Clouds and Earth’s Radiant Energy System (CERES), which has been installed on NASA’s TRMM satellite and on Terra, Aqua and NASA-NOAA’s Suomi National Polar-orbiting Partnership satellites. CERES has been collecting data since December 1997, when it was first launched aboard TRMM.

The team, which consisted of graduate student Kristina Pistone and climate scientists Ian Eisenman and Veerabhadran Ramanathan, used the data from CERES to calculate the changes in Arctic albedo relating to changes in sea ice cover.

The white reflective properties of the sea ice help beam sunlight back into space, keeping the Earth relatively cool. As the sea ice retreats, the darker ocean becomes more prominent and solar energy is hence absorbed into the planet rather than being reflected back out into space.

[ Watch the Video: This World Is Black and White ]

This dance between Arctic ice retreat and solar energy has so far contributed to a 3.6-degree-F warming of the Arctic since the 1970s. Also during this time, the summer minimum Arctic sea ice extent has diminished by 40 percent. This ongoing process is driving the continuing decline in Arctic albedo and NASA’s CERES instruments are able to measure it.

Albedo is measured as a percentage. A perfectly white surface has an albedo of 100 percent, while a perfectly black surface has an albedo of zero percent. Fresh snow has an albedo of around 80 to 90 percent and the darker ocean surface’s albedo is less than 20 percent. The Earth’s albedo is also influenced by clouds, atmospheric aerosols and black carbon.

The overall albedo of the Arctic dropped from 52 percent in 1979 to 48 percent in 2011, according to calculations by the Scripps team. They said that their calculations show surface darkening that is twice the magnitude that has been found in previous studies. The team also checked their calculations against computer models to determine the accuracy of forecasting albedo changes.

Previous studies have used a combination of computer models and observations to estimate how much energy has been absorbed by Earth. The Scripps team opted to take a different route.

The team used the direct measurements taken by the CERES instrument and compared them with observations of sea ice extent as measured by the Special Sensor Microwave Imager (SSM/I) radiometers aboard Defense Meteorological Satellite Program satellites. This approach avoided the possibility of systematic errors that computer models can sometimes make.

"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 study, published in the journal PNAS, comes 45 years after atmospheric scientists Mikhail Budyko and William Sellers theorized that retreating sea ice in the Arctic would amplify global warming.

"Scientists have talked about Arctic melting and albedo decrease for nearly 50 years," said Ramanathan, 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."

The results of this study show that heating resulting from albedo changes caused by retreating sea ice in the Arctic is “quite large,” noted Eisenman. “Averaged over the entire globe, it's one-fourth as large as the heating caused by increasing atmospheric CO2 concentrations during the same period.”

The CERES data used in this study had also included data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, which also flies on the Terra and Aqua satellites. MODIS is able to distinguish between clouds and sea ice, which have similar brightness.

Norman Loeb, CERES principal investigator, said the MODIS capability helps improve the accuracy of the CERES readings.

"By exploiting the unique capabilities of simultaneous CERES and MODIS measurements, the NASA satellite data enable studies on how albedo is changing with unprecedented detail and accuracy," he noted.