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 the oceans have
absorbed. 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.
“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.
The NASA dataset used in this study consists of a
merging of CERES data and measurements from
the Moderate Resolution Imaging Spectroradiometer
(MODIS) instrument, which fli