A trio of papers in the May 6 Science confirms that the amount of sunlight
Earth’s surface is absorbing has been increasing since the early 1990s,
reversing the previous 30-year trend of “global dimming,” during which
surface sunlight diminished by about 5 percent.
Earth’s climate and surface temperature are determined by the balance between
incoming radiation and outgoing radiation. The net energy is mostly governed
by the greenhouse effect that traps heat in the planet’s atmosphere and
the percentage of sunlight reflected back toward space, a parameter called albedo.
Although the former has been well-studied, the dynamics of albedo are not as
well-understood, and albedo is assumed to be a constant in many climate models.
The lower the albedo, the more sunlight is absorbed by Earth’s surface.
A 1 percent decrease in albedo represents an energy gain of 3.4 watts per square
— the equivalent energy effect of doubling the amount of carbon dioxide
in the atmosphere.
“What these papers are showing is that there are two things to take into
account: how much we get in and how much goes out,” says Enric Pallé,
an astrophysicist who studies albedo at the New Jersey Institute of Technology’s
Big Bear Solar Observatory in California. “Until now we were only considering
how much goes out.”
Measuring the amount of sunlight that reaches the surface is one method of studying
albedo. To measure how this surface brightness has changed since 1990, Martin
Wild, a climatologist at the Swiss Federal Institute of Technology in Zurich,
and colleagues used ground-based instruments primarily in the northern hemisphere.
They have found that the amount of sunlight absorbed at the surface has increased
by about 4 percent in the last decade, a reversal that “may substantially
affect surface climate, the hydrological cycle, glaciers and ecosystems.”
The study suggests that changes in the amount of aerosol particles in the atmosphere
may explain why more sunlight is reaching the surface. Aerosols, such as smog,
can absorb or reflect sunlight, thereby cooling the surface. Therefore, global
efforts to reduce smog in the 1990s might actually have eliminated that offset,
unmasking more global warming.
The trend seen in Wild’s team’s ground-based data is corroborated
by another study that used satellite data. “Unlike ground stations, satellites
can uniformly sample the entire globe,” wrote Rachel Pinker, a meteorologist
at the University of Maryland in College Park, in another study in Science.
Pinker and colleagues found an increase in surface brightness of 0.16 watts
per square meter per year between 1983 and 2001. This overall figure represents
a sustained dimming until 1990, when the trend reversed, and solar radiation
at the surface began to increase. Although the direction of the trend is similar
to that found in the ground observations, the team notes that the values “were
much smaller in magnitude.”
A third paper, by climatologist Bruce Wielicki of NASA Langley Research Center
in Hampton, Va., also used satellite measurements to model Earth’s albedo.
From 2000 to 2004, Wielicki and colleagues found a decrease in albedo of 0.6
percent or about 2 watts per square meter. However, Wielicki notes that error
corrections for ultraviolet heating of the satellite itself might reduce this
to 0.9 watts per square meter.
Wielicki’s conclusion contradicts part of a 2004 study by Pallé
that found a long-term trend of decreasing albedo from 1984 to 2000, and an
increasing albedo from 2001 until 2003. Pallé says that the difference
in the length of his study versus the new study may explain the discrepancy
between the two, and he points out that although the various studies covering
the mid-1980s to 2000 differ in their estimates of the size of the albedo changes,
they all agree that albedo decreased over that time, causing an increase in
the amount of sunlight absorbed. “What’s really needed,” he says,
“is a very thorough composite of all the datasets, which has not yet been
done.”
In a commentary accompanying the Science articles, Robert J. Charlson,
an atmospheric scientist at the University of Washington in Seattle, calls for
“sustained, direct, and simultaneous observations of albedo with all methods
that are currently available.” He also notes that this goal is being hindered
by recent budget cuts to earth science programs and launch delays within the
shuttle program.
Sara Pratt
Geotimes contributing writer
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