May
2003
Study
finds lower atmosphere warming
The reanalysis of satellite data could refute
skeptics of global warming.
A new analysis of satellite data collected since
the late 1970s from the lowest few miles of the atmosphere indicates
a global temperature rise of about one-third of a degree Fahrenheit
between 1979 and 1999. The results are at odds with previous analyses
that show virtually no warming in the satellite record over the
20-year period—and they provide more evidence that global
warming is actually occurring.
A
team that includes Climate and Global Dynamics scientists Tom
Wigley, Gerald Meehl, Caspar Ammann, Julie Arblaster, Thomas Bettge,
and Warren Washington reported its findings in the 2 May online
issue of Science. The lead author is Ben Santer of Lawrence Livermore
National Laboratory.
“There is a lot of statistical uncertainty
when you’re trying to estimate a trend from very noisy data,”
Tom says. “But it’s undeniable that the agreement with
both global climate models and surface data is better for the
new analysis than for the old one.”
Over the past 25 years, a series of instruments
aboard 12 U.S. satellites has provided a unique temperature record
extending as high as the lower stratosphere. Each sensor intercepts
microwaves emitted by various parts of the atmosphere, with the
emissions increasing as temperatures rise. These data are used
to infer the temperature at key atmospheric layers.
Since the 1990s, skeptics have pointed to the absence
of a warming signal in the satellite-derived temperatures, which
stood in contrast to a distinct warming trend in average air temperature
at Earth’s surface. A 2000 report from the National Research
Council concluded that both trends might be correct—in other
words, the global atmosphere might be warming more quickly near
the ground than higher up.
Although Tom agreed, he felt there was more to
be explained.
“The real issue is the trend in the satellite
data from 1979 onward,” says Tom. “If the original analysis
of the satellite data were right, then something must be missing
in the models. With the new data set, the agreement with the models
is improved, and the agreement with the surface data is quite
good.”
In order to glean temperatures from the raw satellite
data, several adjustments and corrections had to be made. Until
now, only one group, based at the University of Alabama in Huntsville,
had produced a complete set of global temperatures from the raw
data. It found minimal warming.
For the new study, a group based at Remote Sensing
Systems in Santa Rosa, California, applied a revised set of corrections
to the satellite data. This accounted for the effects of heating
on the radiation sensor itself—the first time this source
of error had been addressed fully, according to the authors—and
made adjustments for the drifting orbit of each satellite and
other factors.
The group found a warming trend of 0.16°F (0.10°C)
per decade in the layer between about 1.5 and 7.5 miles (2.4–12.1
kilometers) high, compared to a trend of 0.02°F (0.01°C)
in the previously published University of Alabama in Huntsville
analysis. Both estimates have a margin of error of plus-or-minus
nearly 0.2°F (up to 0.12°C)
According to the authors, the new results are a
closer match with surface warming, as well with four simulations
of 20th-century climate produced by global-scale models of the
ocean and atmosphere. These simulations were produced by CGD scientists
and their colleagues using the Parallel Climate Model, a global-scale
model of the ocean and atmosphere that was built by NCAR and Los
Alamos National Laboratory. The simulations included solar variations,
volcanoes, greenhouse gases, and sulfate aerosols, all of which
affect climate.
As a further check on the new satellite data set,
the team examined regional patterns. Using a statistical technique,
the group analyzed the 20th-century simulations and searched for
an underlying “fingerprint” of climate change. For instance,
the rates of warming in the satellite-monitored data vary by latitude
from north to south. The authors found that the overall fingerprint
of climate change in the models resembled this and other regional
patterns found in the new satellite data set.
•Bob Henson
Also
in this issue:
In
the midnight hour: BAMEX takes aim at dangerous night storms
The
long riders: How some staffers cope with epic commutes
Study
finds lower atmosphere warming
An
information divide
Building
bridges for Latina students
Short
takes
Delphi
Question: Publications on the Web
