A truce on
Upper atmospheric temperatures recorded over the past 20 years contradict
surface observations that indicate a century of increased warming. Many
scientists have cited this discrepancy as evidence that one or the other
temperature recordings must be faulty. The report, Reconciling Observations
of Global Temperature Change, concludes, however, that both data sets
are real, indicating a more complex relationship between the surface and
upper atmospheric layers.
Climate modelers have declared a truce
to embrace their differences. Although the debate on climate change is
not resolved, a recent report recognizes that the spectrum of opinions
on the issue may share a great deal of common ground.
“We can conclude with a high level of confidence that the surface
temperature is indeed rising,” says climatologist John M. Wallace of the
University of Washington in Seattle. He is chair of the panel of scientists
that wrote a report released Jan. 12 by the National Research Council,
part of the National Academy of Sciences. The Academy had asked the
panel members to examine the apparent conflict between temperature recordings
at the surface and the upper atmosphere. Perhaps the most surprising outcome
of the consensus report was the diverse list of panel members and reviewers:
more than 20 people known in the community for their differing opinions
on global warming.
Satellites, such as this polar orbiter,
and weather balloons are indicating
little warming in the lower to mid-troposphere,
challenging scientists to
rethink the relationship between
the upper atmosphere and Earth's
warming surface. Courtesy
of the National Research Council and NOAA.
The assumption that one or the other data set is incorrect can be put
aside, says Thomas C. Peterson, chief of the scientific services division
of the National Climate Data Center for NOAA. “It’s time to incorporate
both of them to really make sense of what is happening.”
In the past 20 years the average surface temperature over the globe
increased by 0.25 to 0.4 degrees Celsius, raising the estimated warming
during the last century to 0.4 to 0.8 degrees Celsius, according to the
If this warming is a result of greenhouse gases, a similar warming
should be occurring in the upper atmosphere. But since 1979, satellite
and balloon-borne instruments, such as radiosondes, have shown temperatures
have risen very little, 0.2 degrees Celsius at most, in the lower to mid-troposphere
— the atmospheric layer extending about 8 kilometers (5 miles) from Earth’s
It may be that the warming of the surface is not a result of greenhouse
gases but a result of the ocean releasing stored heat, says meteorologist
Richard Lindzen of the Massachusetts Institute of Technology and one of
the reviewers of the report. “For this to be a greenhouse effect it is
crucial for warming to occur in the atmosphere,” he says.
Although uncertainties may exist as large as the disparity in temperature,
human and natural forcings may also be affecting the atmosphere’s vertical
profile. Human induced ozone depletion, from the use of chlorofluorocarbons,
cools the lower stratosphere and this cooling may extend low enough into
the troposphere to influence the satellite data.
The eruption of Mount Pinatubo in 1991 may have had longer-lived influences
in the atmosphere than previously thought as a result of residual cooling
of the ocean. Another influence perhaps may be that an El Niño response
in the lower atmosphere is amplified in the vertical, says research meteorologist
Dian Gaffen of NOAA.
“The vertical structure of the atmosphere is not as tightly linked
as conventional wisdom would have it,” says John R. Christy, professor
of atmospheric science at the University of Alabama in Huntsville. “Vertical
structure, temperature structure, of the atmosphere is just one of the
things in climate models needing improvement.” Are climate models useless
for telling us about how the climate system works? No, he says: “It’s just
that here’s a set of observations that haven’t been well replicated. It
gives us a bit of skepticism about other aspects of climate modeling and
their ability to predict century time scales.”
The report stresses the need to account for the uncertainties in the
surface, satellite and radiosonde data. “We make a strong recommendation
for an improved monitoring system,” Gaffen says. Such a system should include
better measurements of “ozone, water vapor, clouds and aerosols, all of
which have the potential to cause surface and lower to mid-tropospheric
temperatures to change relative to one another,” according to the report.
Wallace is excited about future possibilities for models. “We need
to improve the quality of the observations,” he says. There is a need for
redundancy in the data and continuity of the satellite observations. Twenty
years does not provide a long enough sampling period.
“Even surface data can’t be taken for granted,” he adds, indicating
a needed improvement in the quality control of the data as some surface
stations are moved or shut down.
Understanding the complexities involved in atmospheric responses to
global climate change and narrowing the discrepancy between the surface,
satellite and radiosonde data sets will pose no small challenge.
As Wallace says: “It’s humbling to be faced with a problem like this.”