NEWS NOTES

March 2000

A truce on global warming
 

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.

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.
 
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 report.
 
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 surface.
 
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.”
 

Christina Reed