In May, Rep. Norm Dicks (D-Wash.) sponsored a nonbinding resolution on global warming, stating that the House of Representatives recognizes that warming is real and caused by excessive greenhouse gases in the atmosphere. The resolution reached the floor of the House, but was blocked from a vote. As with other such climate policy decisions, uncertainty surrounding climate research was partially to blame for its failure.

Among many uncertainties that global warming skeptics have cited are past discrepancies between a supposed warming trend at Earth’s surface and a supposed cooling trend in the lower atmosphere in the tropics. A new report, however, issued in April by the U.S. Climate Change Science Program (CCSP) — a White House-sponsored research program on climate change — states that this particular discrepancy has been resolved. The reconciliation removes one of the biggest uncertainties used to challenge the reliability of climate models and observations, says John M. Wallace of the University of Washington in Seattle, who was involved in both the CCSP report and a 2000 National Academy of Sciences report on the topic.

Since the late 1970s, scientists have observed rising temperatures both at Earth’s surface and in the troposphere, the part of the lower atmosphere that reaches to about 10 to 15 kilometers (6 to 9 miles) above the planet. Models predict that temperatures in the troposphere should rise in step with temperatures at the surface, and that the largest upward trends should be in the tropics, Wallace says. But in the tropics, the instruments used to measure tropospheric trends — sensors on satellites and weather balloons called radiosondes — have disagreed somewhat on the relative change, with some even showing a cooling in the troposphere.

A number of reports have come out in the past year that explain this discrepancy, suggesting that the original analyses of the satellite data were incorrect, Wallace says (see Geotimes, October 2005). The new CCSP report is the first time these data have been compiled, however, and is the first report issued by CCSP. The wide variety of experts involved in the report, most of whom have worked on this issue for more than a decade, agreed that errors in observations and analyses were to blame.

The report is important because it is a consensus statement, says Carl Mears of Remote Sensing Systems in Santa Rosa, Calif., who is a lead author on the CCSP report. “We [scientists] are in much more agreement,” he says. “There’s now little disagreement between measurements and models.”

Still, although “this significant discrepancy no longer exists,” as the authors wrote, it is important to note that more work is necessary, especially on tropical temperature datasets, says John Christy of the University of Alabama in Huntsville, who was also a lead author on the report. While the data quality from the satellites and radiosondes is good for measuring short time periods (for which it was designed), it is “poor” for showing temperature trends over years or decades, says Jay Gulledge, a senior research fellow at the Pew Center on Global Climate Change. To figure out what is actually going on in the tropical atmosphere over the long term, he says, “we’ll have to rely on further measurements.”

Many more studies need to be done, including more comparison studies and new, better observations, Christy says. “I hope this will draw attention to the fact that we need to upgrade our observing system,” in addition to surveying past records in finer detail, Wallace adds.

The authors of the CCSP report steered clear of making any policy statements, but the report’s conclusions “will make it harder for Congress to ignore” climate change, Mears says. “The observed patterns of change over the past 50 years cannot be explained by natural processes alone,” the authors wrote, and there is no doubt that humans are affecting the atmosphere.

Whether that will translate to congressional or administrative action remains to be seen, says Vicki Arroyo, director of policy analysis for the Pew Center on Global Climate Change. With each new scientific report that comes out, such as this one, or 2004’s Arctic Climate Impact Assessment, “we keep hoping, expecting that the government will push something through,” Arroyo says, but until “the political will” is there, nothing will happen.

Following the report’s publication, the White House said it welcomed the report’s conclusions. In terms of actions, a White House spokesperson says: “The president believes that while we improve our understanding of climate science, we can also act to reduce emissions of greenhouse gases through investments in cleaner and more efficient energy and technologies.”

Megan Sever

Links:
"Revisiting the satellite record," Geotimes, October 2005
U.S. Climate Change Science Program

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