Satellite data confirms climate change

6 May 2004Nature News Service / Macmillan Magazines Ltd 2004Quirin Schiermeier 

For years, climate researchers have struggled with an apparent discrepancy in the data on global warming: temperatures in the lower atmosphere have been rising far slower than models predict, given how fast the Earth’s surface is heating.

The discrepancy has been central to the arguments of sceptics about global warming. But according to a study in this issue of Nature1 it can be explained by interactions between the troposphere - the first 11 km of the atmosphere - and the stratosphere above it.

In the study, a team from the University of Washington at Seattle and the Air Resources Laboratory of the National Oceanic and Atmospheric Administration (NOAA), based in Maryland, analysed microwave emissions from the atmosphere. The emissions were recorded between 1979 and 2001 by NOAA’s polar orbiting satellites. The data can be used to deduce temperatures in different layers of the atmosphere. And the study finds that stratospheric cooling, a known effect of greenhouse gases,appears to account for discrepancies between temperature trends on the ground and in the troposphere.

The team, led by Qiang Fu, an atmospheric researcher at the University of Washington, subtracted the impact of such cooling from data on the stratosphere and performed a statistical analysis, which found temperature trends consistent with observed warming on the surface and the predictions of climate models.

The finding is “a stunningly elegant and accurate method of clarifying global trends”, says Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder,Colorado.

But it does not impress John Christy, director of the Earth System Science Center at the University of Alabama in Huntsville, whose work established the inconsistency between temperature trends on the surface and in the troposphere2. “You cannot eliminate the stratospheric influence with statistical tools alone,” he says. “If you want to know precisely what happens you need physical measurements.” He says that Fu has overcorrected for the impact of the stratosphere in his analysis.

Other climate scientists welcomed the new findings. “This is the answer - I wish we had recognized it ourselves,” says John Wallace, an atmospheric researcher also at the University ofWashington, who chaired a 2000 survey on reconciling global warming discrepancies for the US National Academies.

The study should be noted by policymakers who justify lack of action on global warming by citing scientific uncertainty, says Wallace. But he is not optimistic about how many minds will be changed. “Single scientific discoveries have little impact in the political arena,” he says. NOAA officials declined to comment on the political implications of the study.

The research comes just after a report from the Pew Center on Global Climate Change predicted that global warming could shrink the US economy.

But neither economic nor scientific analyses are likely to affect US climate change policy, says Henry Jacoby, director of the Massachusetts Institute of Technology’s Joint Program on the Science and Policy of Global Change. “After the Kyoto fiasco, the US administration began to ask for advice from all sides,” he says.“But unfortunately it has never taken the advice it received.”