An article published online in the Proceedings of the National Academy of Sciences reports the first quantitative data showing that global warming is elevating the moisture content of the atmosphere (DOI: 10.1073/pnas.0702872104). The vapor content in each square-meter column of atmosphere has increased since 1988 by about 0.41 kg per decade, says lead author Benjamin Santer of the Lawrence Livermore National Laboratory Program for Climate Modeling & Intercomparison, in California. "Natural variability can't explain that change," he adds. "This is the first identification of a human fingerprint on the amount of water vapor in the atmosphere." Excess water vapor, a greenhouse gas, amplifies the warming effect of higher CO2 concentrations. The authors of the paper used the combined results of 22 different computer models of the climate system and data from a satellite-based microwave imager to show that the recent increase of moisture in the atmosphere is not due to solar forcing or gradual recovery from the effects of the Mount Pinatubo volcanic eruption in 1991. "The climate system is telling us a consistent story. The observed changes in temperature, moisture, and atmospheric circulation fit together in an internally and physically consistent way," Santer adds.