Samplings: December 2007–January 2008

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Atmospheric water vapor over oceans on August 29, 2005, estimated from the satellite-based Special Sensor Microwave Imager. Red areas and the small white areas within them represent locations with high atmospheric moisture content. The highest water vapor values are associated with typhoons Talim and Nabi in the Pacific and with Hurricane Katrina in the Gulf of Mexico. 

Carl Mears and Frank Wentz at Remote Sensing Systems in Santa Rosa, California

It’s the humidity, or so the satellites say. They’ve been measuring a steady rise in atmospheric moisture over the oceans since 1988, when they first started gathering such data. The mugginess seemed a likely hallmark of global warming, and a new study now shows that human activity is definitely the cause. The satellite data indicate that the column of atmosphere above every square yard of ocean now holds nearly three more cups of water than it did two decades ago, according to a team led by Benjamin D. Santer of the Lawrence Livermore National Laboratory in California. Combining results from all twenty-two of the world’s major climate models, Santer and his team discovered that the increase came not from solar radiation, volcanoes, or El Niño—factors that climatologists had considered—but from the greenhouse gases people have been pumping into the air. Greenhouse gases warm the atmosphere and thereby increase its moisture-holding capacity. But water vapor is itself a greenhouse gas—a wicked feedback loop, if ever there was one. Of course, a fraction of the extra vapor condenses and forms clouds, which could offset some of the warming. Beware though: high humidity can trigger intense hurricanes, the kind of cloudy weather we can definitely do without. (PNAS)