Stratospheric water helps explain temperature trends
Earth's surface temperatures have not risen as quickly in the last ten years as they did in the 1980s and 1990s. A team of researchers led by ESRL's Susan Solomon (Chemical Sciences Division, CSD) has now shown that a still-puzzling decline in water vapor 10 miles above Earth's surface may be part of the reason.
Water vapor is a powerful greenhouse gas, well understood to affect warming in Earth's atmosphere.
"Current climate models do a remarkable job on water vapor near the surface," Solomon said. "But this is different - it's a thin wedge of the upper atmosphere that packs a wallop from one decade to the next in a way we didn't expect."
Her team analyzed global satellite observations of stratospheric water, which began in the 1990s, and long-term balloon observations of water vapor from the only available site in the world, Boulder, CO. Sam Oltmans (Global Monitoring Division) and his colleagues have been taking measurements there for nearly 30 years.
Stratospheric water increased in the 1980s and 1990s, and dropped after 2000, globally. The authors used models and calculations to determine the effect of those stratospheric changes on Earth's surface temperatures. Results were published in Science January 28.
Between 2000 and 2009, the drop in stratospheric water content acted to slow the rate at which surface temperatures rose by about 25 percent, compared with what would have occurred otherwise. The more limited balloon data suggest that earlier stratospheric water vapor increases could have enhanced surface warming in the 1990s by about 30 percent, compared with estimates that do not include stratospheric water vapor.
The research team noted that it is not yet clear why stratospheric water vapor levels have changed, and called for further observations of stratospheric water vapor, and careful examination of how climate models represent changes in stratospheric water content.
"The Boulder water data are a remarkable record, and the dedication of those who took those data over decades is one of the things that should be highlighted here- and the need for more!" Solomon said.
Solomon's co-authors were Karen Rosenlof, Robert Portmann, and John Daniel from CSD; Sean Davis and Todd Sanford from CSD and the Cooperative Institute for Research in Environmental Sciences; and Gian-Kasper Plattner from the University of Bern in Switzerland.
Texas A&M University atmospheric scientist Andrew Dessler told ScienceNews the new Science paper identified an important new source of short-term climate variability- but one that may well be temporary. People shouldn't rely on continually declining stratospheric water vapor to slow long-term global warming, Dessler told the magazine. "Water vapor is scarce in the lower stratosphere already, and you can't drop below zero," he said. "This is not going to save our bacon."