What we don’t know.
Posted by Jeff Condon on January 31, 2010
An interesting paper from Science.
Susan Solomon,1 Karen Rosenlof,1 Robert Portmann,1 John Daniel,1 Sean Davis,1,2 Todd Sanford,1,2 Gian-Kasper Plattner3Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here, we show that this acted to slow the rate of increase in global surface temperature over 2000 to 2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% compared to estimates neglecting this change. These findings show that stratospheric water vapor represents an important driver of decadal global surface climate change.
1 NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO, USA.
2 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA.
3 Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland.
“We found that there was a surface temperature impact due to changes in water vapor in a fairly narrow region of the stratosphere,” explains research meteorologist Karen Rosenlof of the National Oceanic and Atmospheric Administration’s (NOAA) Aeronomy Laboratory, one of the authors of the study. “The reason for the water vapor change is the temperature drop at the interface between the troposphere and the stratosphere over the tropics. What we don’t know is why the temperature dropped.”
All told, stratospheric water vapor declined by 10 percent since 2000, based on satellite and balloon measurements, yet that was enough to appreciably affect temperatures at ground level according to climate models. “Reduce the water vapor and you have less long-wave radiation coming back down to warm the troposphere,” Rosenlof says. Conversely, an apparent increase in water vapor in this region in the 1980s and 1990s exacerbated global warming.
Of course, the amount of water vapor in the atmosphere is also affected by another potent greenhouse gas—methane—which has unexpectedly failed to increase in recent years. “The other influence is methane, which breaks down into two water molecules and CO2 in the stratosphere,” explains climate scientist Drew Shindell of NASA’s Goddard Institute for Space Studies (GISS). “Methane’s growth rate has dropped, so it’ll have become a weaker source of stratospheric water, but we don’t fully understand why its concentrations have not increased as rapidly in recent years as they did for the previous several decades.”