Earth's stratosphere is a cold, dry place, above the troposphere—the bottom layer of the atmosphere we breathe on a daily basis. Ruled by winds and hosting everything from bacteria to long-distance jet travel, about the only way that water gets into this high-altitude layer 10 kilometers above the Earth's surface is when it billows up from the humid tropics, rising from the troposphere via the atmospheric interface known as the tropopause. But since 2001 there has been less water vapor in a narrow, lower band of the stratosphere thanks to cooler temperatures in the tropopause, and that may just be holding back global warming at ground level, according to new research published online in Science on January 28.

"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."

That temperature does seem to correlate, however, to sea-surface temperatures in the Pacific that, of course, follow El Niño–La Niña cycles, along with other trends. A new El Niño cycle—warmer surface waters—began last summer, which may mean that stratospheric water levels could change again. So this effect could either be the result of natural variability in Earth's climate, or yet another effect of carbon dioxide and other greenhouse gases like water vapor trapping more heat and thus warming sea-surface temperatures.

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."

In fact, the more than 100 percent increase in overall methane since the 18th century has made the stratosphere a wetter place, notes GISS climate modeler Gavin Schmidt. "What might have caused this effect? I can think of two factors: The 1997–98 El Niño might have moistened the lower stratosphere more than usual, and thus there has been a trend toward drying since then," he says. "A second idea might be related to changes in aerosol emissions from Asia, which have affected temperature profiles in the tropics and the properties of clouds."

And there remains little doubt that average temperatures are getting warmer at ground level; data from NOAA's National Climatic Data Center reveals that the last decade was the warmest since record-keeping began. More monitoring of the entire atmosphere as well as the whole panoply of greenhouse gases over the long term will be required to determine what's behind the lower stratospheric dry out—a set of observations imperiled by the current dearth of Earth observation satellites operated by the U.S. space program.

But one thing remains clear: More greenhouse gases in the atmosphere equals more warming. "It doesn't say that CO2 warming isn't going on," Rosenlof adds. A drier lower stratosphere may simply have slowed the warming caused by the thickening greenhouse gas blanket.