It’s time to re-examine the climate change models – water-vapor is more important than previously thought. That’s the message from a new study by U.S. National Oceanic and Atmospheric Administration (NOAA), which analyzed the effects of water-vapor in the stratosphere on global climate. In particular the study noted that the recent slow-down in global warming, noted since 2000, may be caused by a drop in the amount of stratospheric water vapor.

The new study analyzed the amount of water vapor up to 10 miles into the stratosphere over the past ten years. Water vapor is a known greenhouse gas, a rather potent one at that, but it has generally been assumed to be a relatively small proportion of the factors that affect climate. This study says that effect was underestimated. In years since 2000, there has been a 10% drop in the amount of stratospheric water vapor. Correspondingly the greenhouse gas effect has been reduced. This could have led to balancing the increased CO2 and resulting in a leveling of global temperatures during that period.

The study also considered the period 1980 to 2000 (noting that these observations were made by a single weather balloon). From these data it appears that during that period water vapor increased in the stratosphere. The additional water vapor may have increased global warming by about 30%.

The correlation between stratospheric water vapor and global temperature change is relatively intuitive (stratospheric = global, less vapor = more cooling) but the exact mechanics are unknown. What causes the shifts in available water vapor? The study suggests it might be caused by changes in evaporation rates over the global oceans. If so, then global warming, which increases the oceans’ evaporation might actually produce a kind of natural brake on the rate of warming.

The study was not unaware of the implications of the finding for the ongoing global warming debate:

The new research, led by Susan Solomon, at the US National Oceanic and Atmospheric Administration, who co-chaired the 2007 IPCC report on the science of global warming, is published today in the journal Science, one of the most respected in the world.

Solomon said the new finding does not challenge the conclusion that human activity drives climate change. “Not to my mind it doesn’t,” she said. “It shows that we shouldn’t over-interpret the results from a few years one way or another.”

[Source: The Guardian]