Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming

Dropping a Notch Between 2000 and 2001, the concentration of water vapor in the stratosphere dropped by about 10%. Water vapor is an important greenhouse gas, so did the decrease affect climate and slow global warming? Solomon et al. (p. 1219, published online 28 January) used a combination of data and models to show that lower stratospheric water vapor probably has contributed to the flattening of global average temperatures since 2000, by acting to slow the rate of warming by about 25%. Furthermore, the amount of water vapor in the stratosphere probably increased between 1980 and 2000, a period of more rapid warming, suggesting how important the concentration of stratospheric water vapor might be to climate. Decreases in stratospheric water vapor after the year 2000 slowed the rate of increase in global surface temperature. Stratospheric 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 2000–2009 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% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.

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