Nature 449, 710–713 (2007)

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The amount of water vapour in the atmosphere has increased significantly in recent decades as a result of human activity, according to new research. Because water vapour is a greenhouse gas that traps heat in the atmosphere, increased humidity is likely to heighten the affect of human-induced climate change.

A study led by Katherine Willett of the Climatic Research Unit at the University of the East Anglia, UK, used a new gridded observational data set of surface-specific humidity with a coupled climate model to investigate whether changes in humidity between 1973 and 1999 were due to human or natural influences on climate. The researchers found a significant global increase of 0.07 g kg−1 per decade in surface-specific humidity over this period that was primarily due to human activity.

Although an upward trend in atmospheric water vapour in the late twentieth century has been previously reported, this study is the first to link the increase directly to anthropogenic warming. The findings have important implications for understanding future increases in atmospheric humidity, which would be likely to influence the intensity of precipitation and tropical cyclones, as well as effecting human heat stress and water availability.