It is getting steamy, scientists say

A global-scale increase in surface humidity over the late twentieth century can be attributed mainly to human-induced global warming, reports a paper in Nature this week.

The specific humidity is a measure of how much water vapour there is in a given volume of air. The water vapour content of the atmosphere is an important component of the Earth's climate system - affecting the distribution and maximum intensity of rainfall, the potential intensity of cyclones, and surface hydrology. It is the most important contributor to the natural greenhouse effect.

Scientists have observed significant increases in specific humidity at the Earth's surface over the last few decades, but it has been unclear whether these changes represent a natural or human influence on climate.

Nathan P. Gillett and colleagues combined a new data set of observations with a coupled climate model and found that a significant increase in global mean surface specific humidity over the late twentieth century was mainly due to anthropogenic warming. They conclude that this response to human-induced climate change may have important implications for extreme precipitation, tropical cyclones and human heat stress.