THE discovery that greenhouse gas emissions are making Earth鈥檚 atmosphere wetter is adding weight to the argument that global climate change is upon us.
When the Intergovernmental Panel on Climate Change first announced in 1996 that the balance of evidence suggested that human-induced climate change had begun, sceptics pointed out that the data showed only an increase in global temperature.
To make a convincing case, they argued, climatologists needed to show that other, related aspects of climate such as winds and moisture were also changing as a result of greenhouse gas emissions.
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Benjamin Santer, a climate scientist at Lawrence Livermore National Laboratory in California, and his colleagues studied satellite measurements of the water content of the atmosphere. They found that total moisture content over the oceans had increased since satellite records began in 1988, but the question remained whether or not this was due to human activity.
So Santer鈥檚 team combed through a database of computer simulations involving 22 different climate models, far more than any previous study. This reduced the risk that the idiosyncrasies of any particular model would influence their conclusions.
Some of the simulations included greenhouse gas emissions, some included natural factors such as volcanic eruptions and fluctuations in solar radiation, and some included both or neither.
When they compared the results of the simulations and matched them to the satellite data, the researchers found that the simulations for increased greenhouse gas emissions gave the closest match (, ). Moreover, the amount of extra moisture in the atmosphere was very close to that predicted from increased evaporation as a result of the temperature increase so far.
鈥淭he climate system is telling us an internally consistent story, so some of the criticisms we received 10 years ago are no longer valid,鈥� says Santer.
鈥淭he climate is telling us an internally consistent story, so some of the criticisms of 10 years ago are no longer valid鈥�
Extra atmospheric water vapour fuels the development of hurricanes and also acts as a potent greenhouse gas in its own right, says Santer.
