Rain and Climate

Jul 23, 2007 4:07pm

It’s been raining, unusually hard, in several parts of the world–England, China, India, and parts of the U.S.–and, quite by coincidence, there is a paper coming in this Thursday’s edition of the journal Nature, reporting that, "for the first time, climate scientists have clearly detected the human fingerprint on changing global precipitation patterns over the last century." 

That phrase comes from the lead authors at Environment Canada, who worked with other scientists in Britain, Japan and the U.S.  They compared rainfall patterns since 1925 with the changes that fourteen different computer models of the climate said ought to have happened, and found that in large parts of the world, they match pretty well.

"We show," they write in the Nature paper, "that anthropogenic forcing [i.e., changes caused by human activity] has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing."

They found much of the Northern Hemisphere and the southern tropics getting wetter over time, and some tropical regions just north of the Equator–notably the Sahel region in Africa–getting drier.

The abstract of the paper is HERE.  The full paper is not online without a subscription, but Nature does have a news piece of its own HERE

"This is a very important paper," says climate researcher Myles Allen of Oxford, in the Nature news piece. "It identifies the fingerprint of human influence."

There are some important caveats to remember:

–The rainfall records used in the paper are only on land (records at sea, before weather satellites, are spotty), so about two thirds of the planet cannot be measured. 

–Weather is not climate, so it’s dangerous to say the bad rains today are necessarily connected with global changes.

–Could there be other factors at play?  Yes, of course, but this paper found the best match between actual rainfall and the computer models came when the models accounted for "anthropogenic" factors–the increase in greenhouse gases such as carbon dioxide.

There’s been a lot of discussion over whether variations in the Sun’s intensity could explain the warming of the last 20 years.  To that end, there’s now a paper in Proceedings of the Royal Society, by a pair of British and Swiss scientists, saying that if anything, "all the trends in the Sun that could have had an influence on the Earth’s climate have been in the opposite direction to that required to explain the observed rise in glboal mean temperatures."  The full paper is HERE.

Climate is always politically contentious.  Thoughts welcomed.

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