The Island of Doubt

From "Detection of human influence on twentieth-century precipitation trends" by Xuebin Zhang et al (doi:10.1038/nature06025):

We show that anthropogenic forcing 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. We estimate that anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics.

Now, that's not a forecast per se. But if one assumes that continued global warming will only continue to exaggerate the observed trends. And as study team member Francis Zwiers, told New Scientist, "If you're able to reproduce the past, you also have greater confidence for predictions of the future."

The study concentrated on 40° S to 70° N, because that's where most of the data are. Some more excerpts:

It is expected that wet tropical regions would become wetter and dry regions drier if there were an increase in tropospheric temperature from anthropogenic forcing but no change in lower-tropospheric relative humidity or flow...

If the finding that observed changes are larger than simulated is robust, then projections may also underestimate future precipitation changes....

The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation, such as the Sahel.

And from the New Scientist story:

Zwiers, of Environment Canada in Toronto, says that the pattern shows a substantial drying of the region from the equator up to 30° north. This band encompasses all of north Africa, as well as India, southeast Asia, Mexico and northern South America. Some of these regions, such as the Sahara and Sahel in Africa, are already among the world's driest.

Regions further to the south, including the rainforest regions of central Africa and South America, have begun to get increased rainfall and will continue to get wetter.

The findings are important, Zwiers says, because "as humans, our activities are much more constrained by limits of water than by temperature. In places where agriculture is marginal, it will become more marginal in the future".

In other words, if you live in Scottsdale, Ariz., don't expect water to get any more plentiful. And if you live in the former Lake Chad watershed in the neighborhood of Darfur, things are probably going to get worse before they get better.