With the huge amounts of extreme weather and rainfall in Australian this year, it seemed appropriate to ask questions about rainfall and water vapor in relation to climate science. What areas of the plan will receive more rainfall as climate change continues and will this offset the desertification in other areas? Do water vapor emissions need to be reduced?

Got a question about climate science? Crikey’s environment blog Rooted is still running the Ask a climate scientist series. Keep the questions coming by emailing me directly.

The answers to these questions come from American Geophysical Union’s Climate Science Q&A service, where more than 700 volunteer scientists provide factual and peer-reviewed climate science information to journalists. This service was extended again — it’s still in semi-pilot stage — until March 1, 2011 so get in while you still can. The AGU only comments on science, not climate policy.

Crikey reader Lindsay B asks:

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What areas of the planet are likely to receive more rainfall as a result of a 2, 4 and 6 degrees of warming, and how much of the desertification would this offset?

Spencer Weart, Director Emeritus at the American Institute of Physics, responds:

This is a very important question, of course. Generally speaking, climate models are a lot better at predicting general global changes; when it comes to specific regions, different models may give different results. But overall the models tend to agree that parts of the planet that now get a lot of precipitation will get more, and parts that are dry will get drier. It’s pretty basic: warmer air holds more water, so moisture will evaporate faster where there tend to be droughts, and it will rain harder where there tend to be floods.

I don’t think enough modeling work has been done to give useful separate precipitation estimates for 2, 4 and 6 degrees of warming. The most reliable results that we have at present come from the big push of modeling, and comparison among different models, that was done for the IPCC’s 2007 “AR4” assessment report. These projections were done for the end of this century, assuming the “A1B” scenario for how much greenhouse gas emissions humans will put out; A1B projects a roughly 3 degree mean global warming (more or less, nobody is really sure, it might be 2 or 4).

A1B, by the way, is rather optimistic: as things are now going, we will probably emit more greenhouse gases and thus probably get above 3 degrees. Anyway, you can see the precipitation projections for this scenario here, at the online version of the 2007 report, where there is a table and a diagram. You will see that most of the increase of precipitation is expected in the far north (Canada, Siberia) and around Antarctica (global warming = more snow!) and, in the winter, in a tropical band including parts of Brazil, West Africa and Indonesia.

“Desertification” usually means an expansion of deserts due to human activity, such as overgrazing. The region where historically there has been the most worry about “desertification” is the Sahel, in Africa. However, this lies in between regions that we are pretty sure will get wetter (tropical Africa) and ones that will likely get drier (North Africa and the Mediterranean), so it’s one of the regions where models do not give consistent projections — it may get wetter or drier on average. However, desertification increases mainly during droughts, which hit the Sahel every now and then. The increased evaporation due to global warming will make any drought more severe, so the prognosis is not good.

This answer was reviewed by computational climate scientist Robert Jacob from the Argonne National Laboratory.

Crikey reader Tim asked:

I have often heard denialists argue that since water vapour is the major greenhouse gas, carbon emissions can’t really be a problem. Is it true that water vapour is the major gas, and if so (or not) what role does it actually play in global warming? Also could we actually reduce ‘water vapour emissions’ in the same way as we could reduce carbon emissions?

Spencer Weart, Director Emeritus at the American Institute of Physics, responds:

I agree, it’s important to point out that the Earth is a wet planet. Water cycles in and out of the air, oceans, and soils in a matter of days, exquisitely sensitive to fluctuations in temperature. By contrast CO2 (and other, less important greenhouse gases like methane) linger in the atmosphere for much longer timesclaes (decades or centuries). Thus it is these gases that act as the “control knob” that sets the level of water vapor. If all the CO2 were somehow removed, the temperature at first would fall only a little. But then less water would evaporate into the air, and some would fall as rain. With less water vapor (and also less clouds retaining heat at night) the air would cool further, bringing more rain… and then snow.

Within weeks, the air would be entirely dry and the Earth would settle into the frozen state that can be calculated for a planet with no greenhouse gases. This report by Lacis et al, Science 330 (2010) describe “water vapor and clouds as the fast feedback processes in the climate system,” whereas the “noncondensing greenhouse gases… provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds.” In terms of direct effects on radiation, water vapor accounts for about half the greenhouse effect, clouds for about a quarter, CO2 for 20% and other greenhouse gases 5%.

This answer was reviewed by Clare Murphy (Paton-Walsh) from the Centre for Atmospheric Chemistry at the University of Wollongong

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