Rainfall and global warming

[lionking]

In south-east Australia we have been suffering a drought for several years with some areas having had less than half the long term average rainfall. Politicians, green groups and journalists blame global warming and even argue that it is futile to build desalination plants as emissions would only compound the problem.

I am not a global warming "denier" and accept the IPCC's conclusions that the earth is warming by a fraction of a degree per decade and that man made greenhouse gasses most likely contribute. But does this make drought inevitable, as the media and the greens contend?

I understand that the amount of water in the biosphere is more or less constant (please correct me if this is not so) and that global warming would, if anything, increase rainfall through increased evaporation. Is there any modelling which shows that some regions would receive more rainfall and others less? Or will this drought break like all others?

 

[Professor Yaffle]

http://www.msnbc.msn.com/id/19463513/?open=antarctica

Try the above site - click on "warming scenarios" and then on the area of the world you are interested. It will tell you the forecasted changes in rainfall in different parts of the world. For example, it says Australia will get drier, but for example Northern Europe will have more winter floods.

 

[lionking]

http://www.msnbc.msn.com/id/19463513/?open=antarctica

Try the above site - click on "warming scenarios" and then on the area of the world you are interested. It will tell you the forecasted changes in rainfall in different parts of the world. For example, it says Australia will get drier, but for example Northern Europe will have more winter floods.

Thanks Yaffle, I was not aware of this. Is this generally accepted and are the mechanisms of such rainfall variations understood?

 

[Professor Yaffle]

TBH I don't know much more about it that you - I just like googling...

I don't think the forecasts are set in stone - there will be an amount of uncertainty around them, they are just the best guesstimates based on the current knowledge. For example I came across one abstract that said that current models were predicting a certain increase in water in the atmosphere per increase in temperature, but a smaller rate of increase in precipitation, whereas the paper argued that up to now increases in atmospheric water and precipitation has increased roughly equally.

http://www.sciencemag.org/cgi/content/abstract/317/5835/233

Anyway, I'm sure someone who actually knows what they are talking about will be along sometime soon!

 

[mhaze]

Those forecasts are most likely entirely wrong due to recent research with pretty surprising results.

Check here for a discussion on precipitation.

http://www.internationalskeptics.com/forums/showpost.php?p=2986231&postcount=1353
Here is a good overall discussion.


Precipitation Systems: Nature's Air Conditioner?

Check the links below for the one entitled "How much more rain does GW bring". This is Wentz 2007. Looks like the precipitation is 3x over the models.

There is a lot unsettled in this entire area of understanding the effects of the water cycle on global warming.

 

[lenny]

Is this generally accepted and are the mechanisms of such rainfall variations understood?

i expect one could say it is "generally accepted" that the models do a poor job interms of getting realistic rainfall. (i realise your question was on the understanding of mechanisms, that one is harder to answer.)

Those forecasts are most likely entirely wrong due to recent research with pretty surprising results.

does the fact that the IPCC makes (made) it quite clear that the models often fail to agree on even the sign of the change in amount of precip (whether it will increase or decr3ease) count in their favor or against them, in your book?

(see Figure SPM7 of the AR4 Summary for Policy Makers).