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PostPosted: Wed Nov 28, 2012 11:23 pm 
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A recent study published in the nature journal claims that droughts will not be more extreme in a warmer world.

http://www.nature.com/nature/journal/v491/n7424/full/nature11575.html?WT.ec_id=NATURE-20121115

Audio from the ABC (Australia)
http://mpegmedia.abc.net.au/rural/telegraph/201211/r1034840_11871233.mp3

As I understand it, they are claiming that higher temperatures do not of themselves lead to drought, as this also increases rainfall, so globally there is no net increase in the severity of droughts.
This I find problematic, as I understand it the subtropical highs have moved further South (or North) and become more intense which in turn means more severe droughts in these areas so on balance it would appear to me that the statement that climate change will increase the severity of droughts is still is correct.

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PostPosted: Wed Nov 28, 2012 11:54 pm 
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I think the key problem is the jet streams... these fast-moving high-altitude winds circle the globe and affect the direction of air and the temperatures of those air masses. When the temperature drops, the air can hold less moisture so it causes rain or snow. The movement of that jet stream is mostly effected by hot and cold areas of the globe including the oceans. When the jet stream location changes, so does the locations the air decides to rain or snow. When the air is dried out, it will not rain or snow anymore and especially if it is warming up. Changing weather patterns from changing temperature means that the areas that are wet and the areas that are dry are going to change.

Long story short... there will be the same amount of water in the air as it starts across the land but where the water goes is going to change. Checking the average rainfall is going to show minimal change. Seeing your land go from high productivity to semi-desert grazing land in a dozen years will be the story of many areas of farmland while places that were long desert areas will start to grow different grasses before someone figures they can grow crops on it.


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PostPosted: Thu Nov 29, 2012 4:11 pm 
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They are using an observational dataset to have determined that droughts have not increased as temperatures increased. Interesting study.

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PostPosted: Fri Nov 30, 2012 1:54 am 
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Ann Vole wrote:

Long story short... there will be the same amount of water in the air as it starts across the land but where the water goes is going to change. Checking the average rainfall is going to show minimal change. Seeing your land go from high productivity to semi-desert grazing land in a dozen years will be the story of many areas of farmland while places that were long desert areas will start to grow different grasses before someone figures they can grow crops on it.


The first part of your post re jet streams I broadly agree with. The bit above is not what I would expect. As the temperature rises the air can hold more water vapour (7% more per degree C). The warmer and the wetter the air the higher it is likely to rise and as it rises it will cool to the point where it condenses back to rain. So I think that the the period from evaporation to the rain falling back to the surface should stay about the same the net result I would expect is heaver or more intense rainfall. While it is very difficult to point to any specific weather event and show that there is a direct link to climate change and I believe that recent heavy flooding in numerous places may well be connected to climate change.
My view at this stage is still that deserts will become drier and wet areas will become wetter. Broadly what happens is the air around the equator is warm and wet so we get a large area of rising air which after it has dropped it water on the wet tropics then descends around the 20s deg latitudes which is where we see some of the worlds great deserts. What I expect is that the wet tropical zone will increase in size and so will the very dry zone further away from the equator.

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PostPosted: Fri Nov 30, 2012 2:03 am 
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Snowy123 wrote:
They are using an observational dataset to have determined that droughts have not increased as temperatures increased. Interesting study.


The problem is they are putting a different interpretation on the data. To me it seems as if they are really saying that because the wet areas are getting wetter it makes up for the fact that the dry areas are getting drier. The more I have looked into this the more difficult it becomes to define what exactly we mean by the term drought.

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PostPosted: Fri Nov 30, 2012 8:16 am 
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On a global moisture basis you would be hard pressed to show any change in droughts as they are, like floods, localized events. The only way I could see is to compare the areas classified as being in a drought condition and comparing that data for the same period over time. It is not an easy or simple method by any means.

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