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Will the Wet Get Wetter and the Dry Drier?

Posted on 30 August 2012 by Rob Painting

"The wet get wetter and the dry drier", or "The rich get richer and the poor poorer",  these are phrases that pop up from time to time when discussion focuses on future rainfall or drought. But what does it actually mean?

The animation below, from the NOAA Geophysical Fluid Dynamics Laboratory, explains the basic scientific premise behind this turn of expression and features a nifty climate model simulation to boot. And yes, this process appears to be well under way.


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Comments 1 to 18:

  1. I trust all the Australians kept an eye on the wide, even-browner land? Wet-wetter and dry-drier ain't likely to bring much joy to the southern continent in particular! You know; where we all, um, live?...
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  2. Well spotted Bill. But note that, in the simulation, there are successive years where greater-than-normal rainfall occurs over Australia. Sound familiar?
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  3. The simulations, I guess, include MEI (ENSO index), so it runs under "hopeful" assumption thet ENSO variability is to stay, as opposed to the suggestions that permanent LaNina could potentially develop. However, in the other side (and the other coast), by mid-2050, Perth WA seems to be entering the permanently "red" territory, and even edging "dark brown".
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  4. Yeah, Rob - it means certain parties can always dust off Dorothea Mackeller...
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  5. Chriskoz - I'm not familiar with research suggesting a permanent future La Nina state. A permanent El Nino was implied during the Pliocene (around 5-2.5 million years ago) but more recent work indicates otherwise, i.e ENSO existed throughout that time too. The increased precipitation variability (mainly ENSO as you point out) in the simulations is due to increased specific humidity (greater moisture holding capacity) in a warmer atmosphere. This drives greater moisture convergence & divergence - see: 1.Evaluating the rich-get-richer mechanism in tropical precipitation change under global warming - Chou (2009) 2. Does global warming cause intensified interannual hydroclimate variability? - Seager (2011) So a warming climate means greater extremes in precipitation even if La Nina & El Nino don't change much. And if you look at the abstract from Durack (2012)in the post above you'll note that actual trends are double those projected by the climate models.
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  6. Categorizing the increased precipitation, where it occurs, as 'rich get richer' might be inadvertently misleading. Whether that is an apt description would depend on whether the precipitation arrives in forms amenable to food production or not. Obviously, that objection does not apply to 'wet get wetter' (since the two are, on this topic, synonymous, as far as I can see).
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  7. I'm reminded of the way Richard Feynmann dissected the Challenger disaster. Specifically, the way NASA calculated the risk of a disaster. The increase in droughts and floods increases the risk that one day most of the world's agriculture will be wiped out simultaneously by either one or the other. Russia/Australia and Pakistan managed that trick just 2 years ago. In time, the disasters will line up, and that will be that.
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  8. It's tricky to get a decent handle on overall effect by eyeballing a global graphic showing annual changes in 5 year trends, but I was played it multiple times looking at a number of regions. Others can try the same and see if they had similar perceptions. Europe & North Africa: worst of everywhere. Mega-droughts. This was the most alarming feature of the whole presentation. Europe still produces very significant amounts of global food production and North Africa is experiencing some of the fastest population growth anywhere on the planet. China: mixed, but experienced bands of both wetter and drier than 20thC average. That will hurt when they have already had some pretty brutal floods and droughts that are going to be exceeded. India: Not quite as bad as Europe, but some major browning in regions of very high population density where access to water and groundwater depletion are already *huge* issues. US: Although the SW saw some dark brown, this was not as extreme as I had expected, since I thought the drying of SW US was one of the major climate concerns. Perhaps I've simply gained that impression by looking at too many US-centric analyses. Mexico is pretty dire. Sub-Saharan & southern Africa: Ouch. Pretty severe drying here. Though my memory is that sub-Saharan African rainfall is one of the bits of climate models where there is least agreement. What did others see?
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  9. Rob @5, Indeed I was referring to a possibility (unconfirmed) of permanent El Nino in Pliocene-like conditions. My typo, sorry. Jeffrey @7, Why are you bringing the Challenger disaster to the context of droughts and floods increases? Can you quote what exactly Richard Feynmann had said? I vaguely remember that incident as it was 25+ yago.
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  10. Byron @8 I did much the same as you, stopping the video at various points to better focus on some of the details. I also compared it to Figure 2 of Dai's August 5, 2012 Nature Climate Change letter. There are multiple differences between the two calculations, notably Dai has the drought in the US much worse than the GFDL simulation. The Sahel region is the complete opposite for Dai, he shows increased precipitation. Dai also has the southern part of Africa and the northern part of South America much worse off.
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  11. re:9 and Feynman I was thinking in terms of the difference in danger estimates between the engineers and NASA management. Management loves to see things move forward. "It appears that there are enormous differences of opinion as to the probability of a failure with loss of vehicle and of human life. The estimates range from roughly 1 in 100 to 1 in 100,000. The higher figures come from the working engineers, and the very low figures from management. What are the causes and consequences of this lack of agreement? Since 1 part in 100,000 would imply that one could put a Shuttle up each day for 300 years expecting to lose only one, we could properly ask "What is the cause of management's fantastic faith in the machinery?" I was using NASA management as a stand-in for all present, GDP-first government types who are dragging their feet on the issue of mitigation. Maybe I was a little too elliptical, but I'd thought that Feynman's analysis of risk had become part of modern thinking about complex systems. I think modern government types believe that droughts and floods will behave as they've always behaved: as remote dangers at a great remove in space and time. Australia, Russia, and Pakistan two years ago convinced me otherwise.
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  12. Rob Painting, As I wrote above, the results presented by GFDL differ in several ways from the very recent paper by Dai that can be found here. Would you know what assumptions go into the models that account for the differences. The Sahel region of Africa stands out in particular as one where the predictions diverge radically. Thanks.
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  13. JoeT - Sorry, I don't know the details of how these two models differ. What I can tell you is that the 19 model ensemble used in Seager (2011), linked to above, did not find a change over the Sahel that was statistically significant.
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  14. Question: The"denial machine" now seems to be reduced to trying to prove the C.S. is small. I do not believe it is small, I think there is now good experimental evidence that the fast feedback CS is 3 deg C. BUT say the CS WAS at the lowest possible end, which we might take as 1.5 deg C. Why would this be an excuse for doing nothing about AGW? Does anyone have any crystal ball into denier logic that would help me out here?
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  15. re: 14 Why would you prepare for the lowest possible danger? (And why you would believe it?) Responsible drivers buy more automobile insurance than the minimum required by law. We don't fund the army on the cheap. People opt for aggressive therapies to treat their cancers. Responsible people who are wealthy enough don't count on only their Social Security check, but save/invest more instead. And on and on. As for the minimum climate sensitivity, the 1.5C figure demands that there be NO feedback response. The 1.5C figure consists only of the amount of heating that a doubling of CO2 will produce. (And why assume we'd stop at 560ppm if we refuse to do anything now?) There's nothing magical about the energy produced by increasing GHGs. It's just energy, and there are always feedbacks. But we've had around 1C of warming already and we're at ~390ppm from a base of 280ppm. At 2ppm/year (the current rate), we'll hit 560ppm in around 80 years. Only .5C more warming over the next 80 years? Who could possibly believe that?
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  16. curiousd@14, that is a topic about which Ph.D.'s have been attained; My take is this (and I only gots a BSc in geology), that people almost always, will deny the very *existence* of the tiger that has them by their own tail, until they see the gnashing teeth of that tiger arriving at their throat. NO one wants to admit they are the problem; said another way, only wet babies like change. Clearly the 'facts' of the case, asserted by something north of 97% of the scientific opinion that is relevant to this issue have made no difference to the Moncktons, the Lindzens, the Christys, the____________________(fill in the blank), and frankly, they are the ones who most loudly decry us earth scientists as the ones who're are on the 'gravy train.' Extant data, and the persistent yelling of those folks, belies that bit of evidence. So, the *only* answer I have for you is this: keep asserting scientific support of the problem, kepo debunking the deniers, again and again and (Mod, please pardon the caps) AGAIN, and maybe, just *maybe*, the tiger of cAGW will be short-circuited at our.....crumbs....before they arrive at our jugulars. For me the *single* most important resource I have to help me make the point to those who deny it all, and to which I read and study every day, is SkS. Sorry for the OT post, but thought I'd take a shot at addressing your question.
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  17. Discussions of climate sensitivity are best taken to one of the CS threads, such as this one.
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  18. Rob -- Thanks for the Seager reference. I had trouble getting the paper you linked to. However this worked for me. The paper is 2012, btw. I haven't got to the paper yet, but Seager looks like he would be an excellent source for understanding the physics of drought. He has an interesting website and wrt comments @3 and @5 about whether greenhouse gases would bring on El Nino or La Nina and how that would affect the US Southwest, he had this to say : Currently climate models are all over the map in how the tropical Pacific Ocean responds to rising greenhouse gases. The climate modeling group at Lamont has argued that rising greenhouse gases will warm the western tropical Pacific Ocean by more than the eastern ocean because, in the west, the increased downward infrared radiation has to be balanced by increased evaporative heat loss but in the east, where there is active upwelling of cold ocean waters from below, it is partially balanced by an increase in the divergence of heat by ocean currents. As such, the east to west temperature gradient increases and a La Niña-like response in induced. This is the same argument for why, during Medieval times, increased solar irradiance and reduced volcanism could have caused a La Niña-like SST response, as seen in coral based SST reconstructions. If the Medieval period is any guide as to how the tropical Pacific Ocean and the global atmosphere circulation respond to positive radiative forcing then an induced La Niña could regionally intensify the general projected subtropical drying and the American West could be in for a future in which the climate is more arid than at any time since the advent of European settlement.
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