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Climate change could push risk of ‘megadrought’ to 99% in American southwest

Posted on 28 October 2016 by Guest Author

This is a re-post from Carbon Brief by Robert McSweeney

A megadrought spanning several decades could be almost certain to hit the American southwest this century if greenhouse gas emissions are not curbed, a new study says.

Rising temperatures will “load the dice” in favour of a megadrought in the region, the researchers say. Combined with a decline in rainfall, warming conditions could put risk levels at 99% for much of the region, the study finds, while moderate increases in rainfall would still leave risk levels above 70%.

But there is a “glimmer of hope” another scientist tells Carbon Brief. The study also shows that keeping global temperature rise to no more than 2C above pre-industrial levels would cut this risk by half.


In the last few days, state officials have confirmed that California’s drought has now ticked over into its sixth year. The costs of the ongoing dry spell have run into billions through lost crops and livestock, and the energy demands of pumping water from ever-retreating groundwater stores. Today, more than 60% of the state remains in “severe” drought or worse.

Now a new study, published in Science Advances, says unchecked climate change is upping the risk that California and the wider American southwest could see droughts in future that persist a lot longer.

The study looks specifically at “megadroughts”. These are 35-year periods that are at least as dry, on average, as the worst droughts experienced in the 20th century – such as the “Dust Bowl” droughts of midwest America in the 1930s.

The researchers assessed future drought risk using a statistical model to estimate the impacts of temperature and rainfall changes on soil moisture in the southwest. This region includes Arizona, much of California and Nevada, southern Utah and western parts of Colorado and New Mexico.

The Southwestern US states.

The southwestern US states.

The findings show that rising temperature is the predominant factor in increasing the likelihood of a megadrought. Lead author Dr Toby Ault, assistant professor in the Department of Earth and Atmospheric Sciences at Cornell University, explains to Carbon Brief:

“We found that temperature generally ‘wins out’ and drives evaporation rates to higher levels, which in turn makes megadroughts more likely in a warmer climate even if precipitation increases.”

You can see this in the maps below, which show the risk of a megadrought occurring this century under different levels of warming compared to pre-industrial levels: 2C (upper map), 4C (middle) and 6C (lower).

Global emissions are currently tracking just above a scenario that climate models suggest would raise global average temperatures by around 4C or more by 2100.

The left-hand maps indicate how temperature changes the megadrought risk if rainfall amounts stay the same as they are now. The shading indicates the level of risk, from yellows and oranges up to dark brown where the risk reaches 90-100%.

You can see that under 2C, the risk in some southern parts of the region is less than 30%, with around half of the region at greater than 60% likelihood of a megadrought. But the step change to 4C is stark, with all but the southwest corner of the region at a risk of over 90%.

The right-hand maps show how much extra rainfall would be needed to keep megadrought risk below 50% for each temperature increase. The darker the blue shading, the more rainfall that would be needed.

For a 2C warmer world, most of the region would only need “modest” increases in rainfall – less than 10% – to reduce the risk of megadroughts to 50%, the paper says. However, under 4C, this increases to 10-30%, and then up to 40-50% in northern parts of the region with 6C of warming.

The contours on these maps show how much rainfall is actually projected to change by climate models. As you can see, for all but the most northern parts of the region, rainfall is expected to decrease by 5-15%. This means rainfall is unlikely to reduce the risk of megadroughts substantially.

Maps of megadrought risk for the American southwest under different levels of warming (see A to C), and the required increase in rainfall to compensate for that warming (see D to F). Shading in maps A-C shows megadrought risk, from yellow to dark brown. Shading in maps D-F shows rainfall needed to keep megadrought risk to less than 50%, where dark blue indicates the largest increases. The contours on maps D-F shows climate model projections of expected rainfall changes. Source: Ault et al. (2016)

Maps of megadrought risk for the American southwest under different levels of warming (see A to C), and the required increase in rainfall to compensate for that warming (see D to F). Shading in maps A-C shows megadrought risk, from yellow to dark brown. Shading in maps D-F shows rainfall needed to keep megadrought risk to less than 50%, where dark blue indicates the largest increases. The contours on maps D-F shows climate model projections of expected rainfall changes. Source: Ault et al. (2016)

‘Glimmer of hope’

Earlier research has shown that the Earth has experienced megadroughts around once or twice a century during the past millennium. But while these were caused by natural fluctuations in the climate, we’re now adding human-caused climate change into the mix. Ault explains:

“In the past, the ocean is the most likely culprit for driving megadroughts. Persistent ‘La Niña’ patterns, as well as other prolonged sea surface temperature anomalies, are possible culprits. In the future, temperature will play an increasingly important role.”

The findings add to growing evidence that the American southwest is likely to experience worsening droughts under climate change, says Dr Peter Gleick, chief scientist at the global water thinktank, the Pacific Institute, who wasn’t involved in the study. He tells Carbon Brief:

“In particular, rising temperatures will drive changes in water availability beyond what even moderate increases in precipitation can mitigate, and even worse, there may not be any precipitation increases at all.”

This reinforces the importance of efforts to limit global temperature rise, says Ault:

“Future megadrought risk depends strongly on temperature, hence reducing global carbon emissions will limit the total amount of regional temperature change and ultimately lower the risk of megadrought.”

This means there is a ‘glimmer of hope’ to avoid the crippling impacts of a megadrought, says Gleick:

“The risk of these megadroughts can be partly reduced by aggressive efforts to cut greenhouse gases, but we should have begun these efforts years ago, when similar warnings were first sounded.”

Ault, T. R. et al. (2016) Relative impacts of mitigation, temperature, and precipitation on 21st-century megadrought risk in the American Southwest, Science Advances, doi:2016;2: e1600873

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Comments 1 to 10:

  1. It is often a point claimed by some who are uninterested in actually reasoning clearly, that thousands of years ago California and its environs experienced droughts that spanned centuries.   

    Their illogical point is that because the droughts before there was a CO2 issue the CO2 cannot be the cause of droughts.

    That was in the period closest to the holocene optimum.  It was then as warm as it is currently (though we have raised temperatures more suddenly). 

    It is, I always point out, utterly illogical to expect a different result from the same temperatures.    We are returning (briefly, as it appears we are merely passing through them on the way to something much warmer) to the conditions of the climate optimum.    

    There are two questions only.  The first one is how high will it finally go, as it is clear that we're aimed for something significantly in excess of anything our civilization evolved in.  

    The second is the effect of the rate of change.  It is a complex system and we hit it with a step function change of input.  Will it "Ring"?  What would ringing look like? 

    Keep up the good work. 

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  2. Nice comment. Curious, could you explain a bit more the concept of "ringing". That is one I haven't heard before. Thanks!

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  3. Sauerj@2: 'ringing' or Gibbs phenomenon: The result from a step function/pulse on a second (or higher) order system 

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  4. I would argue that the occurrence is normal as well.

    The likelihood of climate change itself having the effect its always had vs. Human influence is illogical. A permanent drought has been effect in Western US since the 1930s, the Grapes of Wrath even addresses this issue with no recourse. The water wars are in full effect and the EPA made it illegal for water conservation to occur. So if water management has been outlawed by Obama, then the only conclusion will be no water for consumption later.

    I have a solution though! While the EPA might regulate human use to exhaust what watersheds might be left, it would be advantageous to farmers and land owners to bring in beavers from Canada to restore the watersheds.

    The beaver has proven its ability in Texas and is the ultimate watershed manager. It can engineer the development of green savannah if allowed to carry out its functions.

    Obama tried really hard to exterminate the US population in a quiet unobvious way. First he stripped people of their rights, then he stripped them of their jobs, then he quietly took away their access to water and food.

    If the watershed in your area is under threat of drying up, you need beavers now to fix it. Beavers, often thought to be a nuisance, a pest, a horrible animal to have near you is actually a clever watershed engineer. They can be effectively communicated with, they can be an effective ally.

    The key is the research conducted in Gatineau Quebec Canada by the Government of Canada.

    The beaver was interfering with run off, road construction, and was deemed a pest. When the lead researcher used tape recorders with tapes playing running water sound, the beaver started to build dams near the sound. The effect was mind blowing. The beaver started to build where the tape recorders were.

    Thus saving the government millions in road construction costs. Plus, the ability to get the beaver to perform vital construction better than any human construct in water was amazing too! Overall, the Canadian watersheds depend on beavers to manage a lot of it. We didn't know until about 5 years ago. Now we rely on beavers to help keep watersheds managed effectively.

    This will work in California and the western US as well. But first, the farmers need to tell the EPA to get lost, collect rain water without penalty, stop exhausting or taxing ground water and ease up on archaic farming principles.

    If all the areas are properly addressed the solution to the problem should fix itself.

    We cannot hope for better outcomes without addressing best practices.

    That includes watershed management, responsible use, responsible legislation, policy and procedure, as well as due diligence.

    To simply blame climate change is a cop out. Its easier to blame stupidity and ignorance and greed. If I take a five hour shower every day, flush the toilet 50 times a day, open a fire hydrant and flood a street every other day, and generally waste water that's on me. But if the EPA says you can't legally collect rainwater, you have to use your water allotments from the watershed or lose them, and make water management impossible, that's on the government.

    This isn't about climate change. The climate always changes. This is about stupidity. If you still think the climate change is at fault, you should also blame the rain spirits for not showing mercy. And the water elves for not bringing moisture and the Eskimos for not bringing ice. Yeah, blame everything else first, shake fingers and further the problem. Or find a trickle of a stream, two inches wide, 1 inch deep, order some beavers, and quietly correct the problem! Within two years you will have your watershed back.

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  5. It is a pity that LaughingMans post was so inflamatory. Because that is in fact one leg of a 3 pronged approach, ecosystem restoration. And none better than keystone and engineer species like beaver to start restoring vast areas and using them as carbon sinks, as well as mitigating drought. Beavers are the stereotypical ecosystem engineer because of the effects their dams have on channel flow, geomorphology, and ecology. The ecological cascade that follows is profound and spreads far beyond the beavers' habitat.

    Unlike LaughingMan, I don't see this as a stand alone solution. But it certainly can be a significant part of the solution.

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  6. bjchip@1: Maybe one form of how Earth will 'Ring' was anticipated in the 1972 film 'Silent Running'.  Bruce Dern plays a biologist who maintains a spaceship filled with the last natural plants from Earth, and responds with rebellion when he's asked to clear them out for cargo.  I don't think many natural ecosystems can respond quickly enough to avoid catastrophe; for them, 'Ringing' brings fracture.  I think humans can respond more rapidly (but probably after much difficulty), and may have to act forcefully to prevent species extinction in a few decades or so.

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  7. LaughingMan@4: Unless the beavers have a wicked 'rain dance', I don't think they're going to be as helpful as you imagine in many places.

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  8. You attention is directed to Bill DeBuy's book "A Great Aridness" for a look at corroborate research on the recurrance of drought in the southwest.  Essentially, he points out that prolonged droughts in that reagon have been occurring about every thousand years since the end of the last Ice Age, the last severe drought beginning around 1050 CE and ending roughly 1200 CE.  Obviouly, its been about 1,000 years since those days.  Of course, the present climate change appears to strongly exacerbate this recurring phenomenon.  He notes that this SW area was essentially depopulated during the anestral megadroughts making the present situation extremely grave for the populations now living in that geography. 

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  9. Swampfoxh @ 8, sounds like an interesting book. Any clues in the book why we would have a regular drought cycle like this?

    I'm curious what would happen every thousand years to trigger this, as its such a long cycle.

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  10. Some time ago, I was encouraged by a poster on Skeptical Science to look at a climate modeling program that was simple enough to run on a P.C., and originated at some university in Australia. I pointed out a couple of glitches in the displays but it looked like an interesting and useful project. If you ran it, a predicted climate change consequence that was imminent enough to strike within my lifetime was the desetification of the American Southwest.

    But I have now forgotten the University and the name of this program. Can anyone remind me?


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