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Sheffield vs. Dai on Drought Changes

Posted on 23 November 2012 by dana1981

A new paper in Nature by Sheffield et al. (2012) has been making waves, because it argues that there has been little change in global drought over the past 60 years, and that the Palmer Drought Severity Index (PDSI), used for example by Dai (2010), overestimates future drought.  Climate Progress has an interesting response, including comments by Trenberth and Dai.  Paleoclimatologist Kevin Anchukaitis also has a very informative post on the subject at Strange Weather, as does John Nielsen-Gammon.  Below is a repost of another very good post on the subject, by Carbon Brief.

Major droughts in the United States and Russia this summer highlighted the risks climate extremes pose to everyday life, and prompted discussion about links between drought and climate change.

Past research supports a link between climate change and drought - suggesting that globally, the area affected by drought has increased in recent decades as global temperatures have risen.

But a new study in Nature challenges this link, suggesting that the relationship might not be that simple, and that older models may have overestimated the change in drought over the last 60 years. In fact, newer models which take a more detailed look at the how droughts occur suggest the world area in drought may not have changed much at all.

We take a look at the new research and why overestimating the past might not affect predictions of how drought may respond to climate change in the future.

What's changed?

The Intergovernmental Panel on Climate Change (IPCC) concluded in its Fourth Assessment Report, published in 2007, that "Globally, the area affected by drought has likely increased since the 1970s".

In its 2012 Special Report on Extreme Events (SREX), the IPCC was more cautious about past trends, concluding "there are large uncertainties regarding global-scale trends in drought".

According to the new study in Nature, those uncertainties are partly down to the way simple drought models simulate water evaporation. Earlier drought models worked out how much water was evaporating based solely on temperatures. But a number of other factors affect evaporation in real life, like wind speed, vegetation cover and aerosols affecting sunlight.

Newer models include more of these factors, giving a more accurate picture of drought. Using the newer models, the study found there's been no significant change in the area affected by drought between 1950 and 2008. This led the researchers to conclude that studies using the earlier models overestimated the global area affected by drought.

Justin Sheffield, lead author on the study, told Carbon Brief:

"Our findings confirm what the SREX report recognized: that at the time of its writing (6 months ago) there was uncertainty about global trends in drought and that the previous IPCC AR4 conclusion [...] was likely overestimated."

Many uncertainties still exist, even with the new models - the meteorological data sets used to calculate levels of drought are incomplete, and the method for using these models differs from one researcher to the next. That makes quantifying change hard, and more studies will be needed to add certainty to these findings.

But this isn't the only research finding little change in droughts over past decades.

Is "little change" new?

Sheffield and his colleagues aren't the first to find little change in the global area affected by droughts over past decades. In 2006, researchers from the Climatic Research Unit (CRU), based at the University of East Anglia, found similar trends even when they used their own versions of the older, simpler models. Their research on European droughts between 1901 and 2002 concluded:

"evidence for widespread and unusual drying in European regions over the last few decades is not supported by the [models]"

Similarly, the CRU researchers found no statistically significant trend in drought trends over North America during the same time period. Using newer models to simulate drought, they've also looked at global drought trends. The findings, yet to be published, suggest drought has not become unusually strong or widespread in recent decades, agreeing with their earlier work.

Although they differ on how much old and new models agree, the research from both CRU and from Sheffield et al. suggest that when averaged out over the globe, drought has changed very little in recent decades. But that's not to say it won't in the future.

Does this affect future predictions of drought?

The finding that some models may have overestimated past drought is unlikely to affect predictions about future droughts. That's because working out how climate change might affect drought in the future is done using different - climate - models. These climate models are more sophisticated and operate in a different way than the simple models this study looked at to analyse past drought.

Lead author Justin Sheffield explained to Carbon Brief:

"[Climate models] respond and feedback with the projected changes in precipitation, temperature, radiative forcing etc. in a more realistic manner."

The authors of the new paper specifically warn that trends identified by the models used to look at past drought shouldn't be extrapolated into the future, not least because the assumptions these models make based on temperature could overestimate future levels of drought.

Climate models however, as the authors explain, are much better at recreating the links between temperature, precipitation and evaporation, so offer a better way to try to estimate future changes. 

What this new study does do is inform the way climate projections are interpreted. At a time when links between climate change and extreme events are under discussion, this research is a reminder that drought is affected by a number of factors. And it's the combination of these factors, rather than simply temperature rise, that will determine how drought changes in the future.

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Comments

Comments 1 to 7:

  1. Perhaps there have been differences in how drought is defined? What is the current definition of drought? Is it x time without y precipitation, over z area?
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  2. Just thet simple amount of precipitation is not the indicative how well the soil has been saturated with moisture. The form/duration is equally important. A sudden heavy and quick storm over bone dry land does not impregnate it at all just wash the surface and may even be detrimental by eroding it. A steady drizzle over few days may saturate deep soil even if its total amount is smaller than the storm would bring.

    A consistent record of drought changes would be done better with a standardised measure of soil moisture. Anyone has been doing it on a large scale consitently enought for the last 60y? I doubt it. I guess chenges in river flows can be used as proxies. Again, you need a long, detailed record. There will be increased floods and bigger droughts in between, depending on ENSO. Naysayers will always argue that "on average, nothing has changed".
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  3. A good source of data, viz. drought.

    USA Drought Information portal.
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  4. Attempting a comment migration, copying relevant comments from another thread.

     

    ubrew12 says - The bizarre truth is that Heartland farmers in Kansas are only now in business thanks to 'Big Government'-backed farm insurance programs that tax the ordinary American to help them through their global-warming-induceddrought.


    Wheres the proof GW caused the drought?

     

    Clyde...  It's a matter of relative chance.  It's possible you'd see such an extraordinary drought without influence from global warming.  But global warming makes it much more likely that such extraordinary doughts will occur.  

    So, you kind of have two choices.  This was an extreme occurence and unlikely to occur again for a long time, or this is a function of human induced changes in theclimate system and more likely to become more frequent, or even normal.

    You choose where you're going to put your money.

     

    DSL at 23:16 PM on 22 February, 2013

    Try Johanson & Fu (2009), Clyde.  Here's the abstract (emphasis mine): 

    "Observations show that the Hadley cell has widened by about 2°–5° since 1979. This widening and the concomitant poleward displacement of the subtropical dry zones may be accompanied by large-scale drying near 30°N and 30°S. Such drying poses a risk to inhabitants of these regions who are accustomed to established rainfall patterns. Simple and comprehensive general circulation models (GCMs) indicate that the Hadley cell may widen in response to global warming, warming of the west Pacific, or polar stratospheric cooling. The combination of these factors may be responsible for the recent observations. But there is no study so far that has compared the observed widening to GCM simulations of twentieth-century climate integrated with historical changes in forcings. Here the Hadley cell widening is assessed in currentGCMs from historical simulations of the twentieth century as well as future climate projections and preindustrial control runs. The authors find that observed widening cannot be explained by natural variability. This observed widening is also significantly larger than in simulations of the twentieth and twenty-first centuries. These results illustrate the need for further investigation into the discrepancy between the observed and simulated widening of the Hadley cell."

    Chris G at 06:41 AM on 23 February, 2013

    ...

    the average bushels of wheat per acre for Texas is 30, Oklahoma 35, Kansas 40, Nebraska 44(?).  Not even counting an increase in extreme heat anddrought events, we are looking at somewhere in the vicinity of a 25% loss in productivity as the Kansas climate becomes more like Texas.  It's mind-boggling how they can not see this as being bad for the local economy.  But, if you ask one of our politicians, Moran for example, he will tell you he does not support a carbon tax because he believes it will hurt the economy.  (Face-palm)

    ...

    The middle of the area growing wheat in Kansas is about the 38th parallel; the middle of the area growing wheat in Texas is about the 34th parallel.  So, if we can expect climate zone shifts coinciding with Hadley cell shifts, then Kansas becomes like Texas at just over 1 degree C of warming, give or take.

    I realize it is not that simple; I'm just looking for a ballpark figure.

    Hmm, since we are approaching 1 degree already, that would mean that the current drought/heat wave is more likely the new normal rather than an exceptional event.  That's a little frightening.

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  5. On that last bit, I was possibly remembering something incorrectly.  I had thought I'd read something to the effect that Hadley Cells could be expected to expand 2-4 degrees of latitude per degree C of warming, but I haven't found that again, and the estimates of expected expansion I did find were substantially lower.  What I found was that Hadley Cells have expanded 5-8 degrees since 1979 (Seidel, et al 2008), but that, while that is greater what is expected with natural variability, that is also more than what is expected, based on GCMs, from greenhouse gas warming alone.

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  6. More related to heat than drought, let's pick a couple of sample cities in wheat-growing regions.

    Wichita Falls, TX

    Wichita, KS

    Note the average number of days above 90 degrees F, and factor in the yield declines observed in this paper, "Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change", and I think it can be safely said that temperature differences between Texas and Kansas play a large role in the differences in average yield.  Note that wheat is typically harvested in June; so, conditions in April, May, and June have an impact on yield, and July-August not so much.

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  7. Another link, "Modelling predicts that heat stress, not drought, will increase vulnerability of wheat in Europe"

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