Climate Change Is Already Worsening Droughts In Many Ways: Nature Gets It Wrong–And Right

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"Climate Change Is Already Worsening Droughts In Many Ways: Nature Gets It Wrong–And Right"

Trenberth slams new Nature article on drought: “The conclusions of the paper are likely wrong.”

A flawed new article in Nature has a title that sums up its controversial conclusion, “Little change in global drought over the past 60 years.”

I generally judge an article at odds with the broad literature in two ways. How well does it cite and respond to the literature? What do the other leading experts in the field say? This new article comes up short in both areas.

Kevin Trenberth, former head of the climate analysis section of the National Center for Atmospheric Research, has sent me a strong critique which is printed below. NCAR’s Aiguo Dai also sent me a critique.

The new article simply ignores or dismisses a considerable amount of the drought literature and focuses instead on one narrow metric of soil moisture. But as I wrote last year in a Comment that reviewed much of the recent literature for Nature, “The Next Dust Bowl” (subs. req’d, full text here), climate change worsens droughts in three synergistic ways:

A basic prediction of climate science is that many parts of the world will experience longer and deeper droughts, thanks to the synergistic effects of drying, warming and the melting of snow and ice.

Precipitation patterns are expected to shift, expanding the dry subtropics. What precipitation there is will probably come in extreme deluges, resulting in runoff rather than drought alleviation. Warming causes greater evaporation and, once the ground is dry, the Sun’s energy goes into baking the soil, leading to a further increase in air temp- erature. That is why, for instance, so many temperature records were set for the United States in the 1930s Dust Bowl; and why, in 2011, drought-stricken Texas saw the hottest summer ever recorded for a US state. Finally, many regions are expected to see earlier snowmelt, so less water will be stored on mountain tops for the summer dry season. Added to natural climatic variation, such as the El Niño–La Niña cycle, these factors will intensify seasonal or decade-long droughts. Although the models don’t all agree on the specifics, the overall drying trends are clear.

There is simply little doubt that many dry areas have gotten drier and/or warmed up and/or seen earlier snowmelt.

I think it bizarre to claim that there is little change in global drought over the past 60 years when there are so many studies and analyses to the contrary directly linking severe droughts to climate change:

The World Bank’s must-read new report, “Turn Down the Heat: Why a 4°C Warmer World Must be Avoided,” gets this right:

One affected region is the Mediterranean, which experienced 10 of the 12 driest winters since 1902 in just the last 20 years (Hoerling et al. 2012). Anthropogenic greenhouse gas and aerosol forcing are key causal factors with respect to the downward winter precipitation trend in the Mediterranean (Hoerling et al. 2012)…. East Africa has experienced a trend towards increased drought frequencies since the 1970s, linked to warmer sea surface temperatures in the Indian-Pacific warm pool (Funk 2012), which are at least partly attributable to greenhouse gas forcing (Gleckler et al. 2012). Furthermore, a preliminary study of the Texas drought event in 2011 concluded that the event was roughly 20 times more likely now than in the 1960s (Rupp, Mote, Massey, Rye, and Allen 2012).

You won’t find any of those studies referenced in the new Nature article. You can find Funk 2012 and Rupp et al 2012 in the The Bulletin of the  American Meteorological Society Special Issue, “Explaining Extreme Events of 2011 from a Climate Perspective.”

I find it is especially surprising that Nature would publish this piece when three months ago its sister publication, Nature Climate Change, published a piece by Dai, “Increasing drought under global warming in observations and model” that is utterly at odds with it. There is simply no way both of these papers can be true — and yet the new Nature piece never discusses the Nature Climate Change piece.

Dai’s paper notes:

Historical records of precipitation, streamflow and drought indices all show increased aridity since 1950 over many land areas….

I conclude that the observed global aridity changes up to 2010 are consistent with model predictions, which suggest severe and widespread droughts in the next 30– 90 years over many land areas resulting from either decreased precipitation and/ or increased evaporation.

The new Nature paper dismisses previous work by Dai, unjustifiably according to Dai and Trenberth, but in any case, Dai’s Nature Climate Change goes much further in reconciling models with observations. Nature shouldn’t have published this new paper without a serious effort first to reconcile these two papers.

I think it unlikely that the new paper will stand up. Here is what Trenberth sent me:

A “drought” in good science

Kevin Trenberth

The new paper recently published in Nature by Sheffield et al “Little change in global drought over the past 60 years” [Nature 491 15 Nov 2012 435-440] has done some impressive work.  But it should not have been published in Nature.  It has 38 pages of dense supplementary material, for heaven’s sake.  Was that reviewed?  Probably not.

The conclusions of the paper are likely wrong. The paper re-examines the Palmer Drought Severity Index (PDSI) in different formulations and how it changes over time.  However, this has been done before in references embedded in the paper:

24. Dai, A. Characteristics and trends in various forms of the Palmer Drought Severity Index during 1900–2008. J. Geophys. Res. 116, D12115 (2011).

25. van der Schrier, G., Jones, P. D. & Briffa, K. R. The sensitivity of the PDSI to the Thornthwaite and Penman–Monteith parameterizations for potential evapotranspiration. J. Geophys. Res. 116, D03106 (2011).

And dismissed with “Recent studies have claimed that there is little difference between the PDSIs that use the Thornthwaite and PM algorithms (PDSI_Th and PDSI_PM, respectively)24,25 but this can be attributed to inconsistencies in the forcing data sets and simulation configuration(see Supplementary Information).”   Yes, but at least some of the supplementary material in this respect is wrong.

The simple PDSI_th relies on temperature to crudely estimate the atmospheric demand for moisture from the soil and thus evapotranspiration.  It can be calibrated to each area and it works reasonably well but it is not perfect.   It is known that actual evaporation depends on solar radiation and other energy sources, as well as surface wind speeds and humidity in addition to temperature.  The problem is these fields are mostly not available in adequate quality, moreover daily fields are required.

The Sheffield et al  Figs. S12-S14 suggest that the “little drying” conclusion is likely due to the use of the CRU precipitation data, which has fewer than 1500 raingauges for the recent years and which differs substantially from the GPCC and GPCP precipitation products that have many more gauge data for the last 10-20 years.  The authors make a big deal of their findings, but in fact van der Schrier et al. have made similar conclusions in their previously published papers using the same CRU data set as the forcing.  There are also major concerns about the reconstruction of the solar radiation data, which depends a lot on how clouds have changed.

Another key point is that while our previous results with PDSI have been compared with other related but independent records, such as soil moisture, streamflow, and GRACE satellite data, Sheffield et al. made only a detailed comparison of various forcing data for the PDSI calculations.

Another important factor not considered, is that precipitation on land is controlled to a large degree by ENSO: in general with La Niña, as experienced in recent years, there is more rain on land and so the past 2 years have been the wettest on record.  That says nothing about whether the extent and intensity of drought is greater or not when it occurs, and so the ENSO signal should be removed before looking at trends associated with climate change.

Sheffield et al have uncovered some minor problems with Dai (2011) but none that explain the differences in the results.  It is evident from all this that there are major issues with the “forcing” data for the more complex form of PDSI, and thus there remains a lot of merit in the simpler but self calibrated version of PDSI_th.

The bottom line is that climate change adds extra heat to the system and that much of heat goes into drying. A natural drought for whatever reason sets in quicker, becomes more intense, and possibly is longer lasting and more extensive as a result.

Dai sent me a long explanation of a major flaw in Sheffield et al, which I’ll summarize as “they normalized away the recent drying.” I think Nature needs to reconcile these papers, but until then, one should probably stick with the large and growing literature on the large and growing rise in aridity over many parts of the world.

Finally, this analysis does not undercut the large and growing literature on the threat to future generations of Dust-Bowlification — see “We’re Already Topping Dust Bowl Temperatures — Imagine What’ll Happen If We Fail To Stop 10°F Warming.”

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21 Responses to Climate Change Is Already Worsening Droughts In Many Ways: Nature Gets It Wrong–And Right

  1. Jim says:

    I’ve been waiting for you to address this paper. Thank you.

  2. BillD says:

    For a high profile journal such as Nature one migh expect the editors to chose at least one reviewer who is likely to be critical of the main conclusions. Maybe the problem in this case is that Sheffield et al. did not adequately cite the recently published studies that reached different conclusions.

    Does the new paper acknowledge the increased aridity in some regions? Or do they simply look at the average precipitation over the globe?

  3. Mike Roddy says:

    We are not privy to the reasons behind this terrible article, but usually the culprit turns out to be ownership.

    The McMillan Group, part of an even larger Europe based conglomerate, owns both Nature and Scientific American, which have both been rather weak on global warming coverage for years. Any time you have large circulation magazines that depend on advertising, subtly pro fossil fuel articles like this one tend to appear. They won’t embarrass themselves by accepting a piece from someone like Michaels or Christy, but will perform more subtle subterfuges of the truth. We have seen a lot of this lately.

    • Mulga Mumblebrain says:

      Precisely, Mike. You pre-empted my contribution, and far more succinctly. In a capitalist society the Boss calls the tune, and, if you have qualms, well you can join the swelling ranks of the ‘losers’. As I have oft pontificated, the owners of society are simply not, not ever, going to allow the ecological catastrophe be addressed, because they fear that to do so would undermine their wealth and power. They are also motivated, (and their propaganda hacks in the MSM exhibit this tendency with malice), by an intense hatred of environmentalists and any other type who put anything before the pursuit of money and power.

      • David Goldstein says:

        oh c’mon, Mulga, it’s Thanksgiving time!..give us a cheerful declaration just this once!

        • Artful Dodger says:

          That WAS his ‘cheerful’, David. You should wait for his ‘realist’. Because that description explains behavior far better than the restraint shown here. No one has said ‘liar’ yet.

        • Dennis Tomlinson says:

          Thanksgiving holds no sway with an Aussie.

          • Merrelyn Emery says:

            Quite right! Can’t think of any part of our forcible transportation and inhuman incarceration we should give thanks for, except perhaps for our enduring anti-authoritarianism, ME

        • Mulga Mumblebrain says:

          Sorry, David- I call it as I have long seen it. Please believe me-I would give anything to be proved to be just a pessimistic curmudgeon, but I weigh the evidence in my mind every day, and I just cannot see any way out. If only there was leadership and clear-sightedness and determination as during WW2, then I’d be much happier and even, tentatively, optimistic. But there is just the opposite. Enjoy your turkey!

    • Joan Savage says:

      The McMillan Group also owns “Nature Climate Change” which published the Dai article mentioned by Joe Romm.

      McMillan and Wiley-Interscience – and perhaps other publishing houses – are homes to an astounding array of the categories of science.

      In this instance, I’d stick with an interest in the editors and the expertise or lack thereor of the peer reviewers.

  4. Rob Painting says:

    Mike Roddy – No need to go down the conspiracy path, that’s the kind of thinking one would expect at WUWT when reality doesn’t go their way (which is regularly). There’s actually sound scientific basis for the recent Nature paper.

    1. Global warming does not equal global drying. One of the co-authors of the paper, Michael Roderick, has been hammering on about this point for some time now. In other words, warmer air temperatures do not necessarily dry out soils. The 3 main factors are: vapour pressure deficit, surface solar radiation, and wind speed.

    2. Greenhouse gases heat the ocean. They accomplish this by lowering the thermal gradient in the cool skin layer of the surface ocean. The surface is heated by shortwave radiation (sunlight) and, by lowering the thermal gradient, heat loss from the (typically) warmer ocean to the cooler atmosphere is diminished. Sum effect is that the oceans warm.

    3. A warmer surface ocean drives greater evaporation rates, and the oceans supply 86% of global evaporation. This increase has been confirmed observationally – the intensifying patterns of salinity and freshening in the ocean demonstrate this. See Durack (2012) – Ocean Salinities Reveal Strong Global Water Cycle Intensification During 1950 to 2000.

    4. This indicates that evaporation, and therefore precipitation has increased globally (sadly global rainfall measurements are of poor quality, so can’t be of much help in this regard). Atmospheric warming enhances the moisture holding capacity of the air, by by way.

    5. The Earth has experienced prolonged periods, in the last half-century(ish), where the amount of surface solar radiation (SSR), or sunlight, has seen marked dimming and brightening periods. This is caused by aerosols, and cloud fraction, changes over time, which control the amount of sunlight reaching the Earth’s surface. From the mid 20th century there was a long period of SSR dimming, switching to brightening in the 1980′s and dimming again from 2001 onwards. These SSR trends are very localised, but globally they would have contributed to a decline in evaporation, counter-acting ,to some extent, the ocean warming driven evaporation.

    So, when you consider all these variables, and a complete lack of quantification of the contribution of each, it’s not really that surprising that the areal extent of drought hasn’t changed much in the last 60 years.

    But does that mean current, and future, drought is no concern? No. The poleward expansion of the subtropical dry zones will afflict agricultural areas in the United States and Australia. The warmer the Earth gets the more devastating drought will get, because without evaporative cooling provided by sufficient soil moisture, more of the sun’s energy will go into heating the already warmer air – creating more powerful heatwaves.

    Another consideration, is the precipitation/drought intensification provided by La Nina and El Nino. The greater moisture holding capacity of the air, fed into a largely unchanging large-scale atmospheric circulation, will/has created greater precipitation anomalies. The whole wet gets wetter, dry gets drier thing. Regions typically deprived of rainfall during El Nino will see more intense drought, and those will see more rainfall during La Nina, will see greater flooding (in general that is).

    This is going to prove very problematic for agricultural production, even if the area of Earth’s surface covered by drought doesn’t change that much.

    • You know what, Rob?

      I think I just take Trenberth’s word for it.

      Oh, and I was in one of the hottest places in the U.S. June through September, and all the corn and soybeans died, and now the trees are dying like flies.

  5. catman306 says:

    So does this mean that the McMillan Group is ‘fair and balanced’?

  6. Aaron Lewis says:

    What has not changed is that an army of tractors and plows can affect the climate of the world. South America is being plowed now. They have a different kind of soil and the resulting problems will be much longer lasting (Laterite soils).

    In the Dust Bowl, they stopped plowing and within a few years the weather started to return to normal.

    The real difference between now, and the Dust Bowl is that they could expect things to return to normal, and in a time of global warming, we can only expect tomorrow to be warmer.

    • Mulga Mumblebrain says:

      I’ve seen pictures of swales dug by work-teams during the Depression, that capture rainwater, prevent its run-off and allow it to percolate down into the soil, that still work today, though long abandoned. The vegetation around them is, of course, richer and more varied than that even a few metres away. Simple techniques, widely applied, could work miracles. Stubble retention as you observe, is another easy gain.

  7. MapleLeaf says:

    Hi Joe,

    Good to hear that Tremberth and Dai are on this. I know from first-hand experience that the GPCC product is the way to go in this case. The CRU precip. data are coarse and do not agree well at all with other gridded products. There are much superior and long-term rainfall products out there, such as the GPCC.

  8. David says:

    Thanks for this. This is the only place to get important information like this.

  9. Joan Savage says:

    Despite the way the Nature article was so immediately criticized, its authors point out that the past 60 years is not a predictor for the future. Their work may prove useful in expediting a move away from dependence on the temperature-based PDSI and towards the more multifactorate aspects of drought mentioned by Trenberth and Dai.

    In the recent National Research Council Report, “Climate and Social Stress: Implications for Security Analysis” Steinbruner et al. also point to the limitations of the PDSI for assessment of critical drought conditions world-wide.

    • Joan Savage says:

      I oversimplified the first sentence. The authors clearly expect to see evidence of climate change.

      They wrote in their abstract:
      “The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years9, 10.”