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Nature paper: Global droughts unchanged in 60 years

Posted By Joanne Nova On November 16, 2012 @ 3:27 am In Global Warming | 76 Comments

How many images have we seen of drought-stricken cracked land, or been told this is the future? How many headlines have suggested that global warming causes droughts?

Since the end of World War II humans have produced some 85% of all their CO2 emissions, but here is a new study showing that for all those emissions, and for all that warming, droughts back then were just as bad globally as they are today.

Essentially, researchers thought that the Palmer Drought Severity Index (PDSI) was the way to measure global drought levels, and they thought that warming would increase global drought conditions. But the PDSI considers only temperature, not humidity, sunlight and wind. This paper shows that when these factors are included, worldwide drought is about the same now as it was in 1950.

Researchers are finally accounting for the fact that a warmer world usually means more evaporation (especially from the ocean) and thus more rain. It’s good to see that someone has crunched those complex numbers on a global scale. Credit to Sheffield, Wood & Roderick.

global droughts

Figure 1 | Global average time series of the PDSI and area in drought. a, PDSI_Th (blue line) and PDSI_PM (red line). b, Area in drought (PDSI ,23.0) for the PDSI_Th (blue line) and PDSI_PM (red line). The shading
represents the range derived fromuncertainties in precipitation (PDSI_Th and PDSI_PM) and net radiation (PDSI_PM only). Uncertainty in precipitation is estimated by forcing the PDSI_Th and PDSI_PM by four alternative global precipitation data sets. Uncertainty from net radiation is estimated by forcing
the PDSI_PM with a hybrid empirical–satellite data set31 and an empirical estimate. The other near-surface meteorological data are from a hybrid reanalysis–observational data set(31). The thick lines are the mean values of the different PDSI data sets. The time series are averaged over global land areas excluding Greenland, Antarctica and desert regions with a mean annual precipitation of less than 0.5mm d-1.

 

The paper notes AR4 was wrong about this too:

“The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) summarized the evidence in the following terms: ‘‘More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Increased drying linked with higher temperatures and decreased
precipitation has contributed to changes in drought’’.

Figure 2 | Trends in the PDSI and PE. a, c, e,Non-parametric trends for 1950–2008 in annual average PDSI (averaged over the results using the four precipitation data sets and, for the PDSI_PM, also over the two net radiation data sets) fromthe PDSI_Th (a)andthePDSI_PM(c), and their difference (e).b,d, f,Non-parametric
trends for 1950–2008 in annual average PE from the Thornthwaite equation (b) and the PM equations (d), and their difference (f). Values are not shown for Greenland, Antarctica and desert regions with amean annual precipitation of less than 0.5mmd21. Statistically significant trends at the 95% level are indicated by  hatching. The difference in trends in e and f and its statistical significance are calculated from the time series of differences between the two data sets.

It’s good to see this being reported on The Conversation, ScienceNews, and NewScientist. Naturally, this dangerous information could be misinterpreted (unlike most previous drought studies eh?) so caveats are rampant on The Conversation. The caveats take the usual meaningless and vague catch-all approach:” this paper should not be misconstrued as evidence that climate change is not happening” type of warning. Where were these caveat-writers when all the photos of cracked plains were showing on the evening news?

Now we find that “Drought has not been an effective way of measuring climate change over the past 60 years,” he said. [Michael Roderick, The Conversation]

Perhaps that’s because things were a bit circular in drought science?

Roger Pielke Jr of the University of Colorado in Boulder says that since the PDSI uses a formula that assumes higher temperatures cause more droughts, it was hardly surprising that it finds a link.       [ NewScientist]

Kevin Trenberth doesn’t think this new method is right:

Simon Brown of the UK Met Office in Exeter says Sheffield’s analysis is probably right. “There has been a growing acknowledgement that the PDSI should not be trusted when doing climate change studies,” he says. But one of the lead authors of parts of the 2007 IPCC report, Kevin Trenberth of the US National Center for Atmospheric Research in Boulder, is sceptical. He backs work by Aiguo Dai of the State University of New York, Albany, who reported last year that using the Penman-Monteith equation “only slightly reduces the drying trend”.  [ NewScientist]

There are other scientists who are not convinced either:

The finding comes in stark opposition to the results of several recent studies. “It presented a somewhat different view of the drying trend for the last 60 years,” says Aiguo Dai, an atmospheric scientist at the State University of New York at Albany, whose own research suggests that the two equations yield very little difference in drought estimates. Dai says the new study fails to consider trends in soil moisture and other variables. He also claims that the new study relies on outdated weather records and questionable radiation data. However, Sheffield and colleagues attribute the disagreement to inconsistencies in the weather data used by Dai and others.[ScienceNews,]

But if it’s right, the new results may have wider implications:

Sheffield’s findings raise important questions, says Steve Running at the University of Montana in Missoula. “If global drought is not increasing, if warmer temperatures are accompanied by more rainfall and lower evaporation rates, then a warmer wetter world would [mean] a more benign climate.” [ NewScientist]

Actually Fred Pearce at NewScientist has done a respectable job of canvassing opinions from all sides. It’s good to see.

If the paper stands up to scrutiny lets hope the information reaches a wider crowd. If they are right there is 20 years of propaganda to undo.

Little change in global drought over the past 60 years
Justin Sheffield1, Eric F.Wood1 & Michael L. Roderick2

ABSTRACT

Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming1–3. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. In particular, calculations of the Palmer Drought Severity Index (PDSI) show a decrease in moisture globally since the 1970s with a commensurate increase in the area in drought that is attributed, in part, to global warming4,5. The simplicity of the PDSI, which is calculated from a simple water-balance model forced by monthly precipitation and temperature data, makes it an attractive tool in large-scale drought assessments, but may give biased results in the context of climate change6. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation7 that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles8 that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. 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.

REFERENCE

Sheffield, Wood & Roderick (2012) Little change in global drought over the past 60 years, Letter Nature, vol 491, 437

H/t John Coochey, Willie Soon.

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