This is about the paper of the same name by A Dai, available from http://www.cgd.ucar.edu/cas/adai/papers/Dai-drought_WIRES2010.pdf.
To keep things in one place, here is the abstract:
This article reviews recent literature on drought of the last millennium, followed by an update on global aridity changes from 1950 to 2008. Projected future aridity is presented based on recent studies and our analysis of model simulations. Dry periods lasting for years to decades have occurred many times during the last millennium over, for example, North America, West Africa, and East Asia. These droughts were likely triggered by anomalous tropical sea surface temperatures (SSTs), with La Ni ˜na-like SST anomalies leading to drought in North America, and El-Ni ˜no-like SSTs causing drought in East China. Over Africa, the southward shift of the warmest SSTs in the Atlantic and warming in the Indian Ocean are responsible for the recent Sahel droughts. Local feedbacks may enhance and prolong drought. Global aridity has increased substantially since the 1970s due to recent drying over Africa, southern Europe, East and South Asia,
and eastern Australia. Although El Ni ˜no-Southern Oscillation (ENSO), tropical Atlantic SSTs, and Asian monsoons have played a large role in the recent drying, recent warming has increased atmospheric moisture demand and likely altered atmospheric circulation patterns, both contributing to the drying. Climate models project increased aridity in the 21st century over most of Africa, southern Europe and theMiddle East, most of the Americas, Australia, and Southeast Asia. Regions like the United States have avoided prolonged droughts during the last 50 years due to natural climate variations, but might see persistent droughts in the next 20-50 years. Future efforts to predict drought will depend on models’ ability to predict tropical SSTs.
As I’ve said elsewhere – I’m sure I have, errm hold on a moment, oh well can’t find a good example never mind – the most severe problems from GW are likely to be ecological impacts, and a good way of getting such impacts is to have a nice big drought.
Anyway, on to the paper. Here is probably the most important figure, fig 10:
what’s wrong with this pic? Mostly, it is an unthinking average of all the IPCC models. Don’t do that! Some of them are poor. Also, look closely at the stippling: those areas stippled are where 80% or more of the models agree on the *sign* of the change. Not on the magnitude, just the sign. One could make a fair argument for asserting that outside those regions (or some other defined threshold) the results should be ignored, as too uncertain.
This is followed by fig 11, which is the actual PDSI for various periods:
Those are the most important pix, assuming that what you care about are the projections for the future. And since I got this from CP (via SB, if I recall correctly) that *is* what people care about. CP are basing their article on the UCAR press release I think.
Before I go any further, I suppose I’d better confess that for a variety of reasons, some reasonable and some less so, I don’t much like the paper. I’ll discuss some of them (for example, there is the almost-complete-waste-of-time EOF analysis around fig 6; everyone does an EOF analysis nowadays, it is fashionable).
Notice that CP, and UCAR, have replaced “PDSI” (Palmer Drought Severity Index) with “condition” in their pic, though the press release does at least tell you they have done so. I don’t know exactly what this variable is; some combination of Precip, Potential Evap, and maybe other stuff: Dai says “The PDSI is the most prominent index of meteorological drought used in the United States… It incorporates antecedent and current moisture supply (precipitation) and demand (PE) into a hydrological accounting system. Although the PDSI is a standardized measure, ranging from about −10 (dry) to +10 (wet), of the surface moisture condition that allows comparisons across space and time, the normal climate conditions tend to yield more severe PDSI in the Great Plains than other US regions.” Yet UCAR says “A reading of -4 or below is considered extreme drought”: yet the pix show *all* of France and Spain with PDSI below 10 in 2090 which (if I believed it, which I don’t) would mean mega-severe drought for those regions. Quite a bit of the paper is trying to decide what drought index to use, or perhaps trying to justify their choice; or to try to show that various different produce the same answers. But there is clearly room for a great deal of uncertainty.
There is a lot of room for wondering if fig11 means anything: as Dai says We emphasize that quantitative interpretation of the PDSI values shown in Figure 11 requires caution because many of the PDSI values, which are calibrated to the 1950-1979 model climate, are well out of the range for the current climate, based on which the PDSI was designed. Nevertheless [we'll interpret it anyway]. Not knowing much about PDSI I really don’t know how to evaluate this: using the index well outside its intended range might be meaningful; or it might not.
What *does* make sense about the paper? Mostly, fig 10 (despite my harsh words earlier) panel a: in a warmer world, the ITCZ (inter tropical convergence zone, on the equator as shown in the pic if not actually in reality, cos it moves with season, probably an argument for not using annual averages) gets stronger (hence more ppn) but so does the descending branch of the Hadley cell (and maybe broadens too) hence the drying in the sub-tropics (descending air is dry, of course). Naturally, none of that is new; see for example IPCC AR4 fig 10.6. But WTF is the massive decrease in evap just S of Greenland? That is sea / sea ice, so can only come from cooling. Do 80+% of models agree that just S of Greenalnd will cool? That seems very very odd. IPCC AR4 fig 10.8 shows a warming minimum there (probably cos it is a deep convection region).
* Floods in Queensland by Stefan Rahmstorf (chrome does a good translate into perfectly readable English)
* The Wild Weather of 2010 Trenberth, Masters and Cullen (but I really don’t believe ” that the climate is beginning to grow unstable”)