The response by NOAA’s Martin Hoerling to James Hansen’s recent op-ed does not reflect the scientific literature.
I’m traveling, so let me focus first on Hoerling’s incorrect statements — posted on this blog and DotEarth — about drought. As readers know, the journal Nature asked me to write a Comment piece on the threat posed by drought after they read one of my posts examining the latest science on prolonged drought and “Dust-Bowlification.”
The Nature article, which is basically a review of recent drought literature, is here (subs. req’d). Most of the text is here.
The research I did for that article — along with the comments of the expert reviewers I sent it to — is why I know Hoerling is quite wrong. Hoerling begins by quoting Hansen’s recent New York Times Op-Ed piece:
“Over the next several decades, the Western United States and the semi-arid region from North Dakota to Texas will develop semi-permanent drought, with rain, when it does come, occurring in extreme events with heavy flooding. Economic losses would be incalculable. More and more of the Midwest would be a dust bowl. California’s Central Valley could no longer be irrigated. Food prices would rise to unprecedented levels.”
Hoerling then asserts:
He doesn’t define “several decades,” but a reasonable assumption is that he refers to a period from today through mid-century. I am unaware of any projection for “semi-permanent” drought in this time frame over the expansive region of the Central Great Plains. He implies the drought will be due to a lack of rain (except for the brief, and ineffective downpours)….
But facts should, and do, matter to some. The vision of a Midwest Dustbowl is a scary one, and the author appears intent to instill fear rather than reason.
That’s a very serious attack on Hansen — if it were true. But it isn’t, and it should be retracted.
The fact is that the recent literature examining warming-driven drought in America could not be clearer in warning about a “semi-permanent” (or worse) drought in both the South West and the Central Great Plains and “More and more of the Midwest.” Here are two studies that lay things out starkly:
I would also add the 2010, Environmental Research Letters article “Characterizing changes in drought risk for the United States from climate change.”
And that’s not even counting the Journal of Geophysical Research study that Hansen himself co-authored in 1990, “Potential evapotranspiration and the likelihood of future drought,” which projected that severe to extreme drought in the United States, then occurring every 20 years or so, could become an every-other-year phenomenon by mid-century.
As an important aside, contrary to what Hoerling states, Hansen was not implying the drought will be due to lack of rain (by itself). Everyone seriously writing about warming-driven drought knows we are talking about a combination of factors, ones that I laid out in my Nature article:
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 temperature. 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.
Obviously, since Hansen coauthored an article titled, “Potential evapotranspiration and the likelihood of future drought,” we know he understands the drought conditions are driven by more than precipitation changes. The whole point of that 1990 paper was to examine the impact of warming-driven evaporation on soil moisture and drought.
It is quite surprising that Hoerling doesn’t appear to know the drought literature given that, as Revkin notes, he “runs an effort by the National Oceanic and Atmospheric Administration to assess the forces contributing to extreme weather events!”
Hoerling says it is reasonable to assume Hansen means “a period from today through mid-century.” Hansen says the “semi-permanent drought” will develop “over the next several decades.” That would clearly seem to mean that these conditions will evolve by just after mid-century, the 2050s and 2060s. This is also the first period of time where aggressive action to reduce emissions today could substantially change the projected climate.
Dai’s analysis does indeed project drought conditions over the Great Plains and Midwest. He is in the process of revising his analysis, but the figure below (which had been his 2030s projection in his original version) is a rough representation of where his analysis projects things will be in Hansen’s time frame for the U.S.
The PDSI [Palmer Drought Severity Index] in the Great Plains during the Dust Bowl apparently spiked very briefly to -6, but otherwise rarely exceeded -3 for the decade (see here).
And this isn’t just Dai’s finding. Michael Wehner et al. find the drying has the same signature. The study is behind a firewall, but you can see a PDF of a PowerPoint presentation here.
Of course, just because several models project this future doesn’t make it a certainty. As I note in the article, “drought models need to be improved. They successfully chart the hydrological changes seen in the US Southwest and the drying seen at the global level7, but regional predictions can be disturbingly variable.”
On the other hand, these models most certainly are not the worst-case scenario. Dai is modeling A1B (720 ppm), whereas we are on track for worse than that. A plausible worst-case scenario is here (and below): Royal Society Special Issue on Global Warming Details ‘Hellish Vision’ of 7°F (4°C) World — Which We May Face in the 2060s!
Hansen’s use of the term “Dust Bowl” is justified since that is the term widely used in the drought literature (see below). We are talking conditions that become as bad as the original Dust Bowl by mid-century and then get much, much worse for a long, long time. The Nature editors made repeated use of the term “Dust-Bowlification,” and I was particularly delighted that one of the leading experts in the field that I sent the piece to, Jonathan Overpeck, also liked the term.
Indeed, Hoerling’s critique is really only about whether the semi-permanent drought conditions will extend outside the U.S. SW to include most of Northern U.S. Great Plains. The literature is very clear that the Southwest is very likely headed for Dust Bowl conditions: