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 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:
- Aiguo Dai of the National Center for Atmospheric Research, “Drought under global warming: a review” (2010)
- Michael Wehner et al., “Projections of Future Drought in the Continental United States and Mexico” (2011)
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:
- In 2007, Science (subs. req’d) published research that “predicted a permanent drought by 2050 throughout the Southwest” — levels of aridity comparable to the 1930s Dust Bowl would stretch from Kansas to California. And they were also only looking at a 720 ppm case.
- In December 2008, the Bush Administration quietly released a US Geological Survey stunner: SW faces “permanent drying” by 2050, which found:
The serious hydrological changes and impacts known to have occurred in both historic and prehistoric times over North America reflect large-scale changes in the climate system that can develop in a matter of years and, in the case of the more severe past megadroughts, persist for decades. Such hydrological changes fit the definition of abrupt change because they occur faster than the time scales needed for human and natural systems to adapt, leading to substantial disruptions in those systems. In the Southwest, for example, the models project a permanent drying by the mid-21st century that reaches the level of aridity seen in historical droughts, and a quarter of the projections may reach this level of aridity much earlier.
- NOAA: Climate change “largely irreversible for 1000 years,” with permanent Dust Bowls in Southwest and around the globe. This January 2009 PNAS paper finds
… the climate change that is taking place because of increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop…. Among illustrative irreversible impacts that should be expected if atmospheric carbon dioxide concentrations increase from current levels near 385 parts per million by volume (ppmv) to a peak of 450-600 ppmv over the coming century are irreversible dry-season rainfall reductions in several regions comparable to those of the “dust bowl” era
So again, the ‘debate’ such as it is, is how far into the northern US Great Plains and Midwest these Dust Bowl conditions will extend — and that’s without even considering the impact of the increasingly early loss of the winter snowpack, which most of these studies don’t even model. Since the recent literature suggests the droughts will extend that far, Hansen’s warning is justified by the literature.
And Hansen’s use of the phrase “semi-permanent” is fully warranted. Given that the drought conditions just keep getting worse and worse as long as we keep warming – and are “largely irreversible for 1000 years” (according to a NOAA-led paper), “semi-permanent” seems like a rather mild word.
Bottom Line: Given how catastrophic it would be to the nation and the world if our breadbasket were indeed hit by these conditions, Hansen’s warning seems fully justified and Hoerling’s response does not.
Finally, it’s always worth repeating that much of human behavior and government policy is driven by the desire to avoid it worst-case scenarios, which is why we have fire insurance and catastrophic health insurance — and a military budget equal to that of the next 16 countries combined.
If we look at the plausible worst case for climate, we get both continuing high levels of emissions and high carbon-cycle feedbacks. That possibility was discussed in a Royal Society Special Issue on a 7°F (4°C) World, which notes “In such a 4°C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world.”
This would be the worst-case for the 2060s, but is in any case, close to business as usual for 2090s:
This is 13-18°F over most of U.S. and 27°F in the Arctic. The drought conditions that would be created by such warming over most of the central and western U.S. are beyond imagining.
And there is every reason to believe that the earth would just keep getting hotter and hotter:
- Science stunner — On our current emissions path, CO2 levels in 2100 will hit levels last seen when the Earth was 29°F (16°C) hotter: Paleoclimate data suggests CO2 “may have at least twice the effect on global temperatures than currently projected by computer models”
Indeed, Steve Easterbrook’s post “A first glimpse at model results for the next IPCC assessment” shows that for the scenario where there is 9°F warming by 2100, you get another 7°F warming by 2300. Of course, folks that aren’t motivated to avoid the civilization-destroying 9°F by 2100 won’t be moved by whatever happens after that.
So if folks want to quibble about whether the semi-permanent Dust Bowl that the U.S. Southwest is headed to by mid-century might not spread to the northern U.S. Great Plains for, say, another few decades after that, well, I must say they are rearranging deck chairs on the Titanic.
How many major scientific articles have to be published before people realized that on our current emissions path we are simply headed towards self-destruction of modern civilization, where feeding 9 billion people will be exceedingly problematic to say the least?
- Climate Story of the Year: Warming-Driven Drought and Extreme Weather Emerge as Key Threat to Global Food Security
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