From Cornell University and the CMIP modeling jockeys comes this claim:

ITHACA, N.Y. – As a consequence of a warming Earth, the risk of a megadrought – one that lasts more than 35 years – in the American Southwest likely will rise from a low chance over the past thousand years to a 20- to 50-percent chance in this century. However, by slashing greenhouse gas emissions, these risks are nearly cut in half, according to a Cornell-led study in Science Advances, Oct. 5.

“Megadroughts are rare events, occurring only once or twice each millennium. In earlier work, we showed that climate change boosts the chances of a megadrought, but in this paper we investigated how cutting fossil fuel emissions reduces this risk,” said lead author Toby Ault, Cornell professor of earth and atmospheric science.

If climate change goes unabated – and causes more than a 2-degree Celsius rise in atmospheric temperature – megadroughts will become very probable, Ault said.

“The increase in risk is not due to any particular change in the dynamic circulation of the atmosphere,” Ault said. “It’s because the projected increase in atmospheric demand for moisture from the land surface will shift the soil moisture balance. If this happens, megadroughts will be far more likely for the next millennium.”

Ault explained a natural “tug-of-war” governing the surface moisture balance between the precipitation supply (rain) and evaporation (transpiration). But he cautions that increases in average regional temperatures could be so dramatic – more than 4 degrees Celsius (7.2 degrees Fahrenheit) – that evaporation wins out. This, in turn, dries out the land surface and makes megadroughts 70- to 99-percent likely.

“We found that megadrought risk depends strongly on temperature, which is somewhat good news,” Ault said. “This means that an aggressive strategy for cutting greenhouse gas emissions could keep regional temperature changes from going beyond about 2 degrees Celsius (3.6 degrees Fahrenheit).”

This lower average warming figure cuts the megadrought risk almost in half, he said.

These tug-of-war scenarios could very well play out in the American Southwest, according to tree ring and geologic records. During sequences of exceptionally dry years, those rings tend to be relatively narrower than in wet years, he said.

“Tree rings from the American Southwest provides evidence of megadroughts, as there are multiple decades when growth is suppressed by dry conditions,” Ault said, pointing to several megadroughts that occurred in North America between 1300 and 1100 B.C.

“We also know they have occurred in other parts of the world, and they have been linked to the demise of several pre-industrial civilizations,” he said.

The tug of war between moisture supply and demand might play out differently in other parts of the world, Ault said.

“Nonetheless, even in the Southwest we found examples of plausible 21st-century climates where precipitation increases, but megadroughts still become more likely,” said Ault, who noted the normally verdant Northeast is in the middle of drought. “This should serve as a cautionary note for areas like the Northeast expecting to see a more-average moisture supply.

“Megadrought risks are still likely to be higher in the future than they were in the past,” he said. “Hence, efficient use of water resources in the drought-stricken American Southwest are likely to help that region thrive during a changing climate.”

“I wouldn’t ever bet against our ability to, under pressure, come up with solutions and ideas for surmounting these challenges,” said co-author Jason Smerdon of Columbia University’s Lamont-Doherty Earth Observatory,” but the sooner we take this seriously and start planning for it, the more options we will have and the fewer serious risks we’ll face.”

###

On the paper, “Relative Impacts of Mitigation, Temperature, and Precipitation on 21st-Century Megadrought Risk in the American Southwest,” Ault is joined by Justin S. Mankin and Benjamin Cook, both of the NASA Goddard Institute for Space Studies, and Smerdon. The National Science Foundation supported this research.

To their credit, they do make the paper available, rather than just expect us to believe the press release.

Abstract Megadroughts are comparable in severity to the worst droughts of the 20th century, but of much longer duration. A megadrought in the American Southwest would impose unprecedented stress on the limited water resources of the area, making it critical to evaluate future risks under different climate change mitigation scenarios, as well as for different aspects of regional hydroclimate. We find changes in the mean hydroclimate state, rather than its variability, determine megadrought risk in the American Southwest. Estimates of megadrought probabilities based on precipitation alone tend to underestimate risk. Furthermore, business-as-usual emissions of greenhouse gases will drive regional warming and drying, regardless of large precipitation uncertainties. We find regional temperature increases alone push megadrought risk above 70%, 90%, or 99% by the end of the century, even if precipitation increases moderately, does not change, or decreases, respectively. While each possibility is supported by some climate model simulations, the latter is the most common outcome for the American Southwest in Climate Model Intercomparison 5 (CMIP5) generation models. An aggressive reduction in global greenhouse gas emissions cuts megadrought risks nearly in half.

Gosh, if only we could take “agressive” action on GHG’s. The Paper is here: https://cornell.app.box.com/v/megadrought-2016/1/11542178339/96890226850/1

They also provide a video, explaining/pushing the idea…but it was 500 mb in size, on a slow server and I don’t have 45 minutes to wait around to upload to WUWT. You can watch it here if you want:

https://cornell.app.box.com/v/megadrought-2016/1/11540712875

But, megadroughts have been a common feature of the southwest, long before “climate change/global warming” was a glimmer in Jim Hansen’s eye:

For example: Roman Period “megadrought” found in the USA southwest

Certainly, California has experienced megadroughts before. We covered a study that showed they were quite common:

The paper:

North American drought: Reconstructions, causes, and consequences, Cook et al. 2007

PDF here: NADrought

Figure 10 is the source of the above graph:

Fig. 10. Long-term aridity changes in the West (A) as measured by the percent area affected by drought (PDSIb−1) each year (B) (redrawn from Cook et al., 2004). The four most significant ( pb0.05) dry and wet epochs since AD 800 are indicated by arrows. The 20th century, up through 2003, is highlighted by the yellow box. The average drought area during that time, and that for the AD 900–1300 interval, are indicated by the thick blue and red lines, respectively. The difference between these two means is highly significant ( pb0.001).

But the most important statement in their study is this one:

Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century…

That’s very true, we just don’t know. Some people claim we’ll see more precipitation and we’ll be in a more El Niño like state rather than La Niña. Just a few months ago we heard this in a story we covered:

Global warming will increase rainfall in some of the world’s driest areas over land, with not only the wet getting wetter but the dry getting wetter as well.

I’ll believe somewhat iffy paleoclimate records over models (especially the poorly skilled CMIP models) any day.

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