21st century megadrought in the West

A new climatic projection for the western states and northern Mexico looks at drought over the past 1200 years and out to 2100. A graph from the study at Nature Geoscience tells the story best, as usual:

Megadroughtwestus
Here's the caption, slightly edited: 

Reconstructed summer PDSI [Palmer Drought Severity Index] from 800 to 2006, five-year mean. Black rectangles show Stine no. 1 and Stine no. 2 megadroughts. Red circles denote five-year drought events as severe as the turn of the century event. Red line denotes the mean [of the lower graph, for normalized precipitation].

The upper graph, of drought severity is not easy, but disturbing, while the lower graph is scary for a resident of the Western U.S. Might have to read beyond the abstract, except that brilliantly Tom Ashcroft of On Point had a conversation on the drought in the western U.S. amd this research today On Point.  

"We may be looking at a century of drought, or megadrought, this century," said Ashbrook, interviewing Christopher Williams, author of a scary recent op-ed: Hundred Year Forecast: Drought

"The worst drought you saw in the West in 800 years?" asked Ashbrook.

Yes, Williams said, the l998-2004 drought in the Colorado basi was the worst in over an eon. As the subtropical dry zones expand, the Southwest will get much more droughty, forced by "human push" — greenhouse gas emissions. 

Program was exemplary, right down to the usual complaints, but also got deep into the science with Richard Seagar, and deep into the politics with part of a speech from Barack Obama in hot, dry Iowa.

Seagar said that his recent study painted an even bleaker picture, but also appeared to doubt the specific claim that the most recent drought was the worst in 800 years. This proved difficult to untangle.

"That sounds like a dissent," said Ashbrook.

"I wouldn't say that," said Seagar. He agreed that the risk of drought had increased, "by large factors," but said it was too early to say it would be the new normal. Bickering ensued, as he suggested that yes, the Southwest would be hard hit, but wnter rains in the Midwest would prevent Dust Bowls. 

The study actually didn't set out to study drought prevalence. It set out to forecast the diminishing ability of landscapes to store carbon.

No one contested that claim, which would add momentum to the warming. And Michael Wehner at Lawrence Livermore pointed out that the amount of climate change we've experienced to date will pale in comparison to what will be experienced this century in the summer if we can't reduce emissions. 

"Hotter and drier summers," that's what to expect, concluded Seagar. 

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