A major study published today, based on 160 climate models compiled by researchers at NOAA, including a leading voice in climate modeling, Martin Hoerling, and Richard Seager, both of whom who have spent years projecting the impact of climate change on the West, concludes that California's epic three-year drought was not — repeat not — caused by climate change. They write:
The severe drought in California over the last 3 years (2011-14) is primarily due to natural climate variability, key features of which appear to be predictable from knowledge of how California precipitation reacts to tropical ocean temperatures. There has been no long-term trend in California precipitation; however, California temperatures have been rising and record high temperatures during the drought were likely made more extreme due to human-induced climate change.
During a press conference, Hoerling and Seager were asked specifically about a study I reported on a couple of months ago, by Daniel Swain, which indicated that the high pressure system, the so-called Ridiculously Resilient Ridge that formed off the coast and prevented storms from reaching California was linked to climate change.
Hoerling said that yes, the "RRR" did prevent precipitation from reaching California, and yes, the very high temperatures in California these past three years — 2-3 degrees above normal — did worsen the drought, but they challenged Daniel Swain's analysis of that high pressure system off the coast.
Hoerling said that in a narrow sense the Swain study had a point, in that as temperatures go up, we should expect to see higher pressures around the world. (Not just in the North Pacific.) But he added that the Swain study that predicted increasing chances of a North Pacific High was "blatantly in contradiction to models that show a persistent low pressure system developing off the west coast," and said the Ridiculously Resilient Ridge is "absolutely not" what the models show for the future.
To quote from the study itself:
Diagnosis of CMIP5 models indicates human-induced climate change will
increase California precipitation in mid-winter associated with an increase in
westerly flow entering the central Pacific West Coast and a low pressure anomaly
over the north Pacific.
In other words, as Hoerling added in the press conference, California, especially central and Northern California, may expect to see more precipitation in the future — not less! (One caveat: our springs are expected to be hotter, so the effect may wear off sooner than in the past.)
This study was challenged by the eminent Michael Mann in the Huffington Post. Mann brings up some conflicting studies, and makes some good points, but I have to say that the graphic the researchers choose to indicate the correspondence between the models and the reality, at a relatively low height, focused on the crucial area of the North Pacific is very impressive. If these same models that successfully modeled the drought of 2012, which was caused by a high pressure system in the North Pacific, indicate that we will have low pressure conducive to rain in that region — well, thank goodness!
All the caveats remain in place, of course — this is just one study, etc. But still. I'll take any good news on precipitation in California I can find, and discuss it until the cows come home and find some green grass to eat.