from p. 1
On the Cusp of Megadrought
March 7 - 20, 2014
Serving the Seven Cities of the Harbor Area
part has been dry and relatively warm for whole Southwest.... This one is notable by its severity. What kind of legs does it have? Is this going to be a multi-year drought?” These are the questions haunting the scientific community. The rainstorm, which followed did nothing to change that. “It’s a welcome relief,” MacDonald said in a follow-up interview, after the storm, “But it certainly is not enough from that one system to make up the deficit that we’ve had from the 2013 calendar year, and the earlier part of the very warm and dry January and February. The worst thing we could do is to sit back now and say, ‘Well, we’ve had a rainstorm, drought’s over. We don’t have to worry about it.’ Because our numbers [rainfall totals] are still low and one lucky rainstorm like that does not a water crisis end.” On Feb. 14, President Barack Obama visited California, speaking at the Los Banos farm of Joe Del Bosque, where he announced a multipronged response to the drought, including a call for Congress to approve a $1 billion “resiliency fund” to help communities develop their capacity to respond to extreme weather events. “As anybody in this state could tell you, California’s living through some of its driest years in a century,” Obama said. “Right now, almost 99 percent of California is drier than normal— and the winter snowpack that
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provides much of your water far into the summer is much smaller than normal. “The planet is slowly going to keep warming for a long time to come….So we’re going to have to stop looking at these disasters as something to wait for; we’ve got to start looking at these disasters as something to prepare for, to anticipate, to start building new infrastructure, to start having new plans, to recalibrate the baseline that we’re working off of.” Despite the predictable chorus of denial from conservatives, scientists now cite several distinctly different ways that global warming is contributing to the severity and negative impacts of the ongoing drought, which is not limited to California or just the past two years, but actually encompasses the entire western United States, throughout a period of almost 15 years now. In California, on average, throughout the past 13 years, half the state—57 percent—has been under some degree of drought conditions. Throughout the past two years, the figures have been 87 percent and 94 percent. From Jan. 1 through mid-February, the figure was more than 98 percent—but at least that’s down from 100 percent, where it stood from late March to midSeptember of last year. It returned to 100 percent for the two weeks preceding the recent storm (the latest figures available at press time). Over the long haul, things have actually been worse for the West as a whole: almost two-thirds (64 percent) under drought conditions over the past 13 years, though “only” 74 percent and 80 percent the past two years. And remember, according to the projections Dai summarized, these will soon be the “good old days.” This prolonged dry spell is on the cusp of qualifying as a “megadrought,” the likes of which haven’t been seen here in several centuries according to Dr. Robert Seager of Columbia University, whose research first put North America’s Medieval megadroughts on the map. Random Lengths interviewed him (along with MacDonald) in 2007, following a particularly fierce fire season. (“The Fire This Time and Next,” RLN, Nov. 2-15, p. 1) “Some part of the West has been in or out of drought since 1998,” Seager said. “We are getting up to that 15-year time frame.” Lack of precipitation is one factor, but not enough to cause such a prolonged drought, he explained. “Just in terms of precipitation, there may have been a few breaks in the megadrought,” Seager said. “But the with higher temperatures, and more evaporation, those gaps appear to have been filled. We’re still doing the research to confirm this, but it appears to be the case.” Dr. Valerie Trouet, assistant professor in the Laboratory of Tree-Ring Research at the University of Arizona, expressed a similar view in a teleconference with four climate scientists on Jan. 31. “What we are seeing now is fundamentally different from previous mega-droughts, which were driven largely by precipitation,” Trouet said. “Now, thanks to higher temperatures driven by climate change, droughts are increasingly temperature-driven, which makes even normal levels of precipitation less effective in relieving drought conditions. In the Sierra Nevada mountain range, for example, higher temperatures and very low snowpack will reduce springtime runoff and make forest fires even more likely.” Droughts—even megadroughts—are a part
of natural climate variability, so scientists will require an extremely high bar of proof before they’ll say that global warming, rather than variability, caused any single drought to occur. Given that multi-decade megadroughts hit North America repeatedly in the medieval period, don’t expect scientists to blame global warming solely and directly any time soon. It’s simply the wrong sort of warning signal to wait for from them. But they’re gaining more and more insight into how global warming can make droughts more likely, more severe, more persistent, and/or more difficult to cope with and recover from. We’ve already discussed how rising temperatures make multi-year droughts more common, even when precipitation levels recover for a year or two. Another way global warming impacts droughts—particularly here in California—is by altering jet stream behavior, as Trouet explained in a followup interview. In the teleconference, Trouet referred to the “ridiculously resilient ridge,” a 2,000-mile long partition of high pressure off Canada’s west coast. That’s been in place for more than 13 months now, blocking cold weather from the Arctic from moving south. When asked to expand on its role, Trouet said, “The ridiculously resilient ridge is related to the position of the jet stream... Jet stream winds at high elevation go around the earth. Normally they flow fairly zonally westward and fast.” Now, however, “With the North Pole warming faster, there’s changes going on in the jet stream. It weakens the jet stream, so it’s flowing less fast than it used to, and it is making bigger meanders, much larger north-south movements than what we’re used to and staying in the same position for longer periods of time. This is responsible for the resilient ridge impacts in California, as well as the polar vortex in the east.” That’s perhaps the greatest irony in terms of know-nothing political discourse, in which conservatives have attempted to use winter storms caused by the polar vortex as an argument against global warming. In reality, those storms are actually part of the same continental-scale weather system as California’s ongoing drought. The jet stream’s distortions due to global warming are responsible for both of them, for the same reason. There’s a lot we don’t know about the jet stream, Trouet explained, since records only go back 50 or 60 years. She has a grant to gather tree ring data from Central California that can help extend that record by hundreds of years. That’s important, because the models don’t seem to be doing a good job with California’s weather, compared to the larger southwest and northern Mexico. More data from a longer record could lead to more accurately drawn models and a clearer picture of what’s to come. She hopes to publish something within a year. Until then, she and Seager have somewhat different views of what’s happening specifically in California—a normal difference of perspective that’s commonplace in science. But for the rest of the western United States, they see things remarkably the same. When I mentioned his analysis of why the multi-state drought had been so persistent, she readily agreed. “Drought is not just a function of how much rain or snow you get,” she said. “It’s also the relation with temperature.” Timing is also a factor, she noted. When temperatures rise sufficiently earlier in the year, snow packs melt earlier and there’s less water available when it’s needed most in the driest, hottest part of the summer. This brings up a point made by another California Megadrought/ to p. 10