Leading Scientists Explain How Climate Change Is Worsening California’s Epic Drought

http://thinkprogress.org/climate/2014/01/31/3223791/climate-change-california-drought/

By Joe Romm on January 31, 2014

Scientists have long predicted that climate change would bring on ever-worsening droughts, especially in semi-arid regions like the U.S. Southwest. As climatologist James Hansen, who co-authored one of the earliest studies on this subject back in 1990, told me this week, “Increasingly intense droughts in California, all of the Southwest, and even into the Midwest have everything to do with human-made climate change.”

Why does it matter if climate change is playing a role in the Western drought? As one top researcher on the climate-drought link reconfirmed with me this week, “The U.S. may never again return to the relatively wet conditions experienced from 1977 to 1999.” If his and other projections are correct, then there may be no greater tasks facing humanity than 1) working to slash carbon pollution and avoid the worst climate impact scenarios and 2) figuring out how to feed nine billion people by mid-century in a Dust-Bowl-ifying world.

Remarkably, climate scientists specifically predicted a decade ago that Arctic ice loss would bring on worse droughts in the West, especially California. As it turns out, Arctic ice loss has been much faster than the researchers — and indeed all climate modelers — expected.

And, of course, California is now in the death-grip of a brutal, record-breaking drought, driven by the very change in the jet stream that scientists had anticipated.

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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.

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As climatologist and water expert Peter Gleick noted to me, quite separate from the impact of climate change on precipitation, “look at the temperature patterns here, which are leading to a greater ratio of rain-to-snow, faster melting of snow, and greater evaporation. Those changes alone make any drought more intense.”

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Beyond the expansion and drying of the subtropics predicted by climate models, some climatologists have found in their research evidence that the stunning decline in Arctic sea ice would also drive western drought — by shifting storm tracks.

“Given the very large reductions in Arctic sea ice, and the heat escaping from the Arctic ocean into the overlying atmosphere, it would be surprising if the retreat in Arctic sea ice did *not* modify the large-scale circulation of the atmosphere in some way,” Michael Mann, director of the Earth System Science Center at Pennsylvania State University, told me this week. “We now have a healthy body of research, ranging from Lisa Sloan’s and Jacob Sewall’s work a decade ago, to Francis’s more recent work, suggesting that we may indeed be seeing already this now in the form of more persistent anomalies in temperature, rainfall, and drought in North America.”

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As the news release at the time explained, they “used powerful computers running a global climate model developed by the National Center for Atmospheric Research (NCAR) to simulate the effects of reduced Arctic sea ice.” And “their most striking finding was a significant reduction in rain and snowfall in the American West.”

“Where the sea ice is reduced, heat transfer from the ocean warms the atmosphere, resulting in a rising column of relatively warm air,” Sewall said. “The shift in storm tracks over North America was linked to the formation of these columns of warmer air over areas of reduced sea ice in the Greenland Sea and a few other locations.”

Last year, I contacted Sloan to ask her if she thought there was a connection between the staggering loss of Arctic sea ice and the brutal drought gripping the West, as her research predicted. She wrote, “Yes, sadly, I think we were correct in our findings, and it will only be worse with Arctic sea ice diminishing quickly.”

This week, Sewall wrote me that “both the pattern and even the magnitude of the anomaly looks very similar to what the models predicted in the 2005 study

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Ostro gave a talk last year (with Franics), and as Climate Desk summarized, “Ostro’s observations suggest that global warming is increasing the atmosphere’s thickness, leading to stronger and more persistent ridges of high pressure, which in turn are a key to temperature, rainfall, and snowfall extremes and topsy-turvy weather patterns like we’ve had in recent years.”

The climate is changing. “All of our weather is now, and increasingly in the future, influenced by climate change,” Gleick wrote me. “The question about attribution (i.e., is this drought caused by climate change) is, of course, the wrong question — easy for deniers to dismiss because it is not easy to show unambiguous links to some kinds of individual events.”

What is especially worrisome is that climate change has only just started to have an impact on Western droughts. We’ve only warmed 1.5°F in the past century. Absent strong climate action, we are on track to warm 10°F over the next century!

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Finally, a 2009 NOAA-led paper warned that, for the Southwest and many semi-arid regions around the world, “the climate change that is taking place because of increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop.” Impacts that should be expected if we don’t aggressively slash carbon pollution “are irreversible dry-season rainfall reductions in several regions comparable to those of the ‘dust bowl’ era.”

[But we still need to do as much as possible to keep it from becoming even worse. Not to do so is unimaginably evil.]