Models Predict “Megadrought” Risk for American Southwest This Century

Near the beginning of the American Association of the Advancement of Science (AAAS) conference in San Jose, CA, on a winter day that happened to be warm, dry, and sunny, research scientists held a press conference to announce the conclusions of their work on predicting the risks of future “megadroughts.” They published their paper with the ominous title, “Unprecedented 21st-Century Drought Risk in the American Southwest and Central Plains,” in the first issue of Science Advances, a new digital, open-access journal. (The publisher, Marcia McNutt, gave brief opening remarks about how the journal will “showcase new and exciting research.”)

Benjamin Cook (NASA Goddard Institute), Toby Ault (Cornell University), and Jason Smerdon (Columbia University) obtained surprising results from computer model simulations. According to Cook and Smerdon, who videoconferenced with a shared microphone, previous models—such as those used for the IPCC’s 4th Assessment Report—underpredicted drought risks. Cook and his colleagues used drought records documented in more than 1800 tree-ring chronologies over the the past millennium, where ring width decreases in dry years, to develop 17 model projections of 21st century climate in the American Southwest and Central Plains. Their disturbing findings include predictions of megadroughts, lasting 35 years or longer, in both regions worse than any seen in the last 1000 years. In short, they expect climate change to increase drought length and severity in the coming decades.

Mean summer soil moisture and Palmer Drought Severity Index out to 2099. Courtesy: Cook et al. (2015)

Mean summer soil moisture and Palmer Drought Severity Index out to 2099. Courtesy: Cook et al. (2015)

The drought risk is twofold, due to reduced precipitation and to warmer temperatures drying out soils of rivers and lakes, in which models predict increasing evaporation. Long droughts due to climate variations have occurred in the past, such as those occurring during the 12th and 13th centuries (the Medieval Climatic Anomaly) that serve as important benchmarks. But with their tree-ring based reconstruction of the climate history, in a “business-as-usual” emissions scenario, they predict a “persistent shift in the future toward longer droughts” that could exceed even those of these extremely dry centuries.

Ault described how risk assessments are made, in terms of the magnitude of impact and the likelihood. “The levels [of risk] that we see are striking,” with an 80% or higher risk of a drought 35 years or longer in duration by the end of this century if climate change is not mitigated. He described the situation like a golf course, which an initial 10% of it consisting of sandpits. If climate change continues unmitigated, the golf course will gradually become almost entirely sandpits!

Risk (%) of decadal and multidecadal drought calculated from three sets of models. Courtesy: Cook et al. (2015)

Risk (%) of decadal and multidecadal drought calculated from three sets of models. Courtesy: Cook et al. (2015)

In the paper, Cook and his co-authors comment on the difficulties people in the Southwest and Central Plains will face when attempting to adapt to these climate conditions. In particular, the current depletion of nonrenewable groundwater reservoirs “will likely exacerbate the impacts of future droughts.” They discussed implications for both the water supply and food supply in the press conference, considering the dependence on agriculture in California and the breadbasket in the Central Plains. “Water security is food security,” as Ault put it, and people need to “take a no-regrets attitude toward preparedness.” Droughts will inevitably occur, and some of them could be as destructive as large earthquakes and hurricanes.

Their ongoing research will focus on examining the future severity, persistence and geographical scope of droughts, and they will attempt to improve the spatial resolution of their simulations, which currently employ coarse-grained averaging. They also plan to consider hydrology and snowpack, in addition to soil moisture. In any case, the soil moisture metrics and PDRI all point to one conclusion: unless people find a way to substantially mitigate climate change and prevent rising temperatures, the American Southwest and Central Plains can expect to face megadroughts like they’ve never seen before.

Finally, if you’re interested, below you can see my photo from the press conference (and I’m the one in the lower left with a cellphone in front of his face.) In other coverage of these results, see this nicely written Science article by Emily Underwood, and I saw a Washington Post reporter writing an article about it too (but I forgot his name), so watch for that.


People’s Climate March in San Diego

Yesterday afternoon was hot, sunny, and dry in southern California, and it was as great a time as any to draw attention to climate change and demand action on it. I was one of 1,500 people who participated in the People’s Climate March and rally in San Diego, which started at City Hall and the Civic Center, went down Broadway past the train station, and ended at the County Administration Park.


It was exciting and inspiring to be involved in what may have been the largest climate protest in history. At least 300,000 people participated in the march in New York, where the UN climate summit is taking place. According to a speaker for SanDiego350, which was one of the groups organizing the local events, there were marches and rallies in over 3000 cities around the world. They were also widely reported in the media, for example in the New York Times, LA Times, Guardian, and Democracy Now. I’m not a good photographer, so I grabbed the photo above from the SD Reader and the NYC photo below came from the Guardian. I’m sure there were a few differences between the people participating in the SD and NYC protests, as I saw many people wearing flip-flops, heard chants of “¡Si Se Puede!”, and saw a few Mexican wrestler masks too.


From what I could see, it was a very diverse crowd in terms of gender, race, class, and age. Climate change is now more than just an environmental issue—many people from unions, religious groups, and students and teachers attended too. People held signs and yelled chants saying a variety of things: demands for clean energy, green jobs, climate justice, and an end to fracking were common. As I mentioned in my previous post, many Californians are concerned about drought and water policies too, and I saw a few signs about these issues as well. Although we can see widespread support for action on climate change, it’s clear that conservatives and Republicans didn’t show up; climate change has become an increasingly partisan issue in the US over the past few years.

Organizers had great speakers and musicians at the beginning, middle, and end of the march. Many political leaders attended, including Rep. Susan Davis, the Congressional representative for our district. Speakers included: Todd Gloria, City Council president and former interim mayor, who gave a rousing speech to kick off the march; Nicole Capretz, Director of Environmental Policy for the city, who cited labor, women’s rights, and civil rights movements as inspiration; Monique Lopez, Environmental Health Coalition advocate, and City Council member David Alvarez. (More details about the speakers are here.)

Capretz and Gloria outlined their Climate Action Plan, which includes ambitious goals in five areas: energy and water efficient buildings; clean and renewable energy; biking, walking, and transit; zero waste; and climate resiliency. From what I can tell, their emphasis is on the first three prongs. The plan would cut San Diego’s greenhouse gas emissions by 15% by 2020 and nearly half by 2035. More than half of San Diego’s GHG emissions come from transportation, which is why investment in public transit, bike- and pedestrian-friendly areas, incentives for car-pooling, and other related measures are important. However, the plan already faces some resistance from business groups, who only approve of voluntary, incentive-based programs (but not mandatory measures) to get property owners to pursue upgrades to improve buildings’ water and energy efficiency. Mayor Kevin Faulconer is preparing to release his own version of the plan. If it’s watered down, I think he can expect San Diegans to organize more climate marches in the future.

[Although I’m a scientist and always try to lay out the facts in my blog posts, I want to be clear that I’m speaking my personal opinions here.]

Californians and the Environment

The Public Policy Institute of California (PPIC), a nonprofit, nonpartisan thinktank based in San Francisco, recently conducted a survey of Californians’ views of environmental issues. This is particularly important in light of the ongoing drought in the southwest and the upcoming elections in November. According to the report (available in PDF format), the results are based on the responses of 1,705 adult residents throughout California, interviewed in English and Spanish by landline or cell phone, and they’re estimated to have a sampling error of 4% (at the 95% confidence level). I’ll describe what I see as their most interesting results, and if you want more information, I encourage you to read the report.

Global warming: A strong majority say they are very concerned (40%) or somewhat concerned (34%) about global warming. Approximately two thirds of Californians (68%) support the state law, AB 32, which requires California to reduce its carbon emissions to 1990 levels by 2020, but the partisan divide (Democrats at 81% vs Republicans at 39%) has grown on this issue. 80% of Californians say that global warming is a very serious or somewhat serious threat to the economy and quality of life for California’s future. Only 45% of people are aware at all about the state’s cap-and-trade system, which took effect in 2012, but after being read a brief description, Californians are more likely to favor (51%) than oppose (40%) the program. Under a recent agreement between the governor and legislature, 25% of the revenues generated by the cap-and-trade program will be spent on high-speed rail, 35% on other mass transit projects and affordable housing near transit, and the rest for other purposes.


Energy policies: overwhelming majorities of adults favor requiring automakers to significantly improve the fuel efficiency of cars sold in the U.S. (85%) and increasing federal funding to develop wind, solar, and hydrogen technology (78%). Strong majorities support the requirement that oil companies produce cleaner transportation fuels and the goal that a third of California‚Äôs electricity come from renewable energy sources. But residents’ support declines significantly if these two efforts lead to higher gas prices or electricity bills. (This is unfortunate, because gas and oil companies are heavily subsidized in the US, and maybe our gas and electricity bills are too low.) Most residents (64%) oppose building more nuclear power plants, as they have since the Fukushima disaster.

The survey includes other contentious issues: 54% of Californians oppose hydraulic fracturing, or fracking, for oil and natural gas extraction. But a majority (53%) support building the Keystone XL pipeline.

Water policies: Asked about some of the possible effects of global warming in California, majorities say they are very concerned about droughts (64%) or wildfires (61%) that are more severe. 35% say that water supply or drought is the most important environmental issue facing the state today (which is 27% higher than the fraction in a 2011 survey), and this is the first environmental survey in which air pollution was not the top issue. In another measure of concern about drought, strong majorities of residents (75%) say they favor their local water districts requiring residents to reduce water use. The CA legislature is discussing a $11.1 billion state bond for water projects that is currently on the November ballot, and a slim majority of likely voters would support it (51% yes, 26% no).

If you’re interested, the PPIC has useful information and publications on water policies and management of resources: see this page and this blog post series. Water policy analysts argue that in the Central Valley, where most agricultural water use occurs, the failure to manage groundwater sustainably limits its availability as a drought reserve. In urban areas, the greatest potential for further water savings lies in reducing landscaping irrigation—a shift requiring behavioral changes, not just the adoption of new technology. Finally, state and federal regulators must make tough decisions about how and when to allocate water during a drought: they must balance short-term economic impacts on urban and agricultural water users against long-term harm—even risk of extinction—of fish and wildlife.

People’s Climate March

This is a different topic and has nothing to do with the survey, but I want to use this opportunity to plug the People’s Climate March, which will be taking place on Sunday. (This website can direct you to events in your area.) One of the biggest marches and rallies will be in New York City, where the UN climate summit will soon be taking place. Even Ban Ki-moon will be participating! For San Diegans, you can find information about Sunday’s downtown events here. Californians also organized a “People’s Climate Train” to take activists and participants by train from the Bay Area through Denver and Chicago to New York, where they’ll be arriving tonight. Finally, I recommend reading this well written piece by Rebecca Solnit on Dr. Seuss’s The Lorax and the need to raise our voices on Sunday.