Warming pushes Western U.S. toward driest period in 1,000 years
Shrinking water supplies have forced western states to impose water use restrictions; aquifers are being drawn down to unsustainable levels, and major surface reservoirs such as Lake Mead and Lake Powell are at historically low levels. This winter’s snowpack in the Sierras, a major water source for Los Angeles and other cities, is less than a quarter of what authorities call a “normal” level, according to a February report from the Los Angeles Department of Water and Power. California water officials last year cut off the flow of water from the northern part of the state to the south, forcing farmers in the Central Valley to leave hundreds of thousands of acres unplanted.
“Changes in precipitation, temperature and drought, and the consequences it has for our society — which is critically dependent on our freshwater resources for food, electricity and industry — are likely to be the most immediate climate impacts we experience as a result of greenhouse gas emissions,” said Kevin Anchukaitis, a climate researcher at the Woods Hole Oceanographic Institution. Anchukaitis said the findings “require us to think rather immediately about how we could and would adapt.”
Much of our knowledge about past droughts comes from extensive study of tree rings conducted by Lamont-Doherty scientist Edward Cook (Benjamin’s father) and others, who in 2009 created the North American Drought Atlas. The atlas recreates the history of drought over the previous 2,005 years, based on hundreds of tree-ring chronologies, gleaned in turn from tens of thousands of tree samples across the United States, Mexico, and parts of Canada.
The release notes that for the current study, researchers used data from the atlas to represent past climate, and applied three different measures for drought — two soil moisture measurements at varying depths, and a version of the Palmer Drought Severity Index, which gauges precipitation and evaporation and transpiration — the net input of water into the land. While some have questioned how accurately the Palmer drought index truly reflects soil moisture, the researchers found it matched well with other measures, and that it “provides a bridge between the [climate] models and drought in observations,” Cook said.
The researchers applied seventeen different climate models to analyze the future impact of rising average temperatures on the regions. They compared two different global warming scenarios — one with “business as usual,” projecting a continued rise in emissions of the greenhouse gases that contribute to global warming; and a second scenario in which emissions are moderated.
By most of those measures, they came to the same conclusions.
“The results … are extremely unfavorable for the continuation of agricultural and water resource management as they are currently practiced in the Great Plains and southwestern United States,” said David Stahle, professor in the Department of Geosciences at the University of Arkansas and director of the Tree-Ring Laboratory there. Stahle was not involved in the study, though he worked on the North American Drought Atlas.
Smerdon said he and his colleagues are confident in their results. The effects of CO2 on higher average temperature and the subsequent connection to drying in the Southwest and Great Plains emerge as a “strong signal” across the majority of the models, regardless of the drought metrics that are used, he said. He added that they are consistent with many previous studies.
Anchukaitis said the paper “provides an elegant and convincing connection” between reconstructions of past climate and the models pointing to the risk of future drought.
Toby R. Ault of Cornell University is a co-author of the study.
— Read more in Benjamin I. Cook, Toby R. Ault, Jason E. Smerdon, “Unprecedented 21st century drought risk in the American Southwest and Central Plains,” Science Advances 1, no. 1 (1 February 2015): e1400082 (DOI: 10.1126/sciadv.1400082)
