Climate Change And The Growing Risk Of Megadroughts In The Southwestern United States

california dam

Back in June 2012 the United States was going through what was then an unprecedented stretch of dry conditions, with 56% of the country experiencing moderate to exceptional drought. Two years later, the situation has improved for some areas, with “just” 31% of the nation suffering from drought. The exception is the southwest, and California in particular: As of August 2014, the entire state was considered to be in drought conditions, with nearly 60% at the highest “exceptional drought” level, defined as causing “widespread crop/pasture losses; shortages of water in reservoirs, streams and wells, creating water emergencies,” according to the National Drought Mitigation Center.

Given the size of California’s agricultural sector and its contributions to the state’s economy, the drought’s persistence and severity have been excruciating for cities and farmers alike. For example, the state produces 80% of the world’s almonds, yet the water required to grow them would supply 75% of the needs of the state’s population. A 2013 study in Water Policy looked at management practices in four U.S. cities and found that conservation was the most cost-effective way to combat the effects of drought, but that many municipalities didn’t explore it until after surface and groundwater supplies had been exhausted. In August California governor Jerry Brown agreed to put a $7.5 billion bond measure on the November ballot; it includes $1.6 billion for groundwater sustainability and water recycling, but also $2.7 billion for dams and reservoirs.

The question is what next year will look like — will the drought break or drag on? A 2014 study from U.C. Davis found that 2015 is “likely to be another dry year in California,” but the projection was based on statistical projections of past data. A 2014 study, “Assessing the Risk of Persistent Drought Using Climate Model Simulations and Paleoclimate Data,” seeks to get a better understanding of the risk in the context of ongoing climate change. The researchers, from Cornell, the University of Arizona and the U.S. Geological Survey, base their work on both historical data and climate simulations.

The Study’s Findings Include:

  • Standard climate models appear to significantly underestimate the risk of extreme, extended drought in the southwestern United States. “State-of-the-art climate model projections suggest the risk of a decade-scale megadrought in the coming century is less than 50%; our analysis suggests that the risk is at least 80%, and may be higher than 90% in certain areas.”
  • Using standard similations, the probability of droughts lasting 35 or more years or more are just 1%. However, taking into account the IPCC’s four representative concentration pathways (RCP) of greenhouse gas emissions, the risks rise substantially in the U.S. Southwest: The probability of a multi-decade megadrought is 10% to 20% in RCP 2.6; 20% to 40% in RCP 4.5; and 30% to 50% in RCP 8.5.
  • Under the most severe warming scenario, RCP 8.5, the risk of a 50-year megadrought is “non-negligible” — 5% to 10%.
  • The researchers conclude: “These findings are important to consider as adaptation and mitigation strategies are developed to cope with regional impacts of climate change, where population growth is high and multidecadal megadrought — worse than anything seen during the last 2000 years — would pose unprecedented challenges to water resources in the region.”

    Related research: An April 2014 report from the IPCC, “Climate Change 2014: Mitigation of Climate Change,” lays out ways to limit or reverse harmful trends in greenhouse-gas emissions. The need for international cooperation and especially a price on carbon are central to the report, but it also highlights the importance of direct actions at every level.

    This article originally appeared on The Journalist's Resource


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