Drought and Climate Change in California — Part I

The June 12, 2012 U.S. Drought Monitor shows continuing extreme drought in many areas of the Southwest, including southwestern Arizona, and eastern New Mexico and western Texas.  The drought monitor also shows severe drought in Nevada and the Central and Southern Sierras.

Map of drought severity in US
June 12, 2012 US Drought Monitor

The final survey of snow water content by the Department of Water Resources showed levels close to the 1976-77 drought in the Central and Southern Sierras.

Many climate change models predict increasing droughts in the Southwest and California.     The graph below is from a 2011 risk assessment by the Bureau of Reclamation, which used an ensemble of 75 global climate models.

USBR 2050s-1990s precipitation change graph
Percent change in annual runoff from 2050s-2090s, USBR 2011

There appears to be a fairly strong correlation between the areas of predicted decrease in precipitation, and current drought patterns.

The graph below shows precipitation decreases in Sacramento under two climate scenarios chosen by Daniel Cayan et. al. for modelling the state of California’s 2009 Climate Adaptation Strategy.

Graph of projected changes in precipitation in the Sacramento Region
Differences in 30-year mean annual total precipitation in Sacramento
of early(2005–2034), middle (2035–2064), and late (2070–2099) 21st century relative to 1961–1990 for each of six GCMs, for A2 and B1 scenarios

Source:   Daniel Cayan et. al.,  Climate Change Scenarios and Sea Level Rise Estimates for the California 2009 Climate Change Scenarios Assessment

The two IPCC climate change scenarios chosen for modelling by Cayan were the A2 scenario, a higher emissions scenario which assumes continuously increasing population, and the B1 scenario, a lower emissions scenario which assumes rapid changes towards resource efficient technologies, and a declining population after 2050.     The A2 scenario is the more likely if current economic and population trends continue.

Under the A2 scenario, all of the global climate models show moderate to significant decreases in precipitation by mid-century.   Under the B1 scenario, two thirds of the global climate models show significant decreases.

The six global climate models (GCMs) used for the modelling were from major research centers around the world:

  1. the French Centre National de Recherches Météorologiques (CNRM) model
  2. the (U.S.) National Oceanic and Atmospheric Administration Geophysical Fluids Dynamics Laboratory (GFDL) model
  3. the MIROC 3.2 medium-resolution model from the Center for Climate
    System Research of the University of Tokyo
  4. the Max Plank Institute ECHAM5/MPI-OM
  5. the (U.S.) National Center for Atmospheric Research,  Community Climate System Model (CCSM)
  6. the National Center for Atmospheric Research,  Parallel Climate Model (PCM)

The models were chosen based on reasonable reproduction of seasonal temperature and precipitation, variability, and the El Nino/Southern Oscillation  (ENSO).

To summarize, the 75 GCM-ensemble model used by the 2011 US Bureau of Reclamation Risk Assessment, and most of the twelve models used in the 2009 California Climate Change Scenarios Assessment point toward a drier California.    This is consistent with recent patterns of drought in the region.   However, only time will tell if these droughts are the beginning of a significant shift in the climate in California and the Southwest.

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