One of the major impacts of climate change will be on water resources. Many scientists believe that the recent, unprecedented droughts in California and other western states could be the beginning of a permanent transition to a drier climate in the region.
In California, many climate change models project significant drying in the Sacramento and San Joaquin River basins in coming decades. A 2012 study by Randy Hanson and Lorraine and Alan Flint et. al. at the USGS California Water Science Center has one of the clearest illustrations of how much drier these basins could get by the end of the century. The map below shows the projected change in discharge (outflow) from the watersheds. The map on the far right is color coded by the reduction in discharge between 2010-2020 and the end of the century. The orange areas show a reduction of 45% or more.
The study used NOAA’s Global Fluid Dynamics Lab climate model, which is a higher sensitivity climate model, projecting more warming for a given level of C02 in the atmosphere. The higher sensitivity climate models are consistent with recent increases in global temperature, and generally project more drying. The study also used the A2 (medium-high) greenhouse gas emissions scenario, which assumes that C02 in the atmosphere continues to increase.
Under the A2 scenario, the GFDL model runs projected that median flows in the entire Sacramento River watershed could decrease by 17% by 2020, and by 34% by 2100. The box and whisker plot below shows the range of projections for the change in median flows for each 20 year period, when compared with 1963-2000. While there is a fair amount of spread in the projections, the overall trend is strongly down. This is shown by the dashed line, which gives the mean change over all model projections for each 20 year period. The change is not uniform, probably because the GFDL climate model captures cycles in sea surface temperatures, including the Pacific Decadal oscillation.
Not all global climate change models predict as much drying in the Sacramento River basin as the GFDL model used in Hanson’s study, but the ones that do are consistent not only with recent increases in global temperature, but also with the severe droughts in California, Texas, and New Mexico. A 2009 study by Jessica Oster and Isabel Montanez et. al. at UC Davis may also corroborate the climate change models that project significant drying. Oster analyzed the composition of layers in stalagmites in Moaning Cavern in the central Sierra Nevada. The changes in isotopes showed that the last time the climate warmed significantly, at the end of the last ice age, it got much drier.
California is entering a new era of water management, with much greater risk and uncertainty due to climate change. Making conservation a way of life is essential to managing that risk. For this reason, Governor Brown’s mandate to increase long term conservation targets above the 20×2020 levels is critically important for California’s cities.