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Climate Change Scenarios for the California Region Daniel R Cayan 1 2 Edwin P Maurer 3 Michael D Dettinger 2 1 Mary Tyree 1 and Katharine Hayhoe 4 1 Scripps Institution of Oceanography University of California San Diego 2 U S Geological Survey 2 Santa Clara University 4 Department of Geosciences Texas Tech University Abstract To investigate possible future climate changes in California a set of climate change model simulations was selected and evaluated From the IPCC Fourth Assessment activities projections simulations of 21st century climates under a B1 low emissions and an A2 a medium high emissions emissions scenarios were evaluated along with occasional comparisons to the A1fi high emissions scenario The climate models whose simulations were the focus of the present study were from the Parallel Climate Model PCM1 from NCAR and DOE and the NOAA Geophysical Fluids Dynamics Laboratory CM2 1 model GFDL These emission scenarios and attendant climate simulations are not predictions but rather are a purposely diverse set of examples from among the many plausible climate sequences that might affect California in the next century Temperatures over California warm significantly during the 21st century in each simulation with temperature increases from approximately 1 5 C under the lower emissions B1 scenario in the less responsive PCM1 to 4 5 C in the higher emissions A2 i scenario within the more responsive GFDL model Three of the simulations all except the B1 scenario in PCM1 exhibit more warming in summer than in winter In all of the simulations most precipitation continues to occur in winter and relatively small less than 10 change in overall precipitation is projected The California landscape is complex and requires that model information be parsed out onto finer scales than GCMs presently offer When downscaled to its mountainous terrain warming has a profound influence on California snow accumulations with snow losses that increase with warming Consequently snow

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