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 +1.5ºC under the lower emissions B1 scenario in the less responsive PCM1 to +4.5ºC in the higher emissions A2 iscenario 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 losses are most severe in projections by the more responsive model in response to the highest emissions. ii1.0 Introduction In May 2005, the California Energy Commission (Commission) and the California Environmental Protection Agency (Cal/EPA) commissioned a report describing the potential impacts of 21st Century climate changes on key state resources. Although precise prediction of the future climate is impossible, selected scenarios representative of possible climate changes, targeted regionally on California, were explored much as in previous and ongoing efforts by the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al 2001), an examination of ecological and related changes in California (Field et al. 1999), the U.S. National Climate Change Assessment (National Assessment Synthesis Team, 2001), and the recent United Kingdom Climate Impacts Programme (www.ukcip.org.uk/resources/publications/documents/UKCIP02_briefing.pdf). Because of resource constraints and a tight timeframe during which this report was completed, the present study focused on a small set of climate scenarios. This work builds upon previous climate model-based studies of possible climate change impacts on various sectors in the California region, including a broad assessment of possible ecological impacts by Field et al (1999); an assessment of a range of potential climate changes on ecosystems, health and economy in California described by Wilson et al. (2003); a study of how a “business-as-usual emissions scenario simulated by a low sensitivity climate model would afftect water resources in the western United States, overviewed by Barnett et al. (2004); and a multisectoral assessment of the difference in impacts arising from high vs. low greenhouse gas (GHG) emission in Hayhoe et al. (2004; hereafter designated H04). 3Global and regional climates have already begun changing, probably from accumulating emissions of anthropogenic greenhouse gases. As reported by the WMO (2005), “since the start of the 20th century, the global average surface temperature has risen between 0.6oC and 0.7oC. But this rise has not been continuous. Since 1976, the global average temperature has risen sharply, at 0.18oC per decade. In the northern and southern hemispheres, the 1990s were the warmest decade with an average of 0.38oC and 0.23oC above the 30-year mean, respectively”. The 10 warmest years for the earth’s surface temperature all occurred after 1990 (Jones and Palutikof 2006) and 2005 was either the the second or first warmest year on record (Jones and Palutikof 2006; Hansen et al. 2006). Much of the warming during the last four decades is attributable to the increasing atmospheric concentrations of GHGs due to human activities (Santer et al. 1996; Tett et al. 1999; Meehl et al. 2003). At the regional scales of California and western North America, signs of changing climate are also evident, in part reflecting the global changes noted above. Over the past 50 years, trends toward warmer winter and spring temperatures (e.g., Cayan et al 2001), smaller fractions of precipitation falling as snow instead of rain (Knowles et al 2006), a decline in spring snow accumulations in lower and middle elevation mountain zones (Mote et al 2005), an advance of snowmelt by 5 to 30 days earlier in the spring (Stewart et al. 2005), and a similar advance in the timing of spring flower blooms (Cayan et al. 2001). A ongoing effort by the international climate-science community to prepare the Fourth IPCC Climate Change Assessment provided important background and crucial inputs for the studies reported here. In particular, that international assessment prompted and provided (through the Lawrence Livermore Laboratory Program for Climate Model 4Diagnosis and Intercomparison) a large number of climate model simulations of climates under selected GHG emission scenarios. The present effort has focused on a few of the IPCC simulations in order to provide concrete examples of possible impacts and has analyzed the large ensemble of projections generated for the IPCC assessment more cursorily for perspectives on the scenarios selected for intense study in terms of two major sources of climate-change uncertainty: our incomplete understanding of how the climate system responds (as represented by differences between different climate models) and the unknowable future of emissions