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Byline: Daniel R. Cayan (1,2), Edwin P. Maurer (3), Michael D. Dettinger (1,2), Mary Tyree (1), Katharine Hayhoe (4) 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, simulations of twenty-first 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 Fluid 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 twenty-first century in each simulation, with end-of-century temperature increases from approximately +1.5degC under the lower emissions B1 scenario in the less responsive PCM1 to +4.5degC in the higher emissions A2 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. Relatively small (less than ~10%) changes in overall precipitation are 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. Author Affiliation: (1) Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA (2) U.S. Geological Survey, La Jolla, CA, USA (3) Santa Clara University, Santa Clara, CA, USA (4) Department of Geosciences, Texas Tech University, Lubbock, TX, USA Article History: Registration Date: 23/11/2007 Received Date: 02/08/2006 Accepted Date: 05/10/2007 Online Date: 26/01/2008

Byline: Daniel R. Cayan (1,2), Peter D. Bromirski (1), Katharine Hayhoe (3), Mary Tyree (1), Michael D. Dettinger (2,1), Reinhard E. Flick (4,1) Abstract: California's coastal observations and global model projections indicate that California's open coast and estuaries will experience rising sea levels over the next century. During the last several decades, the upward historical trends, quantified from a small set of California tide gages, have been approximately 20 cm/century, quite similar to that estimated for global mean sea level. In the next several decades, warming produced by climate model simulations indicates that sea level rise (SLR) could substantially exceed the rate experienced during modern human development along the California coast and estuaries. A range of future SLR is estimated from a set of climate simulations governed by lower (B1), middle--upper (A2), and higher (A1fi) GHG emission scenarios. Projecting SLR from the ocean warming in GCMs, observational evidence of SLR, and separate calculations using a simple climate model yields a range of potential sea level increases, from 11 to 72 cm, by the 2070--2099 period. The combination of predicted astronomical tides with projected weather forcing, El Nino related variability, and secular SLR, gives a series of hourly sea level projections for 2005--2100. Gradual sea level rise progressively worsens the impacts of high tides, surge and waves resulting from storms, and also freshwater floods from Sierra and coastal mountain catchments. The occurrence of extreme sea levels is pronounced when these factors coincide. The frequency and magnitude of extreme events, relative to current levels, follows a sharply escalating pattern as the magnitude of future sea level rise increases. Author Affiliation: (1) Scripps Institution of Oceanography, University of California, La Jolla, CA, USA (2) Water Resources Division, U. S. Geological Survey, La Jolla, CA, USA (3) Department of Geosciences, Texas Tech University, Lubbock, TX, USA (4) California Department of Boating and Waterways, La Jolla, CA, USA Article History: Registration Date: 23/11/2007 Received Date: 02/08/2006 Accepted Date: 05/10/2007 Online Date: 26/01/2008

Byline: Daniel R. Cayan (1), Amy L. Luers (2), Guido Franco (3), Michael Hanemann (4), Bart Croes (5), Edward Vine (6) Abstract: In response to an Executive Order by California Governor Schwarzenegger, an evaluation of the implications to California of possible climate changes was undertaken using a scenario-based approach. The 'Scenarios Project' investigated projected impacts of climate change on six sectors in the California region. The investigation considered the early, middle and later portions of the twenty-first century, guided by a set of IPCC Fourth Assessment global climate model runs forced by higher and lower greenhouse gas emission scenarios. Each of these climate simulations produce substantial impacts in California that would require adaptations from present practices or status. The most severe impacts could be avoided, however, if emissions can be held near the lower end of global greenhouse gas emissions scenarios. Author Affiliation: (1) Climate Research Division, Scripps Institution of Oceanography, UCSD, U.S. Geological Survey, 201 Nierenberg Hall, La Jolla, CA, 92093-0224, USA (2) Union of Concerned Scientists, Berkeley, CA, USA (3) California Energy Commission, Sacramento, CA, USA (4) University of California, Berkeley, Berkeley, CA, USA (5) California Air Resources Board, Sacramento, CA, USA (6) California Institute for Energy and Environment, Sacramento, CA, USA Article History: Registration Date: 18/10/2007 Received Date: 06/08/2007 Accepted Date: 16/10/2007 Online Date: 26/01/2008

Spring snowmelt is the most important contribution of many rivers in western North America. If climate changes, this contribution may change. A shift in the timing of springtime snowmelt towards earlier in the year already is observed during 1948-2000 in many western rivers. Streamflow timing changes for the 1995 2099 period are projected using regression relations between observed streamflow-timing responses in each river, measured by the temporal centroid of streamflow (CT) each year, and local temperature (TI) and precipitation (PI) indices. Under 21st century warming trends predicted by the Parallel Climate Model (PCM) under business-as-usual greenhouse-gas emissions, streamflow timing trends across much of western North America suggest even earlier springtime snowmelt than observed to date. Projected CT changes are consistent with observed rates and directions of change during the past five decades, and are strongest in the Pacific Northwest, Sierra Nevada, and Rocky Mountains, where many rivers eventually run 30-40 days earlier. The modest PI changes projected by PCM yield minimal CT changes. The responses of CT to the simultaneous effects of projected TI and PI trends are dominated by the TI changes. Regression-based CT projections agree with those from physically-based simulations of rivers in the Pacific Northwest and Sierra Nevada.