Emissions pathways, climate change, and impacts on California

Katharine Hayhoe

Daniel Cayan

Christopher Field

Peter Frumhoff

Kimberly Cahill+10 authors

^{ATMOS Research and Consulting, 809 West Colfax Avenue, South Bend, IN 46601;}^{Climate Research Division, The Scripps Institution of Oceanography, and Water Resources Division, U.S. Geological Survey, 9500 Gilman Drive, La Jolla, CA 92093-0224;}^{Department of Global Ecology, Carnegie Institution of Washington, 260 Panama Street, Stanford, CA 94305;}^{Union of Concerned Scientists, Two Brattle Square, Cambridge, MA 02238;}^{Civil Engineering Department, Santa Clara University, Santa Clara, CA 95053;}^{Atmosphere and Ocean Sciences Group, Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720;}^{Environmental and Societal Impacts Group, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307;}^{Department of Biological Sciences and Institute for International Studies, Stanford University, Stanford, CA 94305;}^{Corvallis Forestry Sciences Laboratory, U.S. Department of Agriculture Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97331;}^{Department of Agricultural and Resource Economics, University of California, Berkeley, CA 94720;}^{Center for Climatic Research, Department of Geography, University of Delaware, Newark, DE 19716; and}^{Department of Geography, Kent State University, Kent, OH 44242}

^{To whom correspondence should be addressed. E-mail:}moc.hcraesersomta@eohyah.

Contributed by Christopher B. Field, June 23, 2004

Freely available online through the PNAS open access option.

Abstract

The magnitude of future climate change depends substantially on the greenhouse gas emission pathways we choose. Here we explore the implications of the highest and lowest Intergovernmental Panel on Climate Change emissions pathways for climate change and associated impacts in California. Based on climate projections from two state-of-the-art climate models with low and medium sensitivity (Parallel Climate Model and Hadley Centre Climate Model, version 3, respectively), we find that annual temperature increases nearly double from the lower B1 to the higher A1fi emissions scenario before 2100. Three of four simulations also show greater increases in summer temperatures as compared with winter. Extreme heat and the associated impacts on a range of temperature-sensitive sectors are substantially greater under the higher emissions scenario, with some interscenario differences apparent before midcentury. By the end of the century under the B1 scenario, heatwaves and extreme heat in Los Angeles quadruple in frequency while heat-related mortality increases two to three times; alpine/subalpine forests are reduced by 50–75%; and Sierra snowpack is reduced 30–70%. Under A1fi, heatwaves in Los Angeles are six to eight times more frequent, with heat-related excess mortality increasing five to seven times; alpine/subalpine forests are reduced by 75–90%; and snowpack declines 73–90%, with cascading impacts on runoff and streamflow that, combined with projected modest declines in winter precipitation, could fundamentally disrupt California's water rights system. Although interscenario differences in climate impacts and costs of adaptation emerge mainly in the second half of the century, they are strongly dependent on emissions from preceding decades.

Abstract

California, with its diverse range of climate zones, limited water supply, and economic dependence on climate-sensitive industries such as agriculture, provides a challenging test case to evaluate impacts of regional-scale climate change under alternative emissions pathways. As characterized by the Intergovernmental Panel on Climate Change, demographic, socioeconomic, and technological assumptions underlying long-term emissions scenarios vary widely (1). Previous studies have not systematically examined the difference between projected regional-scale changes in climate and associated impacts across scenarios. Nevertheless, such information is essential to evaluate the potential for and costs of adaptation associated with alternative emissions futures and to inform mitigation policies (2).

Here, we examine a range of potential climate futures that represent uncertainties in both the physical sensitivity of current climate models and divergent greenhouse gas emissions pathways. Two global climate models, the low-sensitivity National Center for Atmospheric Research/Department of Energy Parallel Climate Model (PCM) (3) and the medium-sensitivity U.K. Met Office Hadley Centre Climate Model, version 3 (HadCM3), model (4, 5) are used to calculate climate change resulting from the SRES (Special Report on Emission Scenarios) B1 (lower) and A1fi (higher) emissions scenarios (1). These scenarios bracket a large part of the range of Intergovernmental Panel on Climate Change nonintervention emissions futures with atmospheric concentrations of CO₂ reaching ≈550 ppm (B1) and ≈970 ppm (A1fi) by 2100 (see Emissions Scenarios in Supporting Text, which is published as supporting information on the PNAS web site). Although the SRES scenarios do not explicitly assume any specific climate mitigation policies, they do serve as useful proxies for assessing the outcome of emissions pathways that could result from different emissions reduction policies. The scenarios at the lower end of the SRES family are comparable to emissions pathways that could be achieved by relatively aggressive emissions reduction policies, whereas those at the higher end are comparable to emissions pathways that would be more likely to occur in the absence of such policies.

avg, average; JJA, June, July, August; DJF, December, January, February; SWE, snow water equivalent.

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Acknowledgments

We thank Michael Dettinger and Mary Meyer Tyree for providing assistance with data and analysis, and Frank Davis for providing review of earlier drafts of this manuscript. PCM model results were provided by PCM personnel at the National Center for Atmospheric Research, and HadCM3 model results were provided by Dr. David Viner from the U.K. Met Office's Climate Impacts LINK Project. This work was supported in part by grants from the David and Lucile Packard Foundation, the William and Flora Hewlett Foundation, the Energy Foundation, the California Energy Commission, the National Oceanic and Atmospheric Administration Office of Global Programs, and the Department of Energy.

Acknowledgments

Notes

Abbreviations: DJF, December, January, February; HadCM3, Hadley Centre Climate Model, version 3; JJA, June, July, August; PCM, Parallel Climate Model; SRES, Special Report on Emission Scenarios; SWE, snow water equivalent.

Notes

Footnotes

^{See the U.S. Army Corps of Engineers Flood Control Requirements for California Reservoirs, Sacramento District Water Control Data System, Sacramento, CA (}www.spkwc.usace.army.mil).

^{See Western U.S. Climate Historical Summaries (Western Regional Climate Center) at}www.wrcc.dri.edu/climsum.html.

Footnotes

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