Despite uncertainties in the climate models used to generate these scenarios, they are valuable tools for planning, said Lisa Sloan, an associate professor of Earth sciences at UCSC. The findings of Sloan and her coauthors will be published online on June 7 by the journal Geophysical Research Letters.
The study supports what many have already guessed--global warming will mean warmer temperatures and smaller snowpacks in California, with serious adverse effects on the state's water supply. But the study also reveals more subtle details, and offers regional specificity and precise numbers backed by a statistical analysis.
"Everybody has guessed at the effects on water resources, but now we have numbers and locations. It's a lot different from the standard arm-waving," Sloan said. "Our hope is that this kind of study will give state and regional officials a more reliable basis for planning how to cope with climate change."
Sloan's research group used a sophisticated computer model of the regional climate system to look at the response of California's climate to changing concentrations of carbon dioxide, the heat-trapping "greenhouse gas" released by burning of fossil fuels. Sloan worked with graduate students Mark Snyder, who is first author of the paper, and Jason Bell, now a computer programmer in the Earth Sciences Department, to develop a regional climate model centered over California. The regional model offers much finer spatial resolution than larger models used to study the global climate. Since the regional climate is driven by global processes, the regional model was coupled with a global climate model.
The researchers looked at the effects of doubling the amount of carbon dioxide in the atmosphere compared to the preindustrial level. The latest projections of carbon dioxide emissions indicate that the atmospheric concentration will be twice the preindustrial level well before the end of this century and possibly as soon as 2050.
With atmospheric carbon dioxide doubled, the California regional climate model showed higher average temperatures every month of the year in every part of the state. The extent of the warming varied, however, with the greatest increases in temperature occurring at high elevations in the Sierra Nevada and the Cascade Range. For example, the average temperature in June in the Sierra Nevada increased by 11 degrees Fahrenheit.
The model also showed rainfall increasing in northern California but staying largely the same in southern California, while snow accumulation in the mountains decreased dramatically. In March, for example, it showed an additional 8 inches of rain falling in the central Sierra, while the height of the snowpack at the end of March dropped by 13 feet. By the end of April, the snowpack was almost completely gone.
"With less precipitation falling as snow and more as rain, plus higher temperatures creating increased demand for water, the impacts on our water storage system will be enormous," Sloan said.
Snyder said this report is only the first step in an ongoing effort to understand the implications of climate change for California. The results represent an "average" year, based on 15 years worth of data generated by the model for each carbon dioxide concentration. Additional studies will try to capture the natural variability of the climate and how that variability may change in the future, he said.
"This opens the door for a lot more studies to look at regional climate change in more detail, and to understand the variability that is likely to occur," Snyder said. "It's important to consider not only the average conditions but also what the extremes will be."
The researchers are also exploring refinements in the model that would yield even higher spatial resolution. Already, the model provides more detail than any previous study, showing how climate change is likely to affect different parts of the state, such as the Coast Ranges, the Central Valley, the Mojave Desert, and the Sierra Nevada.
All of the major features of the results were statistically significant, Sloan said. She added, however, that the results are not predictions.
"The model gives us scenarios of what the future may look like," Sloan said. "There are only two ways to tell how good the model is: One is to wait for 50 years and see what happens, and the other is to model the present day. We've done the latter quite rigorously and have satisfied ourselves that the model does a good job of representing the present climate. So our confidence in these scenarios is pretty good."
In addition to Sloan, Snyder, and Bell, the paper's authors include Philip Duffy and Bala Govindasamy, both climate researchers at Lawrence Livermore National Laboratory. Sloan's research is supported in part by a fellowship from the David and Lucile Packard Foundation.