We can begin stabilizing our climate by adopting
renewable energy technologies and reducing emissions
of heat-trapping gases—or have our children face
higher temperatures, more water shortages,
and a growing public health crisis.
by Amy Lynd Luers
Over the coming decades, California’s climate is likely to become much warmer and perhaps drier in ways that could dramatically alter the state’s environment. Furthermore, because much of California’s economy is dependent on climate-sensitive industries such as agriculture and tourism, the financial impact of climate change in the state may be substantial as well.
Continued reliance on fossil fuels has led to increased emissions of the heat-trapping gases (primarily carbon dioxide) that contribute to global warming. California is an important test case for exploring the effects of this warming because the diversity of its climate zones and ecosystems are so central to the state’s economic strength and quality of life. To determine the extent to which climate change will affect Californians’ well-being in this century, a group of
 To read the PNAS article and Climate Change in California: Choosing Our Future, please visit our Climate Choices website. Copies of the state overview ($5) or fact sheets on heat, water use, and solutions ($5 each; $15 for all four pieces) are available through the UCS publications page.
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independent scientists including two UCS experts has examined the impact of increasing temperatures on snowpack and water supply, extreme heat and heat-related health, and other critical issues. Their findings were published in the prestigious journal Proceedings of the National Academy of Sciences (PNAS) in August.
The new study used state-of-the-art climate models to simulate two possible futures, one under a higher-emissions scenario and another under a lower-emissions scenario. The higher-emissions scenario assumes a world with continued, intensive reliance on fossil fuels, resulting in a steady increase of heat-trapping emissions throughout the century. The lower-emissions scenario envisions a transition to clean energy technologies, causing emissions to peak by mid-century then decline.
Some climatic changes are inevitable because of the continuing effects of past heat-trapping emissions. However, the research shows that the extent to which the climate will change, and the severity of the problems caused by climate change, will depend on whether and how quickly we reduce new emissions.
Water: Too Little, Too Soon?
Over the next several decades, California winters are likely to become warmer, precipitation may decrease, and snowpack in the Sierra Nevada will likely decline. More of the precipitation that does fall would take the form of rain instead of snow and snowpack would melt earlier, resulting in a potential increase in both the risk of winter flooding and summer water shortages.
The Sierra Nevada snowpack represents a major natural reservoir, providing up to half the state’s water supply during the dry spring and summer months. Less snowpack would likely mean less water available for electricity generation, agricultural irrigation, and ecosystem support. Toward the end of the century, California could lose 30 to 70 percent of its historic natural water storage capacity due to declining snowpack under the lower-emissions scenario, or a stunning 70 to 90 percent under the higher-emissions scenario.
Decreasing Sierra Nevada Snowpack | 
| By the end of the century, Sierra Nevada snowpack
could be reduced to less than a third of current levels,
even under a lower-emissions scenario. This figure
shows projections of spring snowpack in the Sacramento-
San Joaquin watershed, which provides water to about
28 million agricultural and urban users in California. |
In addition, California’s water supply will likely be further stressed by a decrease in annual stream flow to existing man-made water storage facilities and an increase in the number of critically dry years. By mid-century, the total annual stream flow into major reservoirs in the Sierra Nevada is projected (in most cases) to drop 10 to 20 percent, with the greatest decreases occurring under the higher-emissions scenario. These projected losses of natural water storage, coupled with increased water demand from a growing population, could force water managers to impose restrictions similar to those used during 1991 and 1992, including a mandatory 50 percent reduction in urban water use and widespread fallowing of agricultural land.
The expected delay in snow accumulation and earlier snowmelt could also have a significant impact on winter recreation. Toward the end of the century, the ski season could shorten by as much as a month under the lower-emissions scenario, while the minimum snow conditions for ski resort operation might never occur under the higher-emissions scenario. Resorts would be forced to rely entirely on snowmaking equipment or move their operations.
Heat: Too Much, Too Often?
If heat-trapping emissions continue to increase, summer temperatures could rise a dramatic 8.5 to 18 °F by the century’s end—nearly double the rise expected under the lower-emissions scenario. Warming temperatures will likely produce worsening air quality, a rise in infectious disease, and an increase in weather-related deaths.
As average temperatures rise, the duration and frequency of extreme-heat events are expected to increase as well. By century’s end, heat waves could be two to five times more common in some regions than they are today, and the number of heat-related deaths under the higher-emissions scenario could be more than double what we would expect under the lower-emissions scenario. The most persistent and severe high-temperature conditions are likely to occur in inland locations that are already hot, such as Sacramento and Fresno. However, the impact on human health could be greatest in cooler coastal cities such as San Francisco where extreme-heat conditions have historically been relatively rare.
Consequences for Agriculture
California’s $30 billion agricultural economy may prove particularly sensitive to projected climate changes over the coming decades. Warmer temperatures, reduced water supply, and a potential increase in pest outbreaks are likely to have adverse effects on the quantity and quality of numerous agricultural products. Water-intensive crops such as grapes, cotton, and alfalfa may be especially hard-hit.
Although warmer temperatures are likely to make the cool growing regions of Monterey and Mendocino Counties better producers of some grape varieties over the next few decades, regions that are already warm (such as the Central Valley) are likely to become less hospitable for viticulture. If warming proceeds as projected under the higher-emissions scenario, growing conditions for quality wine grapes could severely decline toward the end of the century throughout California, including the world-renowned Napa and Sonoma Counties.
Choosing a Future
Californians alone cannot stabilize Earth’s climate. But because they live in the most populous state in the nation and boast the world’s fifth largest economy, citizens of the Golden State can continue to play their historic role as leaders in environmental innovation, slowing the pace of climate change and setting precedents for other states and countries to follow.
California recently passed a renewable energy standard that requires 20 percent of its energy to come from renewable resources by 2017, and has enacted landmark legislation to reduce heat-trapping emissions from new cars sold in the state(see “In the Driver’s Seat,” also from this issue of Catalyst). Smart agriculture and forestry policies, such as methane capture and reforestation, can reduce emissions—and the potentially devastating consequences of climate change—even further.
Amy Lynd Luers is a climate impacts scientist in the Global Environment Program.
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