Climate Change Basics

View of snow covered Sierra Nevada mountains in Nevada County, California.

View of snow covered Sierra Nevada mountains in Nevada County, California.

Climate change is impacting California’s water resources, as evidenced by changes in snowpack, sea level, and river flows. As we seek to adapt to and mitigate the impacts of climate change, responsible management of California’s water resources is essential for the long-term health of our state.

Climate Basics

Climate refers to conditions, such as temperature and precipitation, measured over an extended period of time. Most of California has a “Mediterranean climate,” with hot, dry summers and mild, wet winters. For centuries, this climate has remained relatively stable. However, our climate is changing. The main driver behind climate change is carbon dioxide. Since industrialization in the 1850s, we began to burn fossils fuels like coal, oil, and gas for energy, and have released thousands of tons of carbon dioxide (CO2) into the atmosphere. CO2 is a heat-trapping gas, making the atmosphere act like an increasingly thick blanket, which is warming the planet and disrupting the climate. Here in California, we have seen average annual temperatures increase steadily since 1895 with the rate of warming accelerating since the mid-1970s. At the same time, freezing level elevations have risen by about 150 meters (500 feet) and winter chill time, which is essential for many fruit trees to produce flowers and fruit, has decreased. Extreme heat events have also increased in duration and frequency. Our water resources also show signs of stress. Spring snowmelt runoff has decreased, indicating warmer winter temperatures and more precipitation falling as rain rather than snow. Sea levels have also risen over the past century; 8 inches near San Francisco and 6 inches at La Jolla.

Climate Change Vulnerabilities

Extreme Events – Rain, Flood, and Drought

Climate models indicate that we are likely to see larger and warmer storms, resulting in increased rainfall and flooding. The storms, however, may not bring more water overall, leading to more frequent or severe droughts.


Changes in precipitation, reduced snowpack, and more frequent droughts are likely to increase the demand on groundwater sources, risking overdraft, ground subsidence, and decreased water quality.

Sea Level Rise

Sea level rise in California not only threatens coastal communities, but also the health of the Sacramento-San Joaquin Delta, the heart of the California water supply system and the source of water for 25 million Californians and millions of acres of prime farmland. Sea level rise in California could lead to flooding of low–lying areas, loss of coastal wetlands, saltwater contamination of drinking water, impacts on roads and bridges, and increased stress on levees. It may also require increased flows to prevent salt-water intrusion into the Bay-Delta system.
Water Supply & Demand

Climate change is expected to impact our supply and demand for water in critical and non-complimentary ways. Earlier and decreased runoff can reduce water supplies, even when overall rainfall remains the same. This trend could mean less water available for agriculture, the environment, and a growing population. Decreased snowpack is a critical concern. Warmer temperatures will lead to higher snow levels and cause what snow we do get to melt faster and earlier, making it more difficult to store and use. This loss of snowpack means less water will be available for Californians during the hot summer months and growing seasons. At the same time, water demand is expected to grow as higher temperatures and a longer growing season increase the demand for water.

Water System Operations
Changes in precipitation patterns will require modifications in how we operate and manage our dams and reservoirs. Past patterns can no longer be used to confidently forecast the future and we may need to release more water to prevent flooding and not be able to recoup it through spring runoff. Hydroelectric operations may become less reliable, while higher temperatures increase the demand for electricity for air conditioning.

Variable precipitation and increasing temperatures may affect agricultural crops by reducing winter chill-hours, increasing extreme heat days, increasing evapo-transpiration, and reducing water supply.
Ecosystem Functioning
Forests face risks from increased pests, disease, changes in species composition, and fire. In our rivers, increased water temperatures may hurt native fish, especially salmonids.

Responding to Climate Change

California is a national leader in addressing climate change – in response to the threat, the State has enacted legislation, regulations and executive orders, which put the state on track to reduce our heat-trapping gas emissions to 40% below 1990 levels by 2030.

Reducing carbon emissions can help slow the rate of climate change. DWR's Climate Change program guides the Department in its efforts to improve energy efficiency, fuel efficiency, agricultural methods, and water conservation. Taking steps now to prepare for and adapt to climate change will protect public health and safety, our economy, and our future.

DWR Climate Change Program