Overview: Impacts on Physical Systems

Warming temperatures and changing precipitation patterns have altered California’s “physical systems” — the ocean, lakes, rivers and snowpack – upon which the state depends.

Sep 21, 2018

Warming temperatures and changing precipitation patterns have altered California’s “physical systems” — the ocean, lakes, rivers and snowpack – upon which the state depends. Winter snowpack and spring snowmelt runoff from the Sierra Nevada and southern Cascade Mountains provide approximately one-third of the state’s annual water supply.

The amount of water stored in the state’s snowpack — referred to as snow-water content — is highly variable from year to year, ranging from a high in 1952 of about 240 percent of the long-term average to a record low of 5 percent in 2015. Less snowpack accumulates when winter temperatures are warmer because more precipitation falls as rain instead of snow.

The fraction of snowmelt runoff reaching the Sacramento River between April and July has decreased by about 9 percent since 1906. This reduction is influenced by earlier spring warming and more winter precipitation falling as rain. With less spring runoff, less water is available during summer months to meet the state’s domestic and agricultural water demands. These reductions also affect the generation of hydroelectricity, impair cold-water habitat for certain fishes and stress forest vegetation. The latter has consequences for wildfire risk and long-term forest health.

From the beginning of the 20^th century to 2014, some of the largest glaciers in the Sierra Nevada have lost an average of about 70 percent of their area. Reductions ranged from about 50 to 85 percent of each glacier’s area in 1903. Glaciers are important indicators of climate change: winter snowfall nourishes the glaciers, and spring/summer temperatures melt ice and snow. Winter air temperature determines whether precipitation falls as rain or snow, affecting glacier mass gain; summer air temperature affects glacier loss. Glacier shrinkage worldwide is an important contributor to global sea level rise.

Along the California coast, sea levels have generally risen. Since 1900, mean sea level has increased by about 180 millimeters (7 inches) at San Francisco and by about 150 millimeters (6 inches) since 1924 at La Jolla. In contrast, sea level at Crescent City has declined by about 70 millimeters (3 inches) since 1933 due to an uplift of the land surface from the movement of the Earth’s plates. Sea level rise threatens existing or planned infrastructure, development, and ecosystems along California’s coast.

Other indicators of the impacts of climate change on physical systems show that:

Average lake water temperatures at Lake Tahoe have increased by nearly 1°F since 1970, at an average rate of 0.02°F per year. During the last four years, warming accelerated about 10 times faster than the long-term rate. The lake surface warmed faster — almost 0.04°F per year. The warming of Lake Tahoe’s waters can disrupt the lake’s ecosystem by affecting key physical and biological processes.
Coastal ocean temperatures at three sites in California have warmed over the past century. Over 90 percent of the Earth’s observed warming over the past 50 years has occurred in the ocean. Warming sea surface temperatures can alter the distribution and abundance of many marine organisms, including commercially important species. Ocean warming accounts for about half of the sea level rise that has occurred globally over the past century.
Oxygen concentrations at three water depths offshore of San Diego indicate overall decreases as well as low-oxygen events. Declining oxygen concentrations can lead to significant ecological changes in marine ecosystems, including wide-ranging impacts on species diversity, abundance, and marine food webs. Changing ocean chemistry, in concert with changes in temperature, may lead to even greater and more widespread impacts on coastal marine ecosystems.

Updated Information

Since the publication of the 2018 Indicators of Climate Change in California report, updated information is available, as follows:

Lake water temperatures
UC Davis Tahoe Environmental Research Center 2018 - Tahoe State of the Lake: How Clarity, Climate, Forests, and More are Faring at One of the World's Most Famous Lakes
July 26, 2018

Sea level rise, Snowmelt runoff, Snow-water content
California Department of Water Resources 2018 - California Hydroclimate Report, Water Year 2017
August, 2018