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climate change impacts in the Southwest

“There is certainly an indication in the last 10 to 20 years that we are hav- ing those more often and beyond what we thought was statistically probable based on the last 100 to 150 years of historic record that people based their planning on,” he said. For Western communities already acutely aware of the precious value of water, climate change “basically exacer- bates shortage,” said Jones with DWR. Even though the climate of the Southwest is “highly variable across space and over time” because of ocean- land contrasts, mountains and valleys, the position of jet streams, the North American monsoon, and proximity to the Pacifi c Ocean, Gulf of California and Gulf of Mexico, “there is mounting

Impacts on Physical Systems

Climate is a key factor affecting snow, ice and frozen ground, streams, rivers, lakes and the ocean. Regional climate change, particularly warming temperatures, has affected these natural physical systems. • Annual Sierra Nevada snowmelt runoff

Spring snowmelt from the Sierra Nevada to the Sacramento River has declined over the past century. Lower water volumes of snowmelt runoff indicate warmer winter temperatures. More precipitation falls as rain instead of snow and directly fl ows from watersheds before the spring. As a result, the portion of runoff that occurs be- tween April and June has declined by about 9 percent. In addition to its impacts on the state’s water supply, reduced spring runoff can have adverse ecological impacts.

• Snow-water content While no overall trend is dis- cernible in statewide snow-water content (the amount of water stored in snowpack), a decreas- ing trend has been observed in the northern Sierra Nevada, and


an increasing trend in the southern Sierra Nevada. An integral part of California’s water supply, snowpacks store water that is later available to runoff or percolate into soils in spring and summer.

• Glacier change

Glaciers in the Sierra Nevada have decreased in area over the past century, consistent with a world- wide trend in response to a warming climate. A study of seven glaciers found their areal extent in 2004 to range from 22 to 69 percent of their area in 1900. Glacier shrinkage results in earlier peak water runoff and drier summer conditions, and worldwide is an important contribu- tor to global sea level rise.

• Sea level rise

Sea levels measured at stations in San Francisco and La Jolla have risen at a rate of 8 and 6 inches over the century, respectively. Sea level rise in California could lead to fl ooding of low-lying areas, loss of coastal wetlands such as portions of the San Francisco Bay Delta system, erosion of cliffs and beaches, salt- water contamination of drinking water, impacts on roads and bridges

and harmful ecological effects along the coastline.

• Lake water temperature Average water temperatures in Lake Tahoe have risen by nearly 1 degree F in the past 30 years. Warmer waters in Lake Tahoe may be responsible for reduced lake clarity and making condi- tions favorable for certain algae and introduced species. Tempera- ture data derived from satellite observations also show a signifi - cant warming trend since 1992 for summer nighttime temperatures at six lakes in California and Nevada, including Lake Tahoe.

• Coastal ocean temperature Sea surface temperatures at La Jolla have increased by about 1.8 degrees F over the past century at about twice the global rate. Warmer ocean waters contrib- ute to global sea level rise and extreme weather events, and can impact the marine ecosystem and its populations.

Source: California EPA, Indicators of Climate Change in California, August 2013

scientifi c evidence that climate is chang- ing and will continue to change,” ac- cording to Assessment of Climate Change in the Southwest United States. “There is also considerable agreement – at varying levels of confi dence suffi cient to support decision making – regarding why the climate is changing, or will change.” Because of that, water agencies “need to focus more on how they adapt their planning process to the realities of uncertainty rather than wait for climate scientists to somehow fi nally fi gure out how to predict 30 years in the future,” said Groves with the RAND Corpo- ration. Climate scientists “are doing a great job but I think they are best suited to giving us a good range of what it could look like.”

Western Water

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