California

California is 800 miles long from north to south and spans more than 10° of longitude (32.5° to 42° North) extending across a wide gradient of temperatures and precipitation. Temperature records vary from minus 45° (in Boca, 1937) to 134° F (in Greenland Ranch, 1913). The average length of the growing season (above freezing) ranges from 365 days on the southern coast to less than 50 days at the top of the Sierra Nevada. Annual precipitation records range from 161 inches to no measureable rain for more than a year.

The state has over 1,340 miles of coastline along the Pacific Ocean that serves as a current source of cool, humid, sea air during the warm summers and could conceivably be considered as a potential buffer for future regional warming.  Click here to see gallery of warming trend in California.

Western California is dominated by maritime influence with warm winters, cool summers, small daily and seasonal temperature ranges, and high relative humidity. The Coast Range and Sierra Nevada mountain ranges parallel the coast about 50 and 150 miles inland, respectively. Rainfall is heavy on the coast and on the west side of both the Coast Range and the Sierra Nevada but much lighter on the eastern slopes. A continental desert regime prevails east of the mountains with warmer summers, colder winters, large daily and seasonal temperature ranges, and lower relative humidity. The Great Basin extends from Utah to the Sierra Nevada and includes Death Valley and the Mojave Desert, hottest and driest parts of the State. At the southern end of the Sierras, the Tehachapi Mountains join the Coast Range closing off the southern end of the Central Valley where conditions range between these two climatic extremes. Dry “Santa Ana” winds can flow out of the Great Basin into the Central Valley, the Southeastern Desert Basin, and the South Coast and can occasionally lead to serious fire problems.

A dominating factor in the weather of California is the semi-permanent high-pressure area of the North Pacific Ocean. The Pacific high moves north during the warm dry summer, deflecting most Pacific storms to the Pacific Northwest. Occasionally, moist air drifts from the Gulf of Mexico or the Gulf of California causing scattered, locally heavy showers, mostly over the desert and mountains. But often these storms bring lightning strikes that cause fires rather than moisture to mitigate the heat. Northwest winds out of the Pacific high drive the California Current southward almost parallel to the coast causing upwelling as cold water from deeper layers is drawn to the surface. Moist and warmer Pacific air drifting over this cold water form a bank of fog which is often swept inland. The summer fog bank moves inland during summer months, extending at night and receding during the day, but lifts quickly to form low clouds for only a short distance before evaporating completely.

Recent observations have shown a decline in the occurrence of this fog bank along the California coast. Such a trend could greatly affect the habitat suitability for the coastal flora and fauna. The Pacific high decreases in intensity in the winter and moves south, allowing storms to move across the State, producing widespread rain at low elevations and snow at high elevations.

Occasionally a series of storms from the southwest bring heavy rains causing flooding. But typically in Californian summers, there are extended periods with little or no precipitation. The lack of rain only becomes significant when normal winter water supply fails to materialize. Most of the streams flowing west are fed by melting snow from the high slopes of the Sierra Nevada and projected lower snowpacks could greatly affect the seasonality of water availability. Dams hold water supply for irrigation, industrial, and domestic uses in summer and provide flood control during the winter and spring.  Recently dam levels have been low and it is feared those are harbingers of possible future extended droughts. Approximately 90 percent of California’s water supply is used for agriculture. A shortage of irrigation water stored at the beginning of the season in reservoirs is serious, since normal summer precipitation does not provide a sufficient amount of agriculture’s requirements.

In the future, climate change will increasingly interact with and intensify the pressures of a growing population on the natural ecosystems of California. In 2005, the California Climate Change Center initiated the multi-disciplinary “Climate Scenarios” project to analyze potential climate change impacts on different sectors of the state in response to Governor Schwarzenegger’s Executive Order S-3-05 (Cayan et al. 2006). Future climate scenarios were selected from the IPCC Fourth Climate Assessment that provides several simulations generated by General Circulation Models using SRES emission scenarios. Research scientists including James Lenihan (USDA, USFS) used vegetation models to quantify potential impacts of future climate on natural terrestrial ecosystems and found that high elevation alpine and subalpine forest cover would likely decline in the 21st century as snowpacks decreased and temperatures rose. More productive evergreen hardwoods under these warmer conditions could eventually displace the existing pure evergreen conifer forests. Simulations projected an expansion of grasslands at the expense of woodlands and shrublands even under the milder future climate scenarios as increased tree growth was largely offset by increased wildfires. The cooler and less dry scenarios are projected to enhance carbon sequestration while warmer and drier scenarios would likely cause significant carbon losses. The area burned by wildfires across the state was projected to increase under all future scenarios. Click here to see gallery of impacts datasets and maps.


Relevant Links:

• Simulations of climate change impacts in California by Jim Lenihan
• CA climate change portal
• CA Dept of Forestry and fire protection

August 26, 2010

Dominique Bachelet (Conservation Biology Institute)

Photo credit:  Tosha Comendant and Dominique Bachelet (Conservation Biology Institute)