CALIFORNIA

California, the most populous and third largest state, has a diverse climate. The deserts in the south are some of the hottest and driest areas of the United States, while higher elevations can experience low temperatures and heavy snowfall. The North Pacific High, a semi-permanent high pressure system off the Pacific Coast, and the mid-latitude jet stream play dominating roles in California’s seasonal precipitation patterns. During summer, the North Pacific High and the jet stream move northward, keeping storms north of the state and resulting in dry summers. In winter, this system moves southward, allowing storms to bring precipitation to the state. Due to the moderating effect of the Pacific Ocean, coastal locations experience moderate year-round temperatures while inland locations experience a wider range. Average annual temperatures are less than 40°F at the highest mountain elevations. Average temperatures elsewhere range from less than 50°F in the northeast to greater than 70°F in the southeast. Because of its large north-south extent, and the several mountain ranges, extreme climate events often affect only a portion of the state. For example, strong El Niño events often cause excessive precipitation in southern California, but the effects on northern California are not consistent.

   

Figure 1

Figure 1: Observed and projected changes (compared to the 1901–1960 average) in near-surface air temperature for California. Observed data are for 1900–2014. Projected changes for 2006–2100 are from global climate models for two possible futures: one in which greenhouse gas emissions continue to increase (higher emissions) and another in which greenhouse gas emissions increase at a slower rate (lower emissions). Temperatures in California (orange line) have risen about 2°F since the beginning of the 20th century. Shading indicates the range of annual temperatures from the set of models. Observed temperatures are generally within the envelope of model simulations of the historical period (gray shading). Historically unprecedented warming is projected during the 21st century. Less warming is expected under a lower emissions future (the coldest years being about 2°F warmer than the historical average; green shading) and more warming under a higher emissions future (the hottest years being about 9°F warmer than the hottest year in the historical record; red shading). Source: CICS-NC and NOAA NCEI.

Since the beginning of the 20th century, temperatures have risen approximately 2°F (Figure 1). The years 2014 and 2015 were the first and second warmest, respectively, in the 121-year record. The early 21st century (2000–2009) had the second highest frequency of extremely hot days (maximum temperature above 100°F) in the historical record (Figure 2a), after the 1930s. During the most recent 10 years, the state has also experienced the highest number of very warm nights (minimum temperature above 75°F) on record, and since 1995 a below average number of cold nights (minimum temperature below 20°F) (Figures 3 and 4).

Average annual precipitation varies from less than 2 inches in Death Valley to more than 100 inches near Crescent City in the northwest. Precipitation is also highly variable from year to year, with statewide annual precipitation ranging from 7.93 inches in 2013 to 42.46 inches in 1983, a strong El Niño year. The driest multi-year periods were in the 1920s, 1930s, late 1940s, and late 1980s, and the wettest in the 1900s, early 1940s, early 1980s, and late 1990s (Figure 2b). The driest 5-year period was 1928-1932 and the wettest was 1979-1983. Winter precipitation accounts for about half of total precipitation and has been highly variable (Figure 2c)

One of the most serious climate hazards is flooding. Extreme precipitation episodes resulting in damaging flooding periodically occur. In particular, atmospheric rivers, a weather phenomenon in which a narrow band of very moist air is transported from tropical latitudes of the Pacific Ocean to the west coast, are capable of causing torrential rainfall. From December 1996 to January 1997, heavy rains and snow fell in northern California. The period of December 26–January 3 was particularly severe with some weather stations reporting as much as 25 inches of precipitation. In addition to large rainfall, unusually warm temperatures caused tremendous snowmelt, Lake Tahoe reaching its highest level since 1917. The state experienced massive flooding; some of the most notable locations included the Yosemite Valley (first time since 1861-62), and along the Russian, Klamath, and San Joaquin Rivers. This event was one of a number of extreme precipitation events occurring in the late 1990s, with that period having the highest number in the historical record (Figure 2d).

Drought is another serious climate hazard. Since snowpack is an important element in the management of California’s complex water system, some of the most impactful droughts occur during years of abnormally low snowpack accumulation during the winter months. The historical record indicates periodic occurrences of extended wet and dry periods (Figure 7). Drought conditions can be exacerbated by warm temperatures. The record warmth in 2014 and 2015, in combination with multiple years of below average precipitation (Figure 2b), led to one of the most severe droughts on record for the state.

California is the single most productive agricultural state. The agricultural industry relies heavily on reservoir water supplied by snowmelt and rainfall runoff. Yearly variations in snowpack depths have implications for water availability as snowmelt from the winter snowpack feeds a network of reservoirs. Spring snowpack at Donner Summit reached record low levels in 2014, exceeded in 2015 by a remarkable April 1 snow-water-equivalent value of only 5% of average (Figure 5). Decreased precipitation since 2011 has contributed to near-record low levels in the Shasta Reservoir (Figure 6).

Because summer is the dry season, wildfires are a common occurrence, particularly toward the end of summer. Down slope winds, such as the Santa Ana winds of southern California which can gust to 80 mph, are often associated with the most destructive wildfires. Since they usually occur after the summer dry season when there is ample dry vegetation for fuel, they can cause small fires to quickly burn out of control. These Santa Ana winds have been associated with some of the state’s largest fires, including in October 2003 and October 2007, when more than 800,000 and 1,000,000 acres burned, respectively. In the San Francisco Bay area, the comparable Diablo winds can also be devastating, as evidenced by the Oakland Firestorm of 1991 which killed 25 people and caused over $1.5 billion in damages (in 1992 dollars). The denuding of vegetation by wildfires increases the risks of mudslides and flooding on those areas when heavy rain occurs.

Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways and under the lower pathway a few projections are only slightly warmer than historical records. Overall, warming will lead to increased heat wave intensity but decreased cold wave intensity. Future heat waves could particularly stress coastal communities, such as San Francisco, that are rarely exposed to extreme temperatures and therefore are not well adapted to such events.

Winter precipitation projections range from slight decreases in southern California to increases in northern California, but these changes are smaller than natural variations (Figure 8). Rising temperatures, however, are projected to increase the average lowest elevation at which snow falls, reducing water storage in the snowpack, particularly at those lower mountain elevations which are now on the margins of reliable snowpack accumulation. Higher spring temperatures will also result in earlier melting of the snowpack. The shift in snow melt to earlier in the season is critical for California’s water supply because flood control rules require that water be allowed to flow downstream and that water cannot be stored in reservoirs for use in the dry season.

Naturally occurring droughts are expected to become more intense. Even if precipitation increases in the future, temperature rises will increase the rate of soil moisture loss during dry spells, further reducing streamflow, soil moisture, and water supplies. As a result, wildfires are projected to become more frequent and severe.

Increasing temperatures raise concerns for sea level rise in coastal areas. Since 1880, global sea level has risen by about 8 inches. It is projected to rise another 1 to 4 feet by 2100 as a result of both past and future emissions due to human activities (Figure 9). Sea level rise has caused an increase in tidal floods associated with nuisance-level impacts. Nuisance floods are events in which water levels exceed the local threshold (set by NOAA’s National Weather Service) for minor impacts. These events can damage infrastructure, cause road closures, and overwhelm storm drains. As sea level has risen along the California coastline, the number of tidal flood days (all days exceeding the nuisance level threshold) has also increased, with La Jolla experiencing its greatest number in 2015 and in San Francisco in 1983 (Figure 10). Continued sea level rise will present major challenges to California’s water management system. The Sacramento-San Joaquin Delta is the hub of California’s water supply system. Water from reservoirs in Northern California flows through the Delta where it is then pumped into aqueducts to central and southern California. Sea level rise will cause salty ocean water to intrude into the Delta through San Francisco Bay. This would require increased releases of water from upstream reservoirs to keep the salty water out of the Delta. Water that is used to repel salt flows out into the ocean is no longer available for water supply, reducing the overall amount of water.

Resources

1. Kunkel, K.E, L.E. Stevens, S.E. Stevens, L. Sun, E. Janssen, D. Wuebbles, K.T. Redmond, and J.G. Dobson, 2013: Regional Climate Trends and Scenarios for the U.S. National Climate Assessment. Part 5. Climate of the Southwest U.S., NOAA Technical Report NESDIS 142-5, 79 pp. [URL]
2. Lynn, E., ed., 2015: California Climate Science and Data: For Water Resources Management, Department of Water Resources, State of California, 28 pp. [URL]
3. Melillo, Jerry M., T.C. Richmond, and G.W. Yohe, eds., 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. doi: 10.7930/J0Z31WJ2
4. NOAA, cited 2016: Climate of California, National Oceanic and Atmospheric Administration. [URL]
5. NOAA, cited 2016: Climate at a Glance: U.S. Time Series, published October 2016, retrieved on October 18, 2016, National Atmospheric and Oceanic Administration National Centers for Environmental Information. [URL] Hayhoe, K., D.J. Wuebbles, D.R. Easterling, D.W. Fahey, S. Doherty, J. Kossin, W. Sweet, R. Vose, and M. Wehner, 2018: Our Changing Climate. In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 72–144.
6. NOAA, cited 2016: Stratosphere an Accomplice for Santa Ana Winds and California Wildfires, National Atmospheric and Oceanic Administration National Centers for Environmental Information. [URL]
7. NOAA, cited 2016: Climate of California, National Oceanic and Atmospheric Administration. [URL]
8. NOAA, cited 2016: Climate at a Glance: U.S. Time Series, published October 2016, retrieved on October 18, 2016, National Atmospheric and Oceanic Administration National Centers for Environmental Information. [URL]
9. Pacific Institute, cited 2016: California drought: Impacts and solutions. [URL]
10. Parker, D.R., cited 2016: The Oakland-Berkeley Hills Fire: An Overview. Oakland Office of Fire Services. [URL]
11. Parris, A., P. Bromirski, V. Burkett, D. Cayan, M. Culver, J. Hall, R. Horton, K. Knuuti, R. Moss, J. Obeysekera, A. Sallenger, and J. Weiss, 2012: Global sea level rise scenarios for the United States National Climate Assessment. NOAA Tech Memo OAR CPO-1, 37 pp., National Oceanic and Atmospheric Administration, Silver Spring, MD, 33pp. [URL]
12. Sweet, W., J. Park, J. Marra, C. Zervas, S. Gill, 2014: Sea level rise and nuisance flood frequency changes around the United States: NOAA Technical Report NOS CO-OPS 073, 66 pp. [URL]
13. Vose, R.S., D.R. Easterling, K.E. Kunkel, A.N. LeGrande, and M.F. Wehner, 2017: Temperature changes in the United States. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 267-301.