FAQ: Climate Change in California

Aerial view of smoke from a wildfire

How will California be impacted by climate change?

California is the most populous state in the United States and a global economic powerhouse. The state, however, has one of the world’s most varied and volatile climates and scientists and public officials are concerned about the impacts that human-caused climate change will inflict upon California. Already subject to drought, wildfires, and extreme weather, California’s environmental and social problems will be exacerbated by a warmer world. Temperatures will continue to rise in coming decades due to greenhouse gases that are accumulating in the atmosphere from transportation, industry, and other human activity. These gases absorb and trap heat radiating from the Earth’s surface, and bolster the atmospheric greenhouse effect that is increasingly warming the planet.

Scripps scientists have contributed regional downscaled modeling, analysis, and organization to a series of California Climate Change Assessments. Besides providing a comprehensive assessment of possible climate change trajectories and outcomes, the Scripps products served numerous other study teams, underpinning an evaluation of effects of climate change at regional and local scales. In California’s recent Fourth Climate Change Assessment, the Scripps team produced an ensemble of climate change information at unprecedented detail that was made available to government agencies and other decision makers in California.


how will temperature change in california?

Average summer temperatures in California have risen by approximately 3 degrees F (1.8℃) since 1896, with more than half of that increase occurring since the early 1970s. If global greenhouse gas emissions continue at current rates, the state is likely to experience further warming by more than 2 degrees F more by 2040, more than 4 degrees F by 2070, and by more than 6 degrees F by 2100. Some of the most impressive impacts of warming will be felt during short period heat events (e.g. days exceeding 106.6 degrees F). For example, if emissions continue at current rates, Fresno will likely suffer 43 extreme heat days per year between 2050 and 2099; 10 times more than its yearly average between 1961 and 2005.

California’s unique landscape and coastal setting will affect the patterns of warming. For instance, Scripps climate researcher Alexander Gershunov has detected a trend in the flavor of California’s heat waves, with particularly strong impacts along the coast. Specifically, he found that some heat waves have become increasingly humid. These events have produced markedly warmer nighttime temperatures, a trend consistent with climate change projections. Moreover, the mid-summer heat waves are getting stronger in generally cooler coastal areas. This has particular importance to the millions of coastal dwelling Californians whose everyday lives are acclimated to moderate temperatures.


could climate change cause more wildfires in California?

Scripps researchers have found that the number of wildfires could grow significantly over the next 40 years. With an increase in summer temperatures, the area burned by wildfires has risen fivefold from 1972 to 2018. Warmer summer temperatures and climate-driven aridity are likely to fuel more wildfires in the future. One Fourth Assessment model predicts that large wildfires (greater than 25,000 acres) could become 50% more frequent by the end of the century if emissions are not reduced, and the average area burned statewide would increase 77 percent.

To respond to this rise in wildfires, a consortium of universities including Scripps Oceanography developed what is now ALERTCalifornia, a backcountry observation network of more than 600 cameras, a now critical firefighting tool helping first responders confirm and monitor wildfires from ignition through containment.

Scripps climate scientist Janin Guzman Morales’ research on Santa Ana winds – traditionally most noted in the fall, when they spread Southern California’s wildfires – found that this wind season is projected to narrow down onto its natural peak in the winter (December-January). Given decreasing projected precipitation in the fall (see below), vegetation is more likely to remain dry into December, when back-to-back Santa Ana winds are common, resulting in expected later and stronger future wildfire season. The Thomas Fire, which burned through most of December 2017 and into January 2018 ,was fanned by consecutive Santa Ana wind events and grew to be the biggest wildfire in California’s history at the time. This very late-season wildfire can be considered a harbinger of future later and greater wildfire activity.


how will climate change affect California's precipitation?

The last two decades underscore California’s strong propensity for wet and dry periods, with a string of multi-year droughts punctuated by a few spectacular wet years. Scripps downscaled global models indicate that, by the mid-21st century, California’s dry years may become drier, wet years occasionally becoming wetter. On top of its already volatile hydroclimate, these precipitation and drought extremes would exacerbate other climate problems confronting the state, both flood- and drought-related.

Two key climate change signals in the hydroclimate of California have been identified: progressively less frequent precipitation, particularly in the fall and spring, and greater precipitation extremes. Although these signals tend to cancel each other out in the annual mean precipitation, they exacerbate the natural volatility of the region’s hydroclimate by increasing reliance on the largest storms of the year to make up the annual total precipitation.

  • Heavy Precipitation Events: In California, the ups and downs of the annual water supply are dictated by the presence or absence of a few large winter storms. In most cases, these extremely wet storms come in the form of long, narrow bands of water vapor known as atmospheric rivers (ARs). ARs are the source of the West Coast’s heaviest rains. ARs are both a hazard and a benefit – they cause most of the West Coast’s floods but they deliver the majority of their rain and snow that is vital for the region’s water supply. Like hurricanes, atmospheric rivers become more damaging the stronger they are. Scripps researchers found that these storms pose a $1 billion-a-year flood risk in the West. Scientists expect that atmospheric rivers will become even more significant flood risk as global warming trends increase their intensity.

    Scientists at Scripps’ Center for Western Weather and Water Extremes study these storms to improve forecasts, assess their potential for producing rain and snow, and advise water managers who operate critical state reservoirs.
  • Drought: Tree rings and other paleoclimate evidence show that the Southwest is prone to megadroughts that can last for decades. Such droughts will likely increase as global temperatures warm and California precipitation becomes more variable. Under warmer temperatures, more moisture evaporates from plants and soil, leading to drier seasonal conditions even in years with historically average precipitation. Seasonal dryness could become prolonged, with soils drying earlier in spring and persisting longer into fall and sometimes winter. Under current water management operations, modeling indicates that the annual volume of water stored in Shasta and Oroville reservoirs, the two largest in the state, could shrink by one-third by the end of the century. This reduced storage could limit water supplies and thus lower resilience to droughts.


how will climate change affect CALIFORNIA snowpack?

Historically, California has relied heavily on the Sierra Nevada snowpack. Runoff from melting mountain snow is captured and distributed throughout the state via an extensive network of aqueducts. Observations over recent warmer decades reveal a decline in California’s lower elevation snowpack, and climate models indicate considerably greater loss of mountain snowpack as temperatures continue to warm. Water managers may not, on average, be presented with less overall precipitation, but more of it will fall as rain instead of snow, and the snow that does manage to accumulate will melt earlier in the spring. Thus, climate change will jeopardize California’s dependence on mountain snowpack as a natural water reservoir which stores water from winter storms and gradually releases it in spring and summer. As spring snowpack diminishes in future decades the state’s water storage capacity will effectively be reduced. Scripps researchers are confronting this storage issue by developing improved weather and streamflow forecast techniques in order to help manage the state’s increasingly variable water supplies. This technology bears on important questions about when and where to store and consume water, issues especially critical to California agriculture.

Scripps researcher Amato Evan found that winters are becoming increasingly shorter in the mountainous western U.S., with snow disappearing earlier in the year. His research showed that fall and spring seasons are becoming longer, essentially shortening winter from both sides. Other studies show that by 2050, the average water supply from snowpack is projected to decline to 2/3 of historical levels. If greenhouse gas emissions reductions do not occur, water from snowpack could fall to less than 1/3 of historical levels by the year 2100.

ARs bring much of the snow to California. Warmer weather and changes in ARs’ intensity could affect how much snow they bring, as well as affecting existing snowpack on the ground. For example, in 2017 Scripps researchers found that warm, wet weather from an AR resulted in a rush of snowmelt – snow that was delivered by an earlier AR – that strained the capacity of California’s second largest reservoir and contributed to the near-failure of Oroville Dam. Scientists stress that more intense storms in the future could continue to threaten public safety and infrastructure.


how will Sea-Level Rise affect california?

By 2050, almost $18 billion worth of residential and commercial buildings could be flooded due to sea level rise, which will increase significantly over historical rates of about 7” over the 20th Century along the Southern California coast. Increased greenhouse gases that have already accumulated in the atmosphere and ongoing climate warming already commit Earth to continued sea level rise. Increases over recent historical levels may reach 1.5 feet if action is not taken to reduce greenhouse gas emissions. Further projections estimate that more than half a million people and $150 billion in property in California will be at risk of flooding by 2100, with the upper range projections of rise reaching or exceeding four feet, almost twice the level projected if greenhouse gas emissions were substantially reduced.

Coastal impacts will be particularly severe when sea level rise is boosted by high astronomical tides and winter storm waves and short term sea level increases. Already, coastal coastal communities in California are already experiencing impacts from rising sea levels. The City of Imperial Beach in San Diego County, for instance, is a low-lying community surrounded by water on three sides, and so especially vulnerable to sea-level rise. The community regularly faces flooding from higher storm surge and extreme tide events during the winter months. To aid Imperial Beach in adaptation planning, the David C. Copley Foundation funded a partnership with Scripps’ Center for Climate Change Impacts and Adaptation on the Resilient Futures program, a monitoring and flood alert program to forecast these inundation events.

A team of researchers at Scripps led by Adam Young is studying coastal erosion in San Diego County to understand how changes in sea level, wave activity, and rainfall will impact the shoreline. The collapse of coastal cliffs is expected to increase with enhanced wave attack at the cliff base due to sea-level rise. These cliffs support homes, business, railways, and roads. This research will become increasingly important as sea levels continue to rise and threaten California’s beaches and coastal communities. The Fourth California Climate Assessment warns that two-thirds of beaches in Southern California could disappear by 2100 if sea levels continue to rise at the same rate.


how will climate change impact CALIFORNIA'S coast and ocean?

California has recently experienced unprecedented events along its coasts including a historic marine heat wave, record harmful algal blooms, fisheries closures, and a significant loss of northern kelp forests. These events increase concern that coastal and marine ecosystems are being transformed, degraded, or lost due to climate change impacts, particularly sea-level rise, ocean acidification, and warming.

“The Blob,” a very warm patch of ocean water off the coast of California that occured from 2013-2016, demonstrated that anomalously warm ocean temperatures can produce unprecedented events, including the mass abandonment of sea lion pups. Additionally, the Shore Stations Program, which has tracked ocean temperature at Ellen Browning Scripps Memorial Pier since 1916, measured record high ocean temperatures in 2018 and 2020.

Scripps postdoctoral researcher Lillian McCormick found that changes in oxygen levels due to a warmer ocean could blind some California marine species, including the commercially-important market squid. Warmer surface water temperatures can decrease mixing in the ocean, which is crucial for transporting oxygen-rich surface waters into deeper depths. Oxygen losses are especially pronounced in areas of naturally occurring low oxygen and upwelling, such as off the coast of California.

The biodiverse kelp forests that define much of the California coast are crucial for fisheries and the stability of the nearshore marine environment. Commercially-important fish like rockfish make their habitat here, and iconic species like sea otters rely on the health of these habitats for their survival. However, warmer waters, especially in Northern California, have decimated huge swaths of kelp forest. Kelp relies on cold water in order to grow and reproduce, and increased ocean temperatures, combined with explosions in kelp-eating sea urchins, have threatened these ecosystems along the coast.


How will climate change impact californians?

From wildfires to extreme heat, Californians will be affected by a variety of climate impacts. Vulnerable populations already experience adverse health impacts from weather extremes. For example, heat waves are the natural disaster responsible for the most weather-related deaths in California and the World over the last 30 years, and scientists predict warmer temperatures will bring more of them. This is already happening and the trend has been clearly observed. The 2006 heat wave killed over 600 people, resulted in 16,000 emergency department visits, and led to nearly $5.4 billion in damages. Research suggests that mortality risk for those 65 or older could increase ten-fold by the 2090s because of climate change. Scientists at Scripps found that heat waves driven by Santa Ana winds can impact hospitalizations for heat-related illness in fall, spring, and winter. The findings stress that heat-related illnesses are not just limited to the summer here, and could be exacerbated by warming temperatures in the future all year round.

Direct climate impacts like wildfire, drought, and flooding will negatively affect public health, along with additional indirect effects. For example, wildfire smoke leads to increased respiratory illness and is more dangerous to human health than similar levels of pollution from other sources, warmer temperatures lead to the spread of mosquito-borne diseases like Zika and West Nile virus, and increased disasters lead to greater stress and mental trauma.


Expert Reviewers:

  • Daniel Cayan, Climate Researcher: Climate, Atmospheric Science & Physical Oceanography Division at Scripps
  • Julie Kalansky, Program Manager and Post-Doctoreal Researcher: Center for Western Weather and Water Extremes at Scripps
  • David Pierce, Climate Researcher: Climate Research Division at Scripps
  • Alexander Gershunov, Climate Researcher: Climate, Atmospheric Science & Physical Oceanography at Scripps


Further reading:


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  1. Guirguis et al., “The Impact of Recent Heat Waves on Human Health in California.” 
  2. Williams et al., “Observed Impacts of Anthropogenic Climate Change on Wildfire in California.” 
  3. Polade et al., “Precipitation in a warming world: Assessing projected hydro-climate changes in California and other Mediterranean climate regions.”
  4. Ralph et al., “A Scale to Characterize the Strength and Impacts of Atmospheric Rivers.” 
  5. Corringham et al., “Atmospheric rivers drive flood damages in the Western United States.” 
  6. Martin et al.,  "Winters becoming shorter in mountainous western U.S."
  7. Henn et al., “Extreme Runoff Generation from Atmospheric River Driven Snowmelt During the 2017 Oroville Dam Spillways Incident.
  8. Young et al., “Three years of weekly observations of coastal cliff erosion by waves and rainfall.” 
  9. McCormick et al., “Vision is highly sensitive to oxygen availability in marine invertebrate larvae.” 
  10. Gershunov et al., “California heat waves in the present and future.” 
  11. Schwarz et al., “The health burden fall, winter, and spring extreme heat events in Southern California and contribution of Santa Ana Winds.”