Extreme Heat in California

As climate change warms the state, Californians will experience a sharp rise in extreme heat days, multiplying the daily risks to our communities and critical infrastructure. Days where the temperature reaches 95°F or higher pose a danger to outdoor workers¹, and days above 100°F are more likely to result in power outages from electrical equipment exposed to high temperatures combined with increased electricity demand from air conditioning². Climate models project that these extreme heat days will become more frequent across California, with cities in the Central Valley like Sacramento, Fresno, and Bakersfield experiencing an additional 30 days per year above 95°F by the end of the century compared to the early 2000s.

The coastal regions of California generally don’t experience temperatures as high as inland areas due to temperature moderation from the ocean, but even these areas will need to adapt to a new range of conditions under climate change. Temperatures that were previously only reached on the 3-4 hottest days of the year in coastal cities like Santa Barbara are projected to occur up to 10-12 times per year on average by the end of the century. These changes can have significant impacts in these coastal regions where buildings, infrastructure, and ecological communities have not adapted for frequent high temperatures.

The most severe impacts from extreme heat in California result from prolonged heat waves. Each day that a heat wave persists significantly compounds the risks to human health^{3 4}, the increases in electricity demand⁵ to keep buildings cool, and the chances of wildfires^{6 7}. In Sacramento County where heat waves over 100°F already pose significant stress to the electrical grid and human health, the average length of these events is projected to increase by nearly two days by the end of the century.

These long, intense heat waves will also become more frequent. In Sacramento County, heat waves lasting at least five days with daily temperatures exceeding 100°F have historically occurred about three times a year on average. That number is projected to nearly double by mid-century and to exceed eight per year by the end of the century. Knowing how the likelihood of prolonged extreme heat waves will change in the coming decades helps planners and policymakers prioritize investments in a resilient electrical grid and public safety measures.

The impact of extreme heat is multiplied when it is coupled with warm nighttime temperatures that don’t allow people, animals, buildings, or critical electrical infrastructure to cool off overnight before another hot day. These warm night conditions can raise the mortality rate of heat waves³, reduce the productivity of crops that depend on nighttime chill hours⁸, increase the failure rate of electrical transformers⁹, and change the dynamics of the electrical grid by driving electricity demand at hours that previously had very low demand¹⁰.

Across the entire state of California, the number of days per year where the minimum temperature remains above 70°F is expected to increase on average by 13 days per year by mid-century, and 24 days per year by the end of the century. The increase in warm nights is particularly high in urban regions where the built infrastructure retains more heat overnight, and in the foothills surrounding the central valley, where warm air from the valley floor is pushed up into the hills and trapped overnight.

For example, San Bernardino is expected to experience an additional 28 days per year above 70°F by mid century and an additional 46 by the end of the century. Explore the map above to see the spatial variability in warm nights across the entire state.

Extreme heat is more dangerous when it isn’t expected. Wildfire preparation, agricultural planning, and community cooling center operations all depend on understanding the timing and seasonality of extreme heat. Climate models show that as climate change progresses, extreme heat events will occur both earlier and later in the year, expanding the seasonal window for extreme heat. Extreme heat events that occur early in the year before humans, animals, and plants have acclimated to the heat can be much more dangerous³. Extreme heat progressing later into the fall can lengthen the wildfire season and exacerbate the effects of Santa Ana wind events on fire risk¹¹. Understanding this extended “extreme heat season” is crucial for adaptation planning across the state.

In Sacramento County, the part of the year where temperatures are likely (more than a 50% chance) to top 95°F historically ran from late June to the start of September. At 2.0°C of global warming, that season starts about two weeks earlier and ends about three weeks later, running from mid-June to late September.

Whenever temperatures rise across the state, so does electricity demand from keeping buildings cool. Planning a resilient electrical system requires anticipating how much the climate will be driving increased demand, and where that demand will be the highest.

Cooling Degree Days (CDD) are a metric used by the energy sector to estimate how hot temperatures will translate into demand for electricity to keep buildings cool. Using a comfort threshold of 65°F, CDDs measure how far the temperature exceeds that threshold and how often those exceedances happen throughout the year. Although it does not account for all the factors that drive electricity demand for cooling, CDDs are an essential metric for forecasting electricity demand across the state.

Across the entire state the projected median increase in annual CDDs is 37% by mid-century. The most impactful increases in temperature driven demand can come from parts of the state with more moderate temperatures that will likely start experiencing more days above 65°F. For example, in the Greater Bay Area Electricity Forecast Zone, CDDs are projected to nearly double by mid-century and triple by the end of the century.

The examples on this page illustrate that it takes more than one chart or metric to tell the full story of extreme heat impacts. Considering extreme heat through multiple perspectives of timescales, thresholds, and seasons is key to understanding the full picture of how a region, community, or industry will be impacted by climate change. For planners examining electricity demand, changes in Cooling Degree Days (CDD) provide a convenient annual summary of increasing demand, while Warm Nights and Extreme Heat Season metrics can help plan for additional demand during off-peak hours and times of the year.

Cal-Adapt is built to enable users to understand the climate impacts to their own community. As this statewide overview demonstrates, each region of California will experience extreme heat differently. For example, while coastal regions will not face the same threat from record high temperatures as inland California, they must plan for persistent heat waves occurring earlier in the year. Use the detailed tools across Cal-Adapt to see how extreme heat is projected to change in your own community.