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Temperature: Extreme Heat Tool

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The most serious threats to the public health of Californians will stem primarily from the higher frequency of extreme conditions, principally more frequent, more intense, and longer heat waves. An increase in heat waves may increase the risk of directly related conditions such as heat stroke and dehydration.

To Begin
Simply click on an area of interest on the map or enter an address in the search bar above. You will see the first of six different charts that display different aspects of projected extreme heat events. Each chart is accompanied by a brief description below the tool. Note that you can view the results of both “high” (A2) and “low” (B1) emissions scenarios showing different projected outcomes based on varying assumptions.

What is an extreme heat day?
For the purposes of this tool, an extreme heat day is defined as a day in April through October where the maximum temperature (Tmax) exceeds the 98th historical percentile of maximum temperatures based on daily temperature data between 1961-1990. A heat wave is defined as 5 or more consecutive extreme heat days.

About the Data
The data being displayed in this tool have been provided by Scripps Institution of Oceanography.  The data are the results of modeled simulations, consisting of historical model simulations for 1950-1999 and climate change projections for 2000-2099 using the GFDL CM2.1 Global Climate Model.  Note that model data do not represent actual “observed” values for the historical time period, but rather are coupled model simulations that are run over the historical period.  Thus, the simulations include both a retrospective historical period and the 21st Century projected climate run which is driven by a scenario of hypothetical greenhouse gas emissions. Climatologists compare statistics of the model data for the historical period to gage the model’s ability to simulation recent climate and climate variability.   The relative differences between its projected climate and a model’s historical climate is used to determine characteristics of possible climate change, including timing, magnitude, pattern and rates.

Daily timestep for dates in
Historical Avg. # Extreme Heat Days: - Extreme Heat Day Threshold: - All values based on modeled data


This information is being made available for informational purposes only. Users of this information agree by their use to hold blameless the State of California, and its respective officers, employees, agents, contractors, and subcontractors for any liability associated with its use in any form. This work shall not be used to assess actual coastal hazards, insurance requirements, or property values and specifically shall not be used in lieu of Flood Insurance Studies and Flood Insurance Rate Maps issued by the Federal Emergency Management Agency (FEMA).


The data presented in this tool are projections of future climate. They are not weather predictions and should not be treated as such. Climate projections tell us how weather conditions are likely to change on average, but they cannot predict the weather at a particular day and time. Learn more about what climate models can and cannot tell us.

Although climate models are powerful and effective tools for simulating the climate system, there is some uncertainty inherent in any projection of the future, and climate model projections are no exception. For example, climate model projections illustrate how the climate system is expected to behave under specific scenarios of greenhouse gas emissions. Since our emissions of greenhouse gases depend on a variety of different social, political, and economic factors, we cannot be certain how they will change. Therefore, projected climate data may not prove to be accurate if the actual emissions pathway we follow differs from the scenarios used to make the projections.

Another source of uncertainty in climate projections is the fact that different climate models—the tools used to simulate the climate system and produce future climate data—may produce different outcomes. There are more than 30 global climate models developed by climate modeling centers around the world, and they have different ways of representing aspects of the climate system. In addition, some aspects of the climate system are less well understood than others. Climate scientists are constantly working to improve our theories of the climate system and its representation in climate models. In the meantime, one way to account for model differences is to look at projections from many different models to get a range of possible outcomes. You can then take the average of the values across the different models, and this average value is a more likely outcome than the value from any single model. The default visualizations in this Cal-Adapt are based on the average values from a variety of models. Find out more about climate change data.

What does this mean?

Climate data provided by:

Scripps Institution of Oceanography

California Nevada Applications Program (CNAP)

Data Set Contributed: Projected Daily Temperature

Projected daily temperature data for tmax (high) and tmin (low). These data layers were downscaled using a bias correction and constructed analogues (BCCA) approach and include data for the GDFL climate model for two different scenarios (A2, B1).

Associated Publications

Current Chart
chart type:

To zoom into a time range, click on the graph and drag the mouse to your end date. To zoom back out, double click anywhere on the graph.
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