U.S. Heat Waves

Pro: Dangerous Heat Waves are Increasing

summer hot weather season high temperature thermometer with city view. 123rf.com

Climate Change is causing an increase in heat waves as the world continues warming to dangerous levels.  Clearly greenhouse gases in the atmosphere add more heat to the system causing more heat waves.

In addition to changes in extreme temperatures alone, studies of heat stress trends over the late 20th to early 21st century found increases across much of the country in the conditions that cause heat stress for humans.

In a study of changes in extreme heat stress conditions at 187 weather stations across the US between 1949 and 2005, 20 percent of the stations had recorded a substantial increase in the number of one-day, extreme heat stress events (relative to local conditions between 1961 and 1990).

On average, these stations recorded 12 more days with extreme heat stress per year in 2005 than they did in 1949. 

The Union of Concerned Scientists: Heat Waves and Climate Change
The future of extreme heat

In the next few decades, if carbon emissions continue to grow, most of the country could see 20 to 30 more days annually with maximum temperatures above 90 degrees Fahrenheit. The Southeast could be hit even harder, potentially enduring 40 to 50 more such days. 

Con: Heat Waves are Not Increasing

Claims of increased U.S. heat wave frequency and length are often misleading because they ignore the worst heat waves the U.S. ever experienced in the 1930s, even though records are publicly available. According to the best state-of-the-art temperature data available, there has been no sustained increase in maximum temperatures in the United States since 2005. When looking at U.S. high temperatures for the entire record since 1895, there is no statistically significant trend that suggests summer maximum temperatures are increasing.   In fact, NOAA performed a “cherry pick” technique by excluding data prior to 1961 to claim a trend in summer high temperatures almost 5 times greater than reality.

A common claim from global warming alarmists is that heat waves are increasing due to changes in the climate. Heat waves are defined as a period of unusually hot weather1 and are highly dependent on the high temperature of the day. When the high temperature data for the United States is examined, there has been no sustained increase in maximum temperatures in the United States since 2005 as seen in Figure 1. The lack of increased high temperatures is documented by the National Oceanic and Atmospheric Administration’s U.S. Climate Reference Network2, an extremely accurate network of temperature stations throughout the United States that requires no corrective adjustments which began reporting nationwide temperature data in 2005.  The CRN is the Gold Standard of surface climate monitoring tools.

Figure 1: Contiguous U.S. Maximum Temperatures Since 2005

Figure 1, source: plot generated online here, accessed June 2020. Data provided by U.S. Climate Reference Network, National Oceanic and Atmospheric Administration, https://www.ncdc.noaa.gov/crn.

The definitive indicator for heat wave severity can be found in the U.S. Annual Heat Wave Index for the last 120 years, using National Oceanic and Atmospheric Administration (NOAA) surface temperature data. As seen in Figure 2, there has been no recent increase in heat wave intensity or duration since the peaks in the 1930’s. This peak occurred well before Carbon Dioxide concentrations in the atmosphere increased to any level of concern.

Figure 2: Heat Index Data Shows No Increased Heat Wave Intensity or Frequency since the 1930’s

Figure 2, Heatwave Index for the contiguous United States, 1895-2015 (no later data is available). The U.S. Annual Heat Wave Index tracks the occurrence of heat wave conditions across the United States. This index defines a heat wave as a period lasting at least four days with an average temperature that would only be expected to occur once every 10 years, based on the historical record. The index value for a given year depends on how often heat waves occur and how widespread they are. Source: Our World in Data, using data from NOAA.

Note that the all-time high temperature records of the “dust bowl” era4 in the 1930’s produced the largest and longest period in the U.S. heat wave index, and not seen since.

Claims of recently increased U.S. heatwaves essentially vanish when all historical data is considered.

When you access NOAA’s own publicly available data for summer high temperatures (June through September, summer is June 21st through September 20th ) you find that NOAA performed a cherry pick4 using only data from 1961 onward, which complete ignores the all-time high temperature records of the “dust bowl” era5 in the 1930’s.

When all of the data is compared the NOAA “cherry picked 1961 onward data” their claim of increased summer heatwaves essentially vanishes and the trend becomes statistically insignificant.

Figure 3: A Comparison of Summer Maximum U.S. Temperature Trends Using ALL available data versus “Cherry Picked” Data from 1961 to 2018.

Figure 3: a comparison of U.S. Contiguous Maximum Temperatures using all of the available data (left) and the “cherry picked” 1961 to 2018 (right). Note that using only data from 1961 onwards creates a nearly 5 times greater high temperature trend of 0.37°F per decade, creating a false impression. The trend using all available data is only 0.08°F per decade, which is statistically insignificant. Chart provided by meteorologist Anthony Watts. Source for NOAA online generated plots are here and here, accessed June 2020


  1. Definition of a heatwave. Oxford Learner’s Dictionary https://www.oxfordlearnersdictionaries.com/definition/english/heatwave
  2. U.S. Climate Reference Network, National Oceanic and Atmospheric Administration, https://www.ncdc.noaa.gov/crn
  3. U.S. heat wave frequency and length are increasing, U.S. Global Change Research Program https://www.globalchange.gov/browse/indicators/us-heat-waves
  4. Cherry Picking, Logically Fallacious https://www.logicallyfallacious.com/logicalfallacies/Cherry-Picking
  5. The Dust Bowl Heatwave of July 1936 National Weather Servicehttps://www.weather.gov/arx/heat_jul36
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