U.S. Heat Waves are Getting Worse • Watts Up With That?

Pro: Dangerous Heat Waves are Increasing

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 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.
Source: The Union of Concerned Scientists: Heat Waves and Climate Change

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

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

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 21^st through September 20^th ) 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.