A Look at Extreme Heat Waves in the United States over the Past 70 Years
Here is my brief abstract of the results of the analyses reported in this essay: The 1970s were a period of rapid transition in the mean of the Heat Wave Index over the contiguous United States. While the decadal mean increased over the 70 year span by ~4%/year, ratios of the 2020/1970 minimum and maximum values were found to be increasing by 4.73, and 3.76 fold respectively. Those changes are captured within the four components in the Heat Wave Index: the most influential were Season Length (+ 3.06 fold), Frequency (+ 2.83 fold) and Duration (+ 1.35 fold). To date, 2023 is the warmest year since the beginning of the modern industrial revolution: “Globally, September 2023 was the warmest September — and the largest monthly anomaly of any month — since records began in 1850.”
Quick update, July 22, 2023: from the Washington Post, “The summer of 2023 has featured the most intense heat in modern records averaged over the planet. June was Earth’s hottest on record, and the oceans are exhibiting unprecedented warmth.”
Another more recent update: https://berkeleyearth.org/september-2023-temperature-update/ ‘.
The natural world can be a very hostile place. Humans, together with all organisms, have learned this through brutal experience. Extant species, including H. sapiens, are remnants of the millions of adaptive experiments over these past ~ 4 billion years. One constant that we have recognized is that “ambient temperature” must remain within a range where life is sustainable: given universal internal thermometers, shelter and protective coatings, e.g. clothing suitable for the local environment.
Globally: “…it is estimated that at least 15 000 people died specifically due to the heat in 2022.” The World Health Organisation (WHO) has published guidelines for protecting ourselves during extreme heat episodes. There are many countries today in Asia currently suffering through extreme heat waves: https://en.wikipedia.org/wiki/2023_Asia_heat_wave .
Here in western Canada the memories of the “heat dome” over the Pacific Northwest are literally seared into our memories. Record breaking temperatures up to 49.6 C (121.3 F) followed by horrendous wildfires that decimated many areas in central British Columbia even erasing the village of Lytton.
Extremes: Heat Waves in the United States
The United States had a population of 333,287,557 in 2022. While not the largest in the world, it has the largest economy. Thus, the impacts of climate and local weather extremes can have both huge national and international financial impacts.
The National Oceanic and Atmospheric Administration (NOAA) collects and archives many categories of data. Using those data, the long term “extreme heat index” has been released by the US Environmental Protection Agency (EPA). Since 1979 it is estimated that a total of ~ 11,000 US residents have died of heat related causes.
Heat Wave Trends in the contiguous United States over the past 70 Years
Near surface global temperatures after 1950, post World War II, showed significant increases with the extraordinary post-War industrial expansion driven by fossil fuels. For that reason the graphical analyses presented here include the years 1950–2020, i.e. 70 years inclusively, or 7 decades, shown in Figure 1. Over the 70 year period the average linear increase in HI went from 0.032 to 0.120, thus an increase of 0.088 units, or a 2.75 fold increase in heat waves over 70 years.
Table 1.Statistical summary of the seven (7) decades of Heat Wave Indices in the US, 1950–2020.
The 1970s were a period of rapid transition in the mean, minimum and maximum of the Heat Wave Index: ratios of 2020/1970 for the Mean= 4.45, the Minimum= 4.73, and the Maximum= 3.76.
The US Extreme Heat Index (HI) incorporates these components:
- This index defines a heat wave as a period lasting at least four days with an average temperature that would only be expected to persist over four days once every 10 years, based on the historical record.
There are four (4) components (indicators) that make up the Extreme Heat Index (Table 2):
Table 2. Heat Wave Characteristics in the United States by Decade, 1961–2021. Source: EPA’s Climate Change Indicators in the United States: www.epa.gov/climate-indicators; data source: NOAA, 2022; Web update: July 2022.
Summary
Without doubt there were many undefined covariates associated with “Year” contributing to the strong time element in the 70 year trend. Despite limitations we can list these findings:
- Pearson’s correlation between HI and Year was 0.56, P<0.0001
- The average increase per year in the HI was 0. 0009/year , P<0.0001; ; variation explained by the regression is 31% (r²).
- The linear model mean in 1950 was 0.045, in 2022 it increased 2.76 fold to 0.124. That is an increase of approx. 4% per year.
- The four components (indicators) making up the HI showed strong upward trends over the 7 decades. Ranked in decreasing order of change: Season Length (+ 3.06 fold), Frequency (+ 2.83 fold), Duration (+ 1.35 fold) and Intensity (+ 1.19 fold).
- Heat waves have thus increased most in Length, Frequency and Duration.
- Improving the capacity to forecast the increasing likelihood of extreme Heat Waves must become a part of future planning for public health, reliable energy sources for cooling and overall infrastructure.
- While this brief analysis has focused upon the contiguous 48 states, similar trends are developing at the higher latitudes. The Arctic is now warming at a rate exceeding 4x that at lower latitudes.
Statistical analyses were done using NCSS 2022 software, v22.0.5
Thank you for reading my work. Please feel free to comment.
