Extreme Heat: Hotter for Longer
It seems each week this summer brings another news story of broken temperature records and unprecedented heatwaves. In hindsight, however, this is unsurprising as new heat records have become a regular occurrence. Depending on who is compiling the data, July 2024 was either the 14th consecutive month of record-high temperatures or ended a 13-month streak by coming in barely cooler than July 2023.[1] Current temperature trends follow a record breaking year – 2023 was the hottest recorded “by far” since reliable records begin in 1850. Even with several months left, 2024 is on track to beat last year as the hottest on record and is virtually assured a spot as one of the top five hottest years on record. Complicating the picture, average global temperature tells only part of the story. Oceans warm slowly, moderating global temperatures, and temperatures over land are rising faster than the global average. In the U.S. lower 48 States, temperatures have risen by 2.5oF since 1970 (and, since higher latitudes are warming fastest, by 4.2oF in Alaska). This trend of continuously rising temperatures is troubling for those who work or recreate outdoors.
Such a small average global increase seems negligible; most people would hardly notice a difference between 80oF and 82.5oF. Where we really feel rising temperature is in the hot extremes, where maximum temperatures approach or exceed historical records. As average temperatures slowly rise, extreme temperatures are both breaking records and occurring more frequently. Over consecutive days, sustained extreme temperatures create heatwaves. This article explores the science behind heatwave trends, explaining why they are becoming more extreme, occurring more often, and how this affects us.
Historical Trends and Distributions
Two charts, produced by NASA, are useful for understanding temperature change and why it matters for hot extremes. The first shows average daily global temperature for every year since 1980:
Image as of August 2024. Source: NASA Goddard Digital Team.
This chart displays each year’s daily temperature as a continuous line, starting with January on the left and ending with December on the right. Temperature is shown vertically, so cooler temperatures are lower on the chart (note: the big brains at NASA use Celsius; 17oC equals 62.6oF). Record-breaking 2023 temperatures (shown as a thick pink line) are clearly hotter than the preceding 43 years (shown as thin white lines). This year, shown in red and purple,[2] are close to (and sometimes hotter than) 2023 temperatures. In fact, the hottest day ever recorded occurred on July 22, 2024. The recent significant temperature increase since 2023, which exceeded most modeling estimates and can’t be fully explained by natural phenomena, has many scientists wondering if future projections might not be hot enough.
So global average temperatures are rising, and the increase in average temperature has a direct effect on the magnitude and frequency of hot temperature extremes. To understand how, we’ll use another NASA chart (for an animated version and more detailed explanation, click here):
Land surface temperature anomaly, 1962-2022. Source: NASA scientific visualization studio.
This second chart is a bit less intuitive, visualizing global temperature anomalies; that is, how much temperatures in any year differ from a baseline (i.e., historical average) temperature. Cooler temperatures are shown to the left and warmer temperatures to the right (again in Celsius; 4oC corresponds to 7.2oF). The average, or baseline, temperature is shown as “0”, since 0 is neither hotter nor colder than the historical average. The number of times a temperature occurs is shown by the curve, with more frequent occurrences reflected as higher on the chart; average temperatures occur many times (the peak of the curve) while very cold and very hot temperatures occur infrequently (the lower ends, or tails, of the curve).
Understanding this chart provides two important insights:
- Average temperatures are getting warmer. For each of the four years shown, the distribution (i.e., the entire curve) shifts right, indicating overall warming. We can quickly see how much average temperature between years has changed by comparing the curve peaks; between 1962 and 2022, the difference (i.e., increased global land surface temperature) is about 2oF.
- This chart also shows how temperatures are changing. The curve for 2022 hasn’t just shifted right; it is also shorter and fatter than preceding curves, appearing “squished down”. Compared with historical records, average temperatures are occurring less frequently (the curve’s peak in 2022 is lower than the peak in 1962), while extreme hot temperatures are more frequent (shown as the thicker right, “hot” tail of the 2022 curve).
These two charts can be summarized by the following statement: temperatures are rising, and extremely hot temperatures are occurring more frequently than might be assumed as the distribution pattern of temperature changes.
Extreme occurrence of extreme temperatures
Heatwaves are consecutive days of extreme heat. Although there is no formal definition for a “heatwave”, most agree that hot temperatures must last at least two consecutive days (two or three days is commonly used in scientific literature). Heatwaves can be measured according to daily maximum temperature (i.e., the hottest part of a day) or minimum temperature (i.e., the coolest part of the night). While the hottest temperature is an intuitive way to measure a heatwave, health impacts (especially for outdoor workers) often correspond most strongly with elevated overnight temperatures; when hot nights follow hot days, high temperatures when we sleep prevent our bodies from fully recovering.
How temperature is measured also matters. Some heatwave definitions use temperatures above a set threshold while other definitions consider how much the temperature deviates from the local norm. For an example of heatwaves defined on a local scale, consider the severe winter heatwaves in Antarctica over the past couple years. These Antarctic “heatwaves” had temperatures 50oF hotter than normal for that location and time of year but maximum temperatures of only about -4oF. Heatwave warnings in parts of Great Britain start at 77oF; for people living in temperate Scotland and North Ireland where air conditioning is less common, this is justifiable, even if it seems an absurdly low threshold for someone from Miami or Phoenix. On the other hand, the National Weather Service uses fixed thresholds (when heat index exceeds 105-110oF) to issue alerts.[3]
By any definition or location, heatwaves are increasingly common, increasingly severe, and occurring in more places across the globe. Four characteristics help define heatwave trends. Records between 1961-2021 from 50 large U.S. cities provide a good example for considering these characteristics.
- Frequency defines how often heatwaves occur. In the 50-city data set, increased heatwave frequency is found to be “statistically significant” (i.e., not likely to result from random variation). Heatwave frequency in these 50 cities is increasing almost twice as fast when measured by daily minimum temperature (hot nighttime temperatures) compared to maximum temperatures.
- Duration defines how long a heatwave lasts. The data shows average heatwaves in these 50 cities are now more than a day longer than they were in the 1960s.
- Occurrence defines when heatwaves happen. The heatwave season (i.e., the period of the year when heatwaves occur) has grown considerably across the U.S., now lasting about 46 days longer than it did in the 1960s.
- Intensity defines how hot temperatures get during a heatwave. On this characteristic, the average U.S. trend doesn’t necessarily follow global trends (there are some important regional differences, but that’s for another blog). Although the 50-city data set finds an increase in the hottest heatwave temperatures, it’s possible this increase results from natural variation (the trend isn’t “statistically significant”). Measuring heatwaves in these cities by minimum temperature does shows a clear increasing temperature trend, with average minimum heatwave temperature now more than half a degree hotter than in 1960. While a half degree doesn’t seem like much, this increase is on top of already hot temperatures.
Heatwave characteristics for 50 large U.S. cities, as described above. Source: National Oceanic and Atmospheric Administration.
Several indices combine intensity and duration to compare historic heatwaves. One, the heatwave magnitude index, has been used to compare heatwaves globally since the 1980s. Researchers found there has been a threefold increase globally in the area of the world experiencing at least moderate heatwaves, and heatwaves with a magnitude that previously occurred just once every ten years are now likely to occur2.8 times over the same period. Other research, using different data sets and assumptions, also finds increasing heatwave trends in the U.S. For example, Vanos et al looked at warming in eight cities across the U.S. Midwest between 1940-2000, finding an average of one additional severe heatwave occurring each decade, effectively doubling the number of heatwave days in some cities over the 60 years analyzed. Keellings and Moradkhani use an advanced spatial method to study heatwaves across the U.S., finding clear increases in severity, extent, and duration. Across the Northern Hemisphere, Rogers et al found the global area impacted by two or more simultaneous heatwaves increased by 46%, with heatwaves 17% more intense and up to six times more frequent, since 1979.
Heatwave Effects
Heatwaves have wide ranging affects. Agricultural crop yields can drop, often the result of both high temperature and concurrent drought. Economic activity is often reduced as worker productivity declines and consumers avoid leaving air-conditioned home to shop or dine. As an example, Texas’ gross domestic product is estimated to have been reduced by 0.5-1% due to excessive heat in 2023, with declines in job growth greatest for mining and construction– two industries that can’t be moved indoors.
Heatwaves are also deadly. In the U.S., mortality rates increase by nearly 4% during heatwaves compared to non-heatwave days; during the first heatwave of the season, this rate is over 5%. Heat related deaths in the U.S have been steadily increasing, with heat attributed to at least 2,302 fatalities in 2023.
Events this summer in Saudi Arabia provide a grim example of how dangerous heatwaves can be. The 2024 Hajj – an annual five-day pilgrimage to the Saudi Arabian cities of Mecca and Medina performed by Muslim from around the world – coincided with a heatwave. Daily temperatures ranged between 117-120oF, overwhelming safety measures and contributing to many of the at least 1,300 fatalities. As extreme as this event was, other heatwaves have been more deadly. A European heatwave in 2003 has been linked to over 30,000 deaths, while the combination of a 2010 heatwave and associated air pollution in Russia is attributed to more than 55,000 fatalities, with 5,000 more deaths in Moscow alone compared to the same year before.
Conclusions
In recent decades, heatwaves have increased in intensity, occur more frequently, and last longer. These increases are the result of relatively small increases in global average temperature, since small average changes significantly increase the frequency of extreme temperature events. Heatwaves can be deadly and sometimes result in large numbers for fatalities. Understanding the science behind heatwaves and how they are changing across the U.S. and the globe allows better adaptation and preparation.
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About the author: Erik is a doctoral candidate at Duke University where he conducts research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120oF) to Arctic conditions in central Alaska (-42oF).
[1] NASA and NOAA, two U.S. agencies, claim July 2024 was hotter than July 2023, so the monthly high temperature record streak continues (as of August 2024). The European climate agency Copernicus says July 2024 was about 0.01oC cooler than July 2023, so the streak of record setting months was broken. The method each agency uses and associated margins of error matter; for practical purposes, July ‘23 and July ‘24 are statistically “tied”. I’m personally biased towards NASA as an authoritative source- after all, only one of those agencies can claim to have landed a man on the moon!
[2] The different colors for 2024 correspond to different data sets. NASA uses a combined dataset that includes satellite measurements (among others, including thousands of weather stations) to rapidly assess temperature; this is shown as purple. Measurements undergo control checks to ensure only quality data is used, but this takes time. White, pink, and red lines represent temperature measurements from the final, quality-controlled data set.
[3] The threshold value the National Weather Service uses varies by region, so even this fixed threshold method incorporates an element of local relativity.