Special report We humans have gorged ourselves on fossil fuels for well over a century, and the bill for that delicious, civilization-enhancing meal has finally come due.

We’re now paying that tab, and doing so while burdened by scalding inflation: Namely a steadily ballooning exchange rate of punishing heatwaves and their tag-along losses of life and raging wildfires.

As top climate scientist Michael Mann of the University of Pennsylvania said when speaking at the American Geophysical Union Conference in Washington DC (AGU24) in December, “We find ourselves at a critical point where we’re starting to see the impacts of climate change play out in the forms of unprecedented extreme weather events that attribution studies tells us simply would not have been as extreme in the absence of human-caused warming.”

Among those extreme weather events to which Mann referred are those aforementioned lethal heatwaves, which are steadily increasing in number, severity, and duration, and which are clearly coupled to climate change as detailed by the United Nations’ Intergovernmental Panel on Climate Change (IPCC), among many others.

“The frequency and intensity of hot extremes (including heatwaves) have increased, and those of cold extremes have decreased on the global scale since 1950,” the IPCC wrote in its 2021 Sixth Assessment Report. “Human-induced greenhouse gas forcing is the main driver of the observed changes in hot and cold extremes on the global scale (virtually certain) and on most continents (very likely).”

But what exactly is a “heatwave”? Well, Martha and the Vandellas were onto something when they expressed doubt about its exact definition. For one thing, a heatwave in Mecca covers a different temperature range than one in, say, the apple of Donald Trump’s eye, Nuuk, Greenland. Your Register correspondent once spent a 20°C (68°F) day in Antarctica, a sub-Antipodean scorcher that his guides referred to as a shocking heatwave.

The United Nations’ World Meteorological Organization (WMO) perhaps describes heatwaves best — although in an open-ended manner — as “a period where local excess heat accumulates over a sequence of unusually hot days and nights.”

So a heatwave could be as pleasant as a balmy day dodging penguins on the Antarctic Peninsula, or as severe as the recent atmospheric blast furnaces in Siberia (2016 and 2020), China (2017), Europe (2018), Australia (2019-20), western North America (2021), India and Pakistan (2022), and Japan (2023) — “severe” meaning not only hotter, but also longer-lasting and over a broader area. These are the heatwaves that kill, and these are the heatwaves that multiple speakers focused on during AGU24.

Today, hot – tomorrow, hotter

Among the 30,000-plus worthies gathered for that scientific soirée was Erich Fischer, a senior researcher at the Institute for Atmospheric and Climate Science, ETH Zürich, in Switzerland, who spoke of recent record-shattering heatwaves and how they “pose serious problems for adaptation and impacts-planning,” how they’re increasing in severity and duration, and how those increases in severity and duration are themselves increasing in frequency.

“Not only are we seeing extreme temperature anomalies, often record-breaking,” Fischer said, “in many cases they even exceed the previous records by very large margins.” Using a topologically unsound metaphor, he added, “Not only are we seeing more sixes on the loaded weather dice, but more sevens under current conditions.”

Heatwave records are being broken at, well, a record pace. In the past two-dozen years, for example, new records have been set “almost everywhere on the globe,” he said. Those records, of course, can be defined using different time scales, but “since 2010, 57 percent of the Earth’s surface has seen all-time daily temperature records,” he added, “and if you look at monthly time scales, it’s way more.”

In 2021, Fischer and his colleagues published a paper in Nature Climate Change, which opened with an abstract that predicted “week-long heat extremes that break records by three or more standard deviations are two to seven times more probable in 2021-2050 and three to 21 times more probable in 2051-2080, compared to the last three decades.” In retrospect, however, those spooky predictions were too conservative. At AGU24, Fischer said, “They came a lot faster than we thought when at the time we were writing that abstract.”

Mark Risser, a research scientist in the Climate and Ecosystem Sciences Division of the Berkeley National Laboratory in California, also at AGU24, agreed — but his work focused not just on further proving that heatwaves are getting hotter and lasting longer, but more specifically on answering the all-important question: “Exactly how much hotter will heatwaves get?”

Spoiler alert: Pretty damn hot. [Cue The Trammps’ Disco Inferno — ed.]

One standard way of predicting heatwave severity has been what are known as IDF curves — intensity-duration-frequency — borrowed from precipitation studies and used to develop the standard heat index method of predicting and warning the public about the dangers of individual heatwaves. However, Risser cites work [PDF] by David Romps and Yi-Chuan Lu which suggests that estimating these curves in today’s increasingly extreme climate can result in an underestimation of actual heatwave risk and intensity.

Risser suggests a more reliable method would be to use a physics-based model that takes into account the fluid dynamics of the Earth’s atmosphere and how convection limits surface temperature, as detailed in a 2023 paper by Yi Zhang and William Boos, published in the Proceedings of the National Academy of Science (often sniggeringly referred to by its acronym, PNAS). Risser used that method to model what he calls a Probable Maximum Temperature (PMT) for the location and conditions under study.

And the area Risser and his team studied was the data-rich Western United States, east from the Pacific Coast to Montana down to New Mexico. Diving further into the weeds to describe their methodology, they based their projections on the widely used data set provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) known as ERA5 (ECMWF Reanalysis v5) of the global climate covering the period from January 1940 to present.

Reanalyses such as ERA5 combine past observations with models to generate consistent time series of multiple climate variables, and provide a comprehensive description of the observed climate as it has evolved during recent decades, downscaled onto 3D grids at sub-daily intervals. This tight, localized historical detail coupled with predictive modeling can produce reasonable estimates of future heatwaves’ severity under low, medium, and high emission scenarios.

Risser was interested in predicting worst-case temperatures — and the numbers he came up with are worse than worst.

These numbers are pretty large so skepticism is reasonable

First, remember that maximum temperatures in parts of the Western United States are already extremely hot, with some areas in the deep southwest having some of the highest temperatures on the planet. During last July, for example, high temperatures in California’s Death Valley averaged — averaged — 122°F (50°C), and topped out at 129°F (54°C) on July 7. In Phoenix, Arizona, high temperatures remained at or above 100°F (38°C) for 133 straight days in 2024 and at or above 110°F (43°C) for 31 straight days in 2023.

Even considering that toasty environment, however, Risser’s worst-case projections are alarming. How alarming? He projects an average annual worst-case event — heatwave — increase over the area under study to be 18°F (10°C) or more by 2070 to 2100, with uncertainty being in the range of 3.5 to 5.7°F (2 to 3.2°C).

That’s pretty damn hot, to be sure, but even more concerning are his projected summer maximums. In a dry-air scenario, his work concludes that by 2100 those worst-case scenarios could see temperatures of 130° to 150°F (54°C to 66°C), with uncertainties in the 1° to 4.5°F (0.5° to 2.5°C) range. And remember, that average is calculated over most of the entire Western United States — in the deep southwest, Risser projects temperatures maxing out at over 170°F (77°C).

These absurdly high temperatures might be dismissed … but at the same time when we’re talking about long-lived infrastructure-focused planning

But, hey, there’s some hope: If you add a cooling waft of humidity to that air, those scorchers can drop by between two to eight degrees Fahrenheit (one to four-and-a-half degrees Celsius). Whoop-de-doo.

Risser stands behind his worst-case projections, but he concedes that they are rather over-the-top. “These numbers are pretty large,” he said at AGU24, “so skepticism is reasonable. But I do think that we have pretty good evidence that temperatures are going to be warming going into the twenty-first century.”

Adding to that last understatement, Risser concluded, “We might be entering a new regime here,” adding that “these absurdly high temperatures might be dismissed because they are so high, but at the same time when we’re talking about long-lived infrastructure-focused planning, long-probability planning, these sort of numbers might be just what we need.”

As recent events in Los Angeles, California are proving, preparing for disasters of “absurdly high” severity might not be a bad idea.

It’s not the heat, it’s the lethality

While both Fischer and Risser pointed out that temperature extremes are becoming more frequent and more severe than previously projected, Sarah Perkins-Kirkpatrick, a professor of climate science at the Australian National University, also speaking at AGU24, brought to the attention of attendees another underestimated heatwave-induced nastiness: Death.

Heat kills. As an August 2024 article in The New York Times stated, “Heat deaths have doubled in the US in recent decades, study finds.” That study published in the Journal of the American Medical Association reports that 21,518 people died from heat in the United States alone since 1999 — and that figure is likely an undercount. What’s more, the annual number of heat-related deaths has increased by 117 percent during that period, with a sharp increase since 2016.

To be sure, cold-related deaths still greatly outnumber those caused by extreme heat. However, according to a 2021 paper published in The Lancet, that gap is shrinking. As the paper points out, “At a global level, the results indicate that global warming might slightly reduce net temperature-related deaths in the short term [by lessening death by cold], although, in the long run, climate change is expected to increase the mortality burden.”

Perkins-Kirkpatrick asserts that as we develop strategies to prepare for the sure-to-rise number of death-dealing heatwaves, we need to get a handle on who is at risk and at what temperatures — and she argues that we have been underestimating the dangers.

Since 2010, planners have been guided by a paper, published by Steven Sherwood and Matthew Huber in PNAS which concluded that “any exceedence of 35°C (95°F) for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible.”

The 35°C cited by Sherwood and Huber is what’s referred to as a “wet-bulb” temperature: A measure of heat effect, sometimes called a heat index, at an idealized 100 percent humidity — essentially a wet rag wrapped around the bulb of a thermometer. Although real-world humidity won’t hit that idealized wet-bulb extreme, the higher the humidity, the more dangerous the temperature, since high humidity inhibits your body’s ability to cool itself by sweating.

For example, although you’d face death by internal organ failure if you had to endure a temperature of 35°C at 100 percent humidity, you’d need to get the actual temperature up to 46°C (115°F) at 50 percent humidity to reach that same 35°C web-bulb death zone.

It’s important to note, however, that the 35°C wet-bulb mortality threshold describes death’s door for your average healthy man or woman. If you are, say, over 65, under four, have coexisting conditions such as obesity or high blood pressure, live in an area with no access to cooling or shade, or if the temperature remains punishingly high for a long period of time, you and your goose will be cooked at a lower — and in some cases much lower — wet-bulb temperature.

As Perkins-Kirkpatrick pointed out, however, that 35°C standard is highly questionable. Other studies that take into account the expertise of physiologists, such as detailed by Vanos et al, 2023, in Nature Communications, lower that 35°C wet-bulb cliff and differentiate among different populations. A December 2024 paper by Fan and McCall in Nature Communications lowers the danger zone even further, asserting that outdoor workers could face “uncompensable heat stress” at a warming level of a mere 2°C relative to preindustrial temperatures — a level our globe is rapidly approaching.

In addition, Perkins-Kirkpatrick accurately notes: “There are also stages of ‘livability’ before binary ‘survivability’.” Translation: Hyperthermia — heat stress — can permanently mess you up without killing you. As it progresses, some of its effects are irreversible — you start to “cook” as cells in your internal organs overheat and burst. Metaphor: If you fry an egg, you can’t bring it back to its pre-breakfast state merely by cooling it.

So when will vulnerable populations be doomed by the the increasing number and severity of heatwaves? Not for awhile, says Perkins-Kirkpatrick, but it depends upon who you are and where you live. She and her team ran projections modeling different wet-bulb danger zones for six different especially vulnerable cities — Mecca, Bangkok, Karachi, Phoenix, Seville, and Mount Isa in Queensland, Australia — for four different groups: Young adults, and folks 65 and over, each both inside and protected from the sun and outside, unprotected.

But back to Perkins-Kirkpatrick and her team. After testing their model on historical data derived from those six cities, thus showing that the model performed well in reproducing a variety of observed danger thresholds for their four target groups at different temperature and humidity levels, the team then chose a 3°C GWL and ran a series of projection probabilities for the same cities at six-hour increments at a range of temperatures and humidities projected at that GWL, and plotted them as a swarm of dots on one graph per city.

What they discovered was sobering:

A scatter plot of future humidity and temperature events in six vulnerable cities shows dangerous times ahead for both young adults and older folks, especially when stuck outside … Source: Perkins-Kirkpatrick et al, 2025

Bottom line: Don’t be old or homeless when living in one of the cities that Perkins-Kirkpatrick studied, especially if your town is heat and humidity-prone.

Also, don’t be poor or disabled during future heatwaves, as Yuki Miura, assistant professor at New York University’s Tandon School of Engineering detailed in her AGU24 presentation, during which she presented a set of model-based projections describing “how future heatwave patterns will exacerbate inequalities, particularly affecting lower-income populations in urban areas with limited access to cooling resources.”

Miura and her team based their projections on all 34 models comprising the industry-standard Climate Model Intercomparison Project-6, better known in the trade as CMIP6. Since CMIP6 is a global model, the team refined and narrowed their study areas using the NASA Earth Exchange Global Daily Downscaled Projections data set, better known as NEX-GDDP, thus creating a combined data set identified by the truly painfully acronym-initialism of NEX-GDDP-CMIP6, pronounced “NEX-gee-dee-dee-pee-SEE-mip-SIX”. But I digress.

Taking the temperatures projected by that model for the years 2030 through 2060, and selecting from that data only those periods described as heatwaves as outlined by the WMO, Miura and her team divided their results into two projected scenarios, a middle-of-the-road heatwave distribution based on low future emissions, and a high-emissions distribution.

To study the socioeconomic impacts of those projected heatwaves in all US counties, with populations and demographics as enumerated by census data from 2020, the team took the median temperature in their heatwave range — 110°F (43.3°C) — and plotted its effect on each county, totalling up the number of people in different socioeconomic and disability groups affected by 110°F heatwaves.

Bottom line: Approximately 196 million people in the US are projected to be exposed to 110°F heatwaves in the 2030-2060 timeframe. Among that group — about 60 percent of the country’s population as of the 2020 census — are 50 million who live below the poverty line and 50 million who have some form of disability, whether it be mobility, sensory, cognitive, or a combination thereof. Each of these latter groups, as proven by multiple studies, are far more vulnerable to heatwave stresses up to and including death than are their financially secure and physically able fellow citizens.

As the old astronautical saying goes, “Houston, we have a problem,” but in this case not only at Mission Control, but also in Los Angeles, Phoenix, Dallas, San Diego, Atlanta, Miami, Washington DC, Boston, and so on, and so on, and so on — and you can reliably substitute your city or continent of choice.

Even when projecting a low-emissions scenario (SSP 245) large chunks of the US will suffer 130°F (54°C) heatwaves in the 2030-260 timeframe. At high emissions (SSP 585)? Guess… Source: Miura et al, NYU

But as Miura pointed out, “Our goal is not just to identify the risk, but also provide actionable solutions.” Her next research mission will be to examine the access to heatwave-mitigation solutions in the affected areas, and the costs of enhancing mitigations such as easy access to air conditioning (AC) in the areas that need help.

Those AC costs won’t be cheap, Miura acknowledges, but what also needs to be taken into account will be the extra energy needed to power those necessary cooling capabilities. Will those AC energy needs be met in such a way as to not exacerbate the climate crisis that is exacerbating the heatwaves in the first place? Or will they engender more fossil-fuel pollution, which will cause more warming, which will require more energy for AC, which will engender more fossil-fuel pollution, which will cause more warming, which will require more energy for AC, which will engender more fossil-fuel pollution, which will … you get the picture: Ouroboros installed as Secretary of Climate Mitigation.

It’s abundantly clear that you, me, and all of our fellow citizens — rich, poor, disabled, whomever — are vulnerable to the dangers of climate-change exacerbated heatwaves. But what about the rest of nature? Y’know, plants, animals, insects, the environment in general? Well, there’s another heatwave-related aspect of climate change that’s not only transforming our Earthly home, but also increasing rapidly: Fire.