Feeling the Heat in the Extremes: Where To Expect Heat Waves in the United States in the Future

In the summer of 2022, the US saw record-breaking heat waves that stressed electricity grids, put millions of the most vulnerable Americans in uncomfortable and occasionally lethal conditions, and triggered a flurry of health alerts and warnings.

Summers that are oppressively hot and humid will occur more frequently if current patterns continue. This is the key finding of a collection of recent climate estimates produced by scientists from the NASA Jet Propulsion Laboratory (JPL) and several universities. Colin Raymond, a researcher at JPL, and colleagues used projections from 20 climate models to determine how much heat stress Americans may experience between 2075 and 2099 on the hottest summer days in relation to recorded norms between 1980 and 2005.

If we assume a high-end emissions scenario and the world's temperature rises by 3°C to 5°C by 2075, the top 1% of summer days for heat stress will occur for a quarter to half of the summer. That's a big adjustment, Raymond continued. "That one day of oppressive heat you recall as the worst summer day may very likely occur on 30 or even 50 days each year by 2075," the author writes.

Utilizing results from the Coupled Model Intercomparison Project 5, the scientists went beyond simple air temperature estimates (CMIP5). To predict future heat stress, they combined heat, humidity, and sun exposure into a statistic known as an environmental stress index (ESI). Their full findings, which were released in the journal Environmental Research Letters, indicate that major changes are likely to occur soon.

Variations in humidity must be taken into account in the analysis, according to Raymond. Compared to dry heat, the human body has more difficulty controlling its temperature in humid heat because sweating and evaporation are less effective. As a result, humid weather may not only be less comfortable but also raise the possibility of heat-related illnesses and deaths. Heat waves are the worst type of natural disaster in the US. They cause at least 100 deaths year on average, and many more are likely to occur, according to data from the Centers for Disease Control and Prevention (CDC).

As a baseline for May to September 1980 to 2005, the top 1% of days on the ESI—a proxy for excessive heat stress—were initially estimated. The same calculations were then done for the years 2074 through 2099, assuming higher levels of greenhouse gases in the atmosphere as well as higher levels of cloud cover, air temperature, humidity, and sunlight reaching the surface. High latitudes, inland areas, and hilly areas—particularly the Upper Midwest, Northeast, and Pacific Northwest—will probably suffer some of the biggest shifts in extreme heat stress, as can be seen in the chart above. Temperatures that are around 5°C (9°F) higher than the baseline period of 1980-2005 may be seen in the top 1% of days for heat stress in some places.

Increases in high heat stress in coastal regions, particularly in California, would be less severe than in other places because the land warms up faster than the waters owing to climate change and sea breezes are fuelled by upwelling ocean water. Higher elevation places warm up more quickly than lower elevation ones due to a change toward drier air, drier soil, and less cloud cover, which is why they exhibit increased heat stress.

It's anticipated that the Gulf Coast states may experience fewer pronounced changes on the hottest days. But because they have less of a range between highs and lows to begin with than more northern states, they will see the biggest proportional increases in the frequency of days with severe heat stress (as seen in the second map). Southern Florida and Texas may suffer significant heat stress days up to 50 times more frequently than the historical norms (1980-2005). The frequency of days with acute heat stress would increase by closer to 20 times in the Pacific Northwest or Upper Midwest. Although the southern and eastern U.S. also witnessed considerable increases in humidity, a rise in temperature was the primary factor behind the nationwide swings in heat stress. Elevation affects which areas will see increases in heat stress.

Dangerous heat was an issue all around the planet in 2022. Parts of Europe, Asia, and the Middle East also endured high heat together with the United States. There have been cases of runways melting and train tracks sinking in the UK. Authorities in Shanghai, China, started dimming the lights on the Bund and in the industries as a result of electrical issues.

This whole thing wasn't a coincidence. "We're seeing more heat waves, and they're getting more powerful," said Gavin Schmidt, director of NASA's Goddard Institute for Space Studies. "And that's because of climate change." "We have raised the planet's temperature by about two degrees Fahrenheit over the previous century or so. As a result, more places are regularly experiencing temperatures above 90 or 100 degrees Fahrenheit (32 or 37 degrees Celsius). Since the beginning, it has grown four, five, or seven times.

By ADAM VOILAND, NASA EARTH OBSERVATORY