Research Features

Feeling the Heat in the Extremes

Map of US temperature increase on extreme stress days, 2074-2099 vs 1980-2005


In summer 2022, record-breaking heat waves in California and elsewhere have triggered a stream of health alerts and warnings, strained power grids, and left millions of the most vulnerable Americans sweating through uncomfortable and sometimes deadly conditions.

If trends continue, oppressively hot and humid summers like this one are going to become much more common. That is the key finding from a set of new climate projections conducted by a team of researchers from NASA's Jet Propulsion Laboratory (JPL) and several universities. Colin Raymond, a researcher at JPL, and colleagues used projections from 20 climate models to analyze how much heat stress people across the United States might face between 2075-2099 on the hottest summer days compared to observed norms between 1980 and 2005.

"If we assume a high-end emissions scenario and we end up with a 3°C to 5°C increase in global temperatures by 2075, what has been the top 1 percent of summer days for heat stress will be happening for a quarter to half of the summer. That's a huge difference," said Raymond. "That one oppressively hot day you remember as summer's worst could well be happening on 30 or even 50 days each summer by 2075."

Using downscaled output from the Coupled Model Intercomparison Project 5 (CMIP5), the researchers went beyond simple modeling of future air temperatures. They combined heat, humidity, and exposure to sunlight to project future heat stress by calculating a metric known as an environmental stress index (ESI). Their full results, published in Environmental Research Letters, indicate serious changes ahead.

Raymond considers the incorporation of humidity changes into the analysis to be critical. The human body has more difficulty cooling itself in humid heat compared to dry heat because humid conditions make sweating and evaporation less efficient. That's why humid conditions are not only less comfortable, but perhaps also more likely to contribute to heat-related illnesses and deaths. In the United States, heat waves are the deadliest type of natural disaster, leading to an average of at least 100 deaths per year and likely contributing to many more, according to data from the Centers for Disease Control and Prevention.

Map of US frequency of summer days with extreme heat stress, 2074-2099 vs 1980-2005


The researchers first calculated the top 1 percent of days on the ESI, a proxy for extreme heat stress, averaged for May-September for 1980-2005 as a baseline. They then computed the same for 2074-2099, assuming higher levels of greenhouse gases in the atmosphere and changes to air temperatures, humidity, and sunlight reaching the surface due to changes in cloudiness. As seen in the map above, some of the largest changes in extreme heat stress will likely occur in the high latitudes, inland, and in mountainous areas—particularly in the Upper Midwest, Northeast, and the Pacific Northwest. In these areas, the top 1 percent of days for heat stress could have temperatures that are roughly 5°C (9°F) higher than during the 1980-2005 baseline.

Increases in extreme heat stress in coastal areas, especially in California, would be more moderate than other areas because climate change is causing the land to warm up more quickly than oceans, and upwelling ocean water fuels sea breezes that help disperse stagnant heat and air. Higher elevation areas show amplified heat stress because they are expected to warm up more quickly than lower elevation areas due to a shift toward drier air, drier soil, and less cloud cover.

The Gulf Coast states will likely experience less pronounced changes on the hottest days. But since they have less variability between highs and lows to begin with compared to more northerly states, they will see the greatest relative increases in the frequency of extreme heat stress days (as seen in the second map). Southern Florida and Texas could see extreme heat stress days up to 50 times more often compared to the historical (1980-2005) norms. The increase in the frequency of extreme heat stress days in the Pacific Northwest or Upper Midwest would be closer to 20 times. Rising temperatures were the primary driver of the changes in heat stress throughout the United States, but humidity increases were nearly as important in the southern and eastern U.S. Elevation was also an important factor in which areas will see increases in heat stress.

Dangerous heat wasn't only a problem in the United States in summer 2022. Parts of Europe, Asia, and the Middle East broiled along with the United States. At times, runways melted and train tracks buckled in the United Kingdom. In Shanghai, authorities started dimming lights in the Bund and factories due to power shortages.

None of this was a coincidence. "We're seeing more heat waves, and they're becoming more intense—and that's because of climate change," explained Gavin Schmidt, the director of NASA's Goddard Institute for Space Studies. "We have warmed up the planet by about two degrees Fahrenheit in the last century or so. That is juicing the extremes, so the number of times places are exceeding 90 or 100 degrees Fahrenheit (32 or 37 degrees Celsius) is going up—and not just by a little bit. It's gone up four, five, seven times more than before."

NASA Earth Observatory images by Joshua Stevens, using data courtesy of Raymond, C., et al. (2022). Story by Adam Voiland.

This article was originally prepared as an Earth Image of the Day for the NASA Earth Observatory.