In the heart of Texas, where the summer sun beats down without mercy, residents are no strangers to extreme heat. However, a recent study sheds light on a concerning trend that may explain a lot of things: the heat index, or how hot it truly feels, is rising at a rate three times faster than actual temperature readings.

On the hottest days, Texans are enduring “feels like” temperatures up to 11°F (6°C) warmer than historical norms. The same is most certainly the case in other places around the world that experience similar extremes in both temperature and humidity.

Understanding the heat index

The heat index was introduced in 1979 as a way to reflect how the human body responds to heat and humidity. By taking into account the effects of both relative humidity and air temperature, the heat index offers a more accurate representation of the thermal stress on our physiology.

The human body cools itself through evaporation by sweating. However, when the air is saturated with moisture (high humidity), the sweat on our bodies doesn’t evaporate as fast, making it difficult for the body to regulate its temperature effectively. This explains why the air temperature feels much warmer than it is.

Although the introduction of the heat index was much appreciated, the way it’s measured hasn’t kept up with the times. It often fails to account for increasingly frequent edge cases during heat waves. This miscalculation can have dire consequences, causing some people to underestimate the risks associated with hyperthermia and heat-related illnesses.

Feels like an oven

David Romps, a professor at the University of California, Berkeley, argues that current methods of calculating this metric fall short. So, he and colleagues have now proposed a new method that accounts for all combinations of temperatures and humidity. They chose Texas as their case study to showcase this revised heat index.

“I picked Texas because I had seen some high heat index values there that made me think, OK, this is a state that this summer is probably experiencing combinations of heat and humidity that are not being captured properly by NOAA’s approximation to the heat index,” Romps said in a press release.

Using their new method for calculating heat index resulted in some shocking finds. During last summer’s heat waves across the state, Romps was stunned that Houston’s Ellington Airport experienced an extraordinary heat index of 75°C (167°F) on July 23, 2023. He attributed 12°F (6°C) of this to global warming.

“It sounds completely insane,” Romps said. “It’s beyond the physiological capacity of a young, healthy person to maintain a standard core temperature. We think it’s hyperthermic but survivable.”

The researchers think that the human evaporation process would have ramped up during these mind-melting days. This process, however, puts additional stress on the heart by requiring it to pump more blood to the skin. At these temperatures, it seems miraculous that a human could survive. But the researchers have considered survivability before.

Previously, Romps and colleagues published a 2023 study that challenged the commonly accepted survivable temperature limit, arguing that a wet bulb temperature of 35°C (95°F) — the temperature a thermometer reads when wrapped in a wet cloth, simulating the effect of sweating — is not necessarily lethal for a fit adult, despite causing hyperthermia.

“Heat index is very much like the wet bulb thermometer, only it adds the metabolic heat that a human has that a thermometer does not have,” Romps said. “We think if you kept your skin wet and you were exposed to 167 degrees, even though we’re approaching something like a setting on the oven, you’d still be alive. Definitely not happy. But alive.”

The implications of rising heat stress

Although Romps’ current research did not forecast when Texas might face heat waves severe enough to induce widespread hyperthermia, he noted that such conditions are becoming more plausible. He intends to extend his analysis to other areas using the refined heat index scale — anticipating finding similar patterns.

Romps warned of the severe consequences of unabated fossil fuel consumption potentially exposing half of the global population to extreme, life-threatening heat stress. This risk is particularly acute for individuals who are not young or healthy, those engaged in physical labor, or those exposed to direct sunlight.

The new findings highlight a broader issue: the increasing challenge of communicating the real dangers of climate change to the public. Most people think that an average global warming increase of 1.5 degrees Celsius (3 degrees Fahrenheit) doesn’t sound like a big deal. However, climate change tends to amplify things to the extreme in all directions. Record-breaking heat waves on land and in the ocean, drenching rains, severe floods, years-long droughts, extreme wildfires, and widespread flooding during hurricanes are all becoming more frequent and more intense.

Numbers we can feel

Moreover, as the new study shows, on very hot summer days, the air outside can feel like an oven — much hotter than your thermometer reads. And this has tangible and severe consequences. Last summer, more than 300 Texans died due to heat, the most since the state began such record-keeping in 1989.

From 2008 to 2019, the US experienced an average of 54 days each summer where the heat index (not this revised one) exceeded 90 degrees Fahrenheit. This increase in extreme heat days correlated with nearly 1,700 additional deaths annually from cardiovascular issues, a study published in the journal Circulation reported.

The research also indicates that if global fossil-fuel development persists and efforts to mitigate climate change remain minimal, the US could face up to 80 days of extreme heat each summer. Under these conditions, heat-related cardiovascular fatalities could more than triple, rising to approximately 5,500 extra deaths per year.

“I mean, the obvious thing to do is to cease additional warming because this is not going to get better unless we stop burning fossil fuels,” Romps said. “That’s message No. 1, without a doubt. We have only one direction we can really be taking the planet’s average temperature, and that’s up. And that’s through additional burning of fossil fuels. So that’s gotta stop and stop fast.”

The new findings appeared in the journal Environmental Research Letters.