Article image

Heat index, how hot it 'feels' outside, is rising faster than actual temperatures

The impact of global warming in the United States is far more severe than previously thought. This is caused, in large part, by the heat index. While an increase of about 3 degrees Fahrenheit due to global warming might seem minimal, the actual “experience” of the heat tells a different story.

The research, centered on Texas’s climate data from the summer of 2023, reveals that the heat index — the measure of how hot it really feels, considering humidity — has risen about three times faster than the actual temperature.

On the most sweltering days, this means Texans are enduring feels-like temperatures between 8 and 11 degrees Fahrenheit hotter than what would be expected without the influence of climate change.

Rethinking the heat index and heat stress

David Romps, a professor of earth and planetary science at the University of California, Berkeley, and the study’s lead author, highlights a significant communication gap in conveying the real dangers of rising temperatures.

Traditional measures, including the heat index itself, fall short of accurately capturing the heat stress experienced by individuals. This discrepancy is primarily caused by not fully accounting for the extremes of temperature and humidity we now face.

Forgetting these factors often leads to a dangerous underestimation of the risks of hyperthermia and heat-related mortality.

The findings from Texas echo a broader, national concern. For instance, Arizona reported a staggering 50% increase in heat-associated deaths from 2022 to 2023, with the majority of these deaths occurring among individuals aged 50 years and older.

Insights for surviving the “new” heat

These statistics underscore the urgent need for action to address the escalating risks posed by climate change. Romps stresses the imperative to halt further warming by ceasing the burning of fossil fuels.

“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 asserts.

“That’s message No. 1, without 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,” he explained.

In the face of these challenges, Romps offers practical advice for coping with the heat, particularly for those without access to air conditioning.

Strategies like seeking shade, using water, and employing fans can significantly aid in preventing hyperthermia. Simple actions, such as wetting one’s skin and staying hydrated, can provide essential relief.

“You can coat yourself in water. Get a wet rag, run it under the faucet, get your skin wet and get in front of a fan. As long as you are drinking enough water and you can keep that skin wetted in front of the fan, you’re doing a good thing for yourself,” Romps advised.

New method to measure the heat index

Romps, also an atmospheric physicist, alongside Yi-Chuan Lu, a graduate student turned postdoctoral fellow, embarked on a crucial journey to redefine our understanding of heat stress in the face of global warming.

Their research pivots around the heat index, a measure devised in 1979 to gauge the stress exerted on the human body by combined heat and humidity.

Despite its longstanding use, Romps identified a critical gap. As mentioned previously, original calculations used for measuring the heat index did not account for the extreme heat and humidity levels we encounter today.

Addressing this gap, Romps and Lu advanced the calculation of the heat index, extending its applicability to encompass even the most severe heatwaves, such as those Texas experienced in the summer of 2023.

True impact of the scorching summer sun

This enhancement reveals that the traditional method of extrapolating heat index values by the National Weather Service (NWS), part of the National Oceanic and Atmospheric Administration (NOAA), significantly underestimates the stress experienced during extreme conditions.

The duo’s findings come from a detailed case study of Texas, chosen due to its alarming heat index values. A particular focus was on Houston’s Ellington Airport, where on July 23, 2023, they recorded a heat index of a staggering 167 degrees Fahrenheit (75 C), with global warming contributing to 12 degrees Fahrenheit (6 C) of this figure.

“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.”

Wet bulb temperature directly impacts survival

This survivability, as Romps points out, hinges on the body’s ability to cool itself through evaporative cooling — a process demanding significant cardiovascular effort due to intense sweating.

The generally accepted threshold for maximum survivable temperature is a “wet bulb temperature” of 35 degrees Celsius, which is equivalent to a skin temperature when sweating of 95 degrees Fahrenheit, close to the average person’s core body temperature.

Their research suggests that while this may not necessarily result in death for young, healthy adults, it does pose a serious risk of 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 explained, expanding on the concept of the wet bulb temperature.

“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,” he emphasized.

Climate change and the new normal

Looking to the future, Romps acknowledges the increasing risk of hyperthermic conditions due to climate change, without attempting to predict specific occurrences.

The study’s broader implication warns of a future where, if current fossil fuel consumption continues, a significant portion of the world’s population could face life-threatening heat stress levels, especially those who are not young and healthy, or those working or exposed to direct sunlight.

In light of these findings, Romps is keen on investigating other regions using the refined heat index scale, expecting to uncover similar trends across the globe.

“If humanity goes ahead and burns the fossil fuel available to it, then it is conceivable that half of Earth’s population would be exposed to unavoidably hyperthermic conditions, even for young, healthy adults,” Romps said.

“People who aren’t young and healthy would be suffering even more, as would people who are laboring or are out in the sun — all of them would be suffering potentially life-threatening levels of heat stress,” he concluded.

Global implications of a hotter heat index

In summary, the research conducted by David Romps and Yi-Chuan Lu serves as a crucial wake-up call to the underestimated impacts of global warming on heat stress, highlighting the inadequacies of current heat index calculations to capture the extremities of today’s climate conditions.

By extending the heat index calculations to cover all possible combinations of temperature and humidity, their work focuses on the severe, yet survivable, conditions experienced in Texas and around the U.S., offering a glimpse into a future where such extremities could become commonplace.

Their eye-opening study emphasizes the urgency for immediate action to curb fossil fuel consumption while advocating for the adoption of innovative strategies to adapt to the unavoidable increases in global temperatures, thereby safeguarding communities against the escalating threat of heat stress.

The full study was published in the journal Environmental Research Letters.


Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and


News coming your way
The biggest news about our planet delivered to you each day