We've all experienced sweltering summer days that make us want to seek refuge in the air conditioning. But as global warming continues, heat waves become more frequent and intense. And it's not just about higher temperatures on the thermometer. The combination of heat and humidity is the key factor making these heat waves dangerous.

Measuring the Muggy Misery

Scientists have a metric called the heat index that accounts for temperature and humidity to gauge how hot it feels. The heat index represents the temperature that would feel just as oppressive at a standardized humidity level.

It's calculated based on a model of human physiology and how the body copes with heat through sweating and blood flow to the skin. So, while an air temperature of 95°F is brutally hot, if the humidity is also sky-high, the "feels like" heat index could be a stifling 110°F or higher.

An Exponential Increase in Extremes

In a new study, researchers at UC Berkeley examined how the heat index has changed in Texas due to global warming. They found that while average summer temperatures have risen by about 1.5°C (2.7°F) since pre-industrial times, the peak heat index values have skyrocketed much more - increasing on average by 5-6°C (9-11°F)!

In other words, the effects of that seemingly modest 1.5°C temperature rise get amplified into exponentially higher heat index values. That's because the human body has an increasingly difficult time cooling itself through sweat evaporation at higher temperatures and humidity levels.

A Brush With Physiological Limits

The study highlighted just how close some recent Texas heat waves have come to exceeding the limits of human survivability. On July 23, 2023, the Houston area experienced a record heat index of 75°C (167°F)! While that number seems unbelievable, the authors explain that it reflects conditions short of causing heatstroke in healthy adults.

At a heat index of 75°C, a person's skin temperature would reach around 100°F—hazardous but not instantly fatal, thanks to sweat providing evaporative cooling. This is a sobering reminder that some heat waves are already pushing the boundaries of what humans can endure.

Danger Lurking in Humidity

What makes the heat index metric so powerful is how it captures the hazards of humidity that aren't obvious by just looking at the air temperature. For example, on June 20, 2023, in Nacogdoches, Texas, the air temperature was a scorching but survivable 88°F. However, with an oppressive 91% humidity, the calculated heat index was a staggering 140°F!

At those levels, the added 1.5°C of global warming caused an 18°C (32°F) increase in the heat index compared to pre-industrial conditions. The reason? The body struggles to shed enough sweat to avoid heatstroke as atmospheric conditions approach the physiological limit.

A Warning for the Future

While this Texas analysis is just a case study, it highlights an underappreciated risk of global warming - that the heat index is rising several times faster than air temperatures alone. As the authors bluntly state, "Communicating the impact of global warming in terms of changes to the heat index gives the public a more accurate picture of the extent to which global warming has increased heat stress."

So, the next time you see a heat wave forecast, pay close attention to the predicted heat index values and humidity levels. Those two factors combined could mean the difference between an uncomfortable summer day and a life-threatening heat apocalypse. Failing to curb greenhouse gas emissions means many more communities will face the prospect of surviving heat extremes previously thought impossible.

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