Wet-Bulb Temperature

If you’re hot, you sweat. Sweat is one way your body controls its temperature. The evaporating sweat cools your body.

Evaporation relies on the surrounding air not already being saturated. If the humidity is high, the surrounding air can’t absorb much more moisture. If it’s too humid your sweat doesn’t evaporate, and your body loses one of its key methods for regulating temperature. If you can’t sweat you can’t cool yourself, and so lower temperatures than you might expect can be deadly if the humidity is high.

When it comes to the dangers of climate change, we not only have to worry about high temperatures, we also have to worry about high temperatures combined with high humidity, aka high wet-bulb temperatures.

We already know a version of this as the “heat index,” which calculates for shady areas. Wet-bulb temperature focuses on direct sunlight temperatures.

“The WetBulb Globe Temperature (WBGT) is a measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation). This differs from the heat index, which takes into consideration temperature and humidity and is calculated for shady areas. If you work or exercise in direct sunlight, this is a good element to monitor. Military agencies, OSHA and many nations use the WBGT as a guide to managing workload in direct sunlight.”

National Weather Service- WetBulb Globe Temperature

Kim Stanley Robinson’s Ministry for the Future starts with a heat wave in Uttar Pradesh, India, made catastrophic due to a high wet-bulb temperature. There is no way to escape for most of the population. There is no reprieve from the weather. The combination of high heat and high humidity kills millions.

Unfortunately, as in much of the book, KSR isn’t really stretching his forecasting muscles here. We’ve already seen multiple high wet-bulb temperature events, and are guaranteed to see more.

The first time I remember hearing about this was in the 1990s during the 1995 Chicago heat wave. Over 700 people died in that combination of heat and humidity. A younger, more naive, and optimistic version of me presumed that such an event would really capture the nation’s attention and people would start taking climate change seriously. LOL.

Robinson may have had a more recent event in mind when writing MftF. 2015 saw about 4,000 people die in India and Pakistan due to high hot-bulb temperatures.

Heat waves by themselves can be catastrophic climate events. High wet-bulb temperatures are just one more way a screwed up biosphere will harm humanity.

One reason the nation didn’t take deaths in Chicago in 1995 more seriously is because it was mostly old, impoverished people who died. Robinson makes this point in MftF. Because the catastrophic climate event he describes in the first chapter overwhelmingly affects the poor (many of the most wealthy were able to flee), it is quickly forgotten by most of the world (but not India, which is an imporant element of the unfolding plot).

For more about wet-bulb temperature affects on humans, see the following: Raymond, Colin, Tom Matthews, and Radley M. Horton. “The Emergence of Heat and Humidity too Severe for Human Tolerance.” Science Advances 6, no. 19 (2020): eaaw1838.

This post is part of a Ministry for the Future annotation project. Some of the ideas/concepts/terms used by Kim Stanley Robinson in MftF I want to incorporate into the the next edition of the Green New Deal book I published with Hillsborough River Press.

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