Study: Global Warming is Raising the Tropical Wet Bulb Maximum Temperature

Guest essay by Eric Worrall

According to a study, for every degree of warming, peak wet bulb temperature in the tropics also rises by a degree, so even a small amount of warming is a grave threat to human health. But there are these things called thunderstorms which create a non-linear cutoff to maximum temperatures.

Global heating pushes tropical regions towards limits of human livability

Rising heat and humidity threatening to plunge much of the world’s population into potentially lethal conditions, study finds

Oliver Milman
@olliemilman
Tue 9 Mar 2021 03.00 AEDT

The climate crisis is pushing the planet’s tropical regions towards the limits of human livability, with rising heat and humidity threatening to plunge much of the world’s population into potentially lethal conditions, new research has found.

Should governments fail to curb global heating to 1.5C above the pre-industrial era, areas in the tropical band that stretches either side of the equator risk changing into a new environment that will hit “the limit of human adaptation”, the study warns.

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The research team looked at various historical data and simulations to determine how wet-bulb temperature extremes will change as the planet continues to heat up, discovering that these extremes in the tropics increase at around the same rate as the tropical mean temperature.

This means that the world’s temperature increase will need to be limited to 1.5C to avoid risking areas of the tropics exceeding 35C in wet-bulb temperature, which is so-called because it is measured by a thermometer that has its bulb wrapped in a wet cloth, helping mimic the ability of humans to cool their skin by evaporating sweat.

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Read more: https://www.theguardian.com/science/2021/mar/08/global-heating-tropical-regions-human-livability

The abstract of the study;

Projections of tropical heat stress constrained by atmospheric dynamics

Yi Zhang, Isaac Held & Stephan Fueglistaler 

Extreme heat under global warming is a concerning issue for the growing tropical population. However, model projections of extreme temperatures, a widely used metric for extreme heat, are uncertain on regional scales. In addition, humidity needs to be taken into account to estimate the health impact of extreme heat. Here we show that an integrated temperature–humidity metric for the health impact of heat, namely, the extreme wet-bulb temperature (TW), is controlled by established atmospheric dynamics and thus can be robustly projected on regional scales. For each 1 °C of tropical mean warming, global climate models project extreme TW (the annual maximum of daily mean or 3-hourly values) to increase roughly uniformly between 20° S and 20° N latitude by about 1 °C. This projection is consistent with theoretical expectation based on tropical atmospheric dynamics, and observations over the past 40 years, which gives confidence to the model projection. For a 1.5 °C warmer world, the probable (66% confidence interval) increase of regional extreme TW is projected to be 1.33–1.49 °C, whereas the uncertainty of projected extreme temperatures is 3.7 times as large. These results suggest that limiting global warming to 1.5 °C will prevent most of the tropics from reaching a TW of 35 °C, the limit of human adaptation.

Read more (paywalled): https://www.nature.com/articles/s41561-021-00695-3

The claim that maximum wet bulb temperature will rise linearly with global temperature in my opinion is dubious. Unfortunately we can’t see their full study, but in the real world sea surface temperatures exhibit a sharp cutoff. From Willis’ post “Argo and the Ocean Temperature Maximum”.

Since the sea is 70% of the world’s surface, this sharp cutoff has a major impact on global temperatures.

Why is there such a sharp cutoff on sea surface temperature? The reason is if temperatures rise much above 30C, thunderstorms spontaneously form and cool the surface.

This is why the tropical belt wet season is so wet – all those storms are busy pumping excess heat from the surface, punching a hole through the bulk of the greenhouse blanket, and dumping excess heat at the cold edge of space. Anyone who has seen a thunderhead soaring into the sky has personally witnessed water vapour punching a hole through the bottom of the troposphere.

What if you live too far from the ocean or a large body of water to receive much benefit from this ocean cooling effect? In that case, you are not going to experience extreme humidity – so wet bulb temperature will remain at survivable levels. High humidity requires a source of water vapour.

But what if I’m wrong? Thankfully here in Australia we have pioneered a novel solution to surviving extreme heat.

In the tropical far North of Australia, people install solar heating systems for their swimming pools, not to warm the pools, but to run them at night time. These solar “heaters” at night time function as radiators, dumping excess pool water heat into the cooler night air. That way when wet bulb temperatures soar to uncomfortable heights, we have comfortably cool swimming pools available to help maintain a survivable body temperature.

We have also discovered drinking beer for medicinal purposes helps our bodies dump heat by expanding our surface capillary blood vessels, so to get the full benefit of our radiator cooled swimming pools you have to keep a sixpack of beer on ice in a floating cooler box, and drink bottles of beer at regularly intervals.