Global Warming Could Make Life in Tropics Impossible

By Amy Norton
HealthDay reporter

MONDAY, March 8, 2021 (HealthDay News) – Limiting global warming to targets proposed in the Paris Agreement could prevent tropical regions from reaching temperatures beyond human tolerability, according to a new study.

Researchers estimate that if countries are able to cap warming at 1.5 degrees Celsius above pre-industrial levels, the tropics will be spared from temperatures exceeding the “survival limit.” But life in the hottest latitudes of the world could become intolerable if these controls are not respected.

The study focused on a measurement called wet bulb temperature, which takes heat and humidity into account, and is similar to what weather observers call the heat index.

“The general idea is that the body doesn’t just react to temperature, it responds to humidity,” said Kristina Dahl, a climatologist who was not involved in the study.

The body cools down primarily through sweating and the evaporation of sweat from the skin, Dahl explained. At a certain heat-humidity point, she says, it becomes “thermodynamically difficult” for this to happen.


Scientists believe that a wet bulb temperature of 35 degrees C is the upper limit of human tolerance. This is akin to a heat index of 160 degrees Fahrenheit.

People vary in the amount of heat they can take. But at a wet bulb temperature of 35 degrees C, anyone who lingered outside would be in trouble.

The body normally maintains a fairly stable internal temperature of 37 degrees C (98.6 degrees F). The skin temperature should be a little lower to allow the heat from the heart to flow to the skin. If not, a person’s internal temperature could rise rapidly, explained Yi Zhang, the new study’s lead researcher.

“High internal temperatures are dangerous, even fatal,” said Zhang, a graduate student in atmospheric and ocean sciences at Princeton University in New Jersey.

For their study, Zhang and his colleagues made projections on how global warming might affect wet bulb temperatures in the tropics (between 20 degrees north and south of the equator). This includes the Amazon rainforest, much of Africa, the Indian peninsula, and parts of Southeast Asia.


Researchers started with the theory that fairly simple atmospheric dynamics control local wet bulb temperatures in this tropical region. Then they used decades of weather station data to confirm that was the case.

From there, they were able to project that if global warming is limited to 1.5 degrees Celsius, it will prevent “most of the tropics” from reaching intolerable wet-bulb temperatures.

The results were published on March 8 in the journal Geoscience of nature.

Under the Paris Agreement, an international treaty on climate change, the goal is to limit global warming to “well below” 2 degrees Celsius, and preferably 1.5 degrees, above levels pre-industrial.

These latest projections underscore the importance of that, said Dahl, a senior climate scientist at the nonprofit Union of Concerned Scientists.

The results translate a political goal into a potential impact in the real world, Dahl said.

However, avoiding intolerable wet thermometer temperatures doesn’t mean the planet is out of the woods. Human health can certainly suffer under less extreme heat, she noted.


Heat waves regularly cause heat-related illnesses, which are sometimes fatal. Warming also contributes to air pollution, which can exacerbate chronic diseases such as heart and lung disease, Dahl added.

A group of medical organizations called the Medical Society’s Climate and Health Consortium has identified a range of health effects linked to global warming. It can feed insect-borne infections like Lyme and Zika disease, for example, or contaminate food and water supplies by causing sea level rise, heavy rains and flooding.

Zhang said more research was needed to understand any health effects of wet bulb temperatures below the “survival limit” of 35 degrees C. She also noted that the study only looked at countries located between certain latitudes and that the results may not apply to other regions.

What does it take to cap global warming? The short answer, according to Dahl, is less reliance on fossil fuels (oil, coal, and natural gas) and greater use of cleaner energy sources like solar and wind.

Human-made emissions – primarily carbon dioxide, as well as nitrous oxide and methane – have been responsible for the rise in global temperatures since the 1950s. In the United States, most of these emissions come from the burning fossil fuels for energy purposes, according to the US Energy Information Administration.


More information

The World Health Organization has more on climate change and health.

SOURCES: Yi Zhang, graduate student, atmospheric and oceanic sciences, Princeton University, Princeton, NJ; Kristina Dahl, PhD, Senior Climatologist, Climate and Energy Program, Union of Concerned Scientists, Cambridge, Mass .; Nature Geoscience, March 8, 2021, online

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