Monday, July 31, 2023

Wet Bulb Globe Temperature Tipping Point

High Wet Bulb Globe Temperatures (WBGT) are forecast to hit Louisiana, United States, over the next few days. The image below shows a forecast for August 2, 2023, 18 UTC, with WBGT as high as 35°C forecast for a location 10 miles South East of Abbeville, Louisiana, U.S.

WBGT is a measure used by to warn about expected heat stress when in direct sunlight. It takes into account the effect of temperature, relative humidity, wind speed, and solar radiation on humans.

As temperatures and humidity levels keep rising, a tipping point can be reached where the wind factor no longer matters, in the sense that wind can no longer provide cooling. The human body can cool itself by sweating, which has a physiological limit that was long described as a 35°C wet-bulb temperature. Once the wet-bulb temperature reaches 35°C, one can no longer lose heat by perspiration, even in strong wind, but instead one will start gaining heat from the air beyond a wet-bulb temperature of 35°C.

Accordingly, a 35°C wet-bulb temperature (equal to 95°F at 100% humidity or 115°F at 50% humidity) was long seen as the theoretical limit, the maximum a human could endure.

A 2020 study (by Raymond et al.) warns that this limit could be regularly exceeded with a temperature rise of less than 2.5°C (compared to pre-industrial). A 2018 study (by Strona & Bradshaw) indicates that most life on Earth will disappear with a 5°C rise. Humans, who depend for their survival on many other species, will likely go extinct with a 3°C rise, as illustrated by the image below, from an earlier post.

A 2022 study (by Vecellio et al.) finds that the actual limit is lower — about 31°C wet-bulb or 87°F at 100% humidity — even for young, healthy subjects. The temperature for older populations, who are more vulnerable to heat, is likely even lower. In practice the limit will typically be lower and depending on circumstances could be as low as a wet-bulb temperature of 25°C.

The above image shows a Wet Bulb Globe Temperature of 35°C (95°F) forecast for August 11, 2023, 19 UTC, for a location near Baton Rouge, Louisiana, U.S. 

Heat is the leading cause of weather-related deaths in the United States, as illustrated by the above image (credit: NOAA). Heat fatalities may be conservative figures. Recent research finds that where heat is being listed as an official cause of death, this likely underestimates the full toll of these events. Extreme heat can trigger heart attacks and strokes. In addition, some heart disease risk factors, such as diabetes—as well as heart medications, such as diuretics and beta blockers—can affect a person’s ability to regulate their body temperature and make it difficult to handle extreme heat. The study finds that extreme heat accounted for about 600-700 additional deaths from cardiovascular disease annually. A recent study estimates that extreme heat accounted for 12,000 premature deaths in the contiguous U.S. from 2000 to 2010, and a recent analysis calculates that the summer 2022 heatwave killed 61,000 people in Europe alone. 

The image below shows a temperature (°F) forecast for August 1, 2023, from Climate Reanalyzer

The video below discusses this.

Misery Index

The image below show a high reading on the 'Misery Index', the perceived ('feels like') temperature that is used by, combining wind chill and the heat index (which in turn combines air temperature and relative humidity, in shaded areas). A Misery Index temperature of 56.1°C or 133.1°F was recorded at a location off the coast of the United Arab Emirates (green circle) on August 5, 2023.

The temperature at that location at the time was 35.2°C or 95.4°F, lower than the temperature on the land surrounding the Gulf, but the relative humidity at that spot over the water was 78%, and that combination led to this very high 'feels like' temperature. 

This constitutes a warning. The sea, rivers and lakes are traditionally seen as places to go to, to cool off. However, high temperatures combined with high humidity over water bodies can result in conditions that go beyond what humans can bear. 

Climate change danger assessment

The image below, earlier discussed here, expands risk assessment beyond its typical definition as the product of the severity of impact and probability of occurrence, by adding a third dimension: timescale, in particular imminence.

Imminence alone could make that the danger constituted by rising temperatures needs to be acted upon immediately, comprehensively and effectively. While questions may remain regarding probability, severity and timescale of the dangers associated with climate change, the precautionary principle should prevail and this should prompt for action, i.e. comprehensive and effective action to reduce damage and improve the situation is imperative and must be taken as soon as possible.

Rapidly rising temperatures constitute tipping points in several ways 

Firstly, there is a biological threshold beyond which rising temperatures become lethal for humans, as discussed above. 

Secondly, as Gerardo Ceballos describes in the video below and in a 2017 analysis, there is a biological tipping point that threatens annihilation of species via the ongoing sixth mass extinction. Researchers such as Gerardo Ceballos (2020), Kevin Burke (2018) and Ignation Quintero (2013) have for years warned that mammals and vertebrates cannot keep up with the rapid rise in temperature. Humans are classified as vertebrate mammals, indicating that we will not avoid the fate of extinction, Guy McPherson (2020) adds. 

Thirdly, there are further tipping points, e.g. social-political ones. On the one hand, it would be good if people became more aware, as this could prompt more people into supporting the necessary action. On the other hand, as temperatures keep rising, there is also a danger that panic will break out, dictators will grab power and civilization as we know it will collapse abruptly, as warned about earlier, e.g. in 2007.  


In conclusion, to combat rising temperatures, transforming society is needed urgently, along the lines of this 2022 post in combination with declaration of a climate emergency.


• Wet Bulb Globe Temperature

• National Weather Service - Wet Bulb Globe Temperature: How and when to use it

• The emergence of heat and humidity too severe for human tolerance - by Colin Raymons et al. (2020)

• Brief periods of dangerous humid heat arrive decades early

• Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project) - by Daniel Vecellio et al. (2022)
Discussed at:

• NOAA - Weather Fatalities 2022

• The Effects of Heat Exposure on Human Mortality Throughout the United States - by Drew Shindell (2021)

• Heat-related mortality in Europe during the summer of 2022 - by Joan Ballester et al.
Discussed at:

• As Temperatures Spike, So Do Deaths from Heart Disease (2022 News release)

• Association of Extreme Heat and Cardiovascular Mortality in the United States: A County-Level Longitudinal Analysis From 2008 to 2017 - by Sameed Khatana et al. (2022)

• Co-extinctions annihilate planetary life during extreme environmental change, by Giovanni Strona and Corey Bradshaw (2018)
Discussed at:

• When will we die?

• Climate Reanalyzer - Hourly Forecast Maps

• PBS video - Too HOT and HUMID to Live: Extreme Wet Bulb Events are on the Rise

• Nullschool

• How agriculture hastens species extinction | 60 Minutes (CBS News) | Gerardo Ceballos

• Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines - by Gerardo Ceballos, Paul R. Ehrlich and Rodolfo Dirzo (2017)

• Vertebrates on the brink as indicators of biological annihilation and the sixth mass extinction - by Gerardo Ceballos, Paul Ehrlich, and Peter Raven (2020)
Discussed at:

• Rates of projected climate change dramatically exceed past rates of climatic niche evolution among vertebrate species - by Ignatio Quintero et al. (2013) 

• Pliocene and Eocene provide best analogs for near-future climates - by Kevin Burke et al. (2018)
Discussed at:

• Earth is in the Midst of Abrupt, Irreversible Climate Change - by Guy McPherson (2020)