Heat Stress in the US

Wet bulb globe temperatures as high as 93°F or 34°C are forecast for July 28, 2025 18:00 UTC.

[ Wet bulb globe temperature forecast ]

The above image, adapted from NOAA National Weather Service, shows extreme heat stress for a large part of the U.S. (inset), with a location in Florida highlighted on the main image.  

The forecast for this location is:

[ Extreme heat stress ]

- Wet bulb globe temperature : 93°F or 34°C
- Temperature : 100°F or 38°C
- Apparent Temperature : 116°F or 47°C
- Dew Point : 77°F or 25°C
- Relative Humidity : 50%
- Forecast for : July 28, 2025 18:00 UTC

The image on the right is a same-day forecast for July 28, 2025 18:00 UTC, showing wet bulb globe temperatures as high as 93°F forecast for eight locations on the US East Coast (North and South Carolina and top part of Georgia). 

Heat stress

Heat stress is the leading cause of weather-related deaths in the United States, as illustrated by the image below (credit: National Weather Service).

Numbers of heat fatalities may be conservative. 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 study estimates that extreme heat accounted for 12,000 premature deaths in the contiguous U.S. from 2000 to 2010, and an analysis calculates that the summer 2022 heatwave killed 61,000 people in Europe alone.

Wet-bulb temperature

The human body can cool itself by sweating and the stronger the wind, the more one can cool off by sweating. As temperatures and humidity levels keep rising, a threshold can be reached where the wind factor no longer matters, in the sense that wind can no longer provide cooling. This physiological limit was long described as a 35°C wet-bulb temperature. i.e. once the wet-bulb temperature reaches 35°C, one can no longer lose heat by perspiration, even in strong wind, but instead the human body 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 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.

High temperatures

The image below shows an image, adapted from Climate Reanalyzer, with maximum temperatures forecast as high as 109°F for the three days from July 26, 2025.

[ maximum three-day temperature forecast ]

The image below is adapted from NOAA. 

[ Maximum daily heat index forecast ]

A 96°F or 36°C temperature is forecast for a location near Jackson, Mississippi, US, on July 30, 2025, 18:00 UTC. At a relative humidity of 64%, this corresponds with a wet bulb globe temperature of 94°F or 34°C and a 'feels like' temperature of 121°F or 49°C, as illustrated by the image below.

A 102°F or 39°C temperature is forecast for a location near Texarkana, Arkansas, US, on July 31, 2025, 18:00 UTC. At a relative humidity of 47%, this corresponds with a wet bulb globe temperature of 94°F or 34°C and a 'feels like' temperature of 119°F or 48°C, as illustrated by the image below.

The above forecasts illustrate the huge difference that a higher relative humidity percentage can make.   

Climate Emergency Declaration

The situation is dire and the precautionary principle calls for rapid, comprehensive and effective action to reduce the damage and to improve the situation, as described in this 2022 post, where needed in combination with a Climate Emergency Declaration, as discussed at this group.

Links

• NOAA (National Oceanic and Atmospheric Administration), National Weather Service
https://digital.weather.gov

• NOAA - heat risk graphics
https://www.wpc.ncep.noaa.gov/heatrisk/graphics

• Climate Reanalyzer
https://climatereanalyzer.org

• NOAA Weather Prediction Center, Day 3-7 Heat Index Forecast
https://www.wpc.ncep.noaa.gov/heatindex/heatindex.php

• High feels like temperature forecast (2024) 

https://arctic-news.blogspot.com/2024/08/high-feels-like-temperature-forecast.html

• Wet Bulb Globe Temperature Tipping Point (2023)
https://arctic-news.blogspot.com/2023/07/wet-bulb-globe-temperature-tipping-point.html

• Transforming Society
https://arctic-news.blogspot.com/2022/10/transforming-society.html

• Climate Plan
https://arctic-news.blogspot.com/p/climateplan.html

• Climate Emergency Declaration
https://arctic-news.blogspot.com/p/climate-emergency-declaration.html

Carbon dioxide crosses 422 ppm

Carbon dioxide (CO₂) reached an average daily concentration of 422.06 ppm on April 26, 2022, at Mauna Loa, Hawaii.

Furthermore, very high methane (CH₄) concentrations were recorded recently at Mauna Loa, Hawaii, with surface flask readings appearing to be as high as 1955 ppb.  

Clouds tipping point

A methane concentration of 1955 ppb corresponds, at a Global Warming Potential (GWP) of 200, with a carbon dioxide equivalent (CO₂e) of 391 ppm. Together with the above daily average CO₂ concentration of 422.06 ppm this adds up to a joint CO₂e of 813.06 ppm, i.e. less than 387 ppm away from the clouds tipping point (at 1200 ppm CO₂e) that on its own could raise the global temperature by 8°C.

Such a 387 ppm CO₂e could be added almost immediately by a burst of seafloor methane less than the size of the methane that is currently in the atmosphere (about 5 Gt). There is plenty of potential for such an abrupt release, given the rising ocean heat and the vast amounts of methane present in vulnerable sediments at the seafloor of the Arctic Ocean, as discussed in posts such as this one.

The 1200 ppm CO₂e clouds tipping point could also be crossed even without such an abrupt seafloor methane release. Carbon dioxide and methane levels are rising rapidly. The above image shows carbon dioxide concentration with a trend added, based on NOAA 1980-2021 mean global annual carbon dioxide data, illustrating how carbon dioxide concentration could cross 750 ppm by the end of the year 2029.

[ see also the importance of methane ]

The above image shows methane concentration with a trend added, based on NOAA 2008-2021 mean global annual methane data, illustrating how methane concentration could cross 4100 ppb by the end of the year 2029 and how methane's impact could cross 820 ppm CO₂e by the end of the year 2029.

As illustrated by the image below, 750 ppm carbon dioxide and 820 ppm CO₂e methane would together yield a joint CO₂e of 1570 ppm and thus would have already raised the global temperature by 8°C due to the clouds feedback much earlier than 2029, while the temperature rise would also have been driven up by the higher carbon dioxide and the methane concentrations. 

Furthermore, nitrous oxide is also rising and there are many further forcers, as discussed at the Extinction page. Altogether, there is the potential for a temperature rise of well over 18°C by 2026, as discussed in an earlier post.

Such high carbon dioxide concentrations could occur due to forest fires causing soils to burn (especially peat soils), which can also add vast amounts of methane to the atmosphere. 

The IPCC does contemplate high carbon dioxide scenarios (see image right), but as discussed in an earlier post, does not mention the clouds tipping point.

High carbon dioxide scenarios typically stop at the year 2100 and rarely do concentrations reach higher than 1200 ppm.

In the image on the right, from a 2020 analysis by Malte Meinshausen et al., the SSP5-8.5 scenario is extended to the year 2300 and a carbon dioxide concentration of well over 2100 ppm is reached around 2240.

In conclusion, there is plenty of scientific consideration of the potential for high concentrations of carbon dioxide and methane to eventuate, but it is typically ignored or waved away as too distant in the future to worry about. 

In other words, what's lacking is analysis of abrupt catastrophic climate change.

Climate change danger assessment

The image below expands risk assessment beyond its typical definition as the product of the severity of impact and probability, by adding a third dimension: timescale.

Water in soil and atmosphere

The image on the right, from a news release associated with a recent study, shows changes in atmospheric thirst, measured in terms of reference evapotranspiration from 1980-202 (in mm).

As temperatures rise due to people's emissions, more evaporation will take place over both land oceans, but not all water will return as precipitation, so more water vapor will stay in the air.

[ click on images to enlarge ]

The water-holding capacity of the atmosphere increases by about 7% for every 1°C (1.8°F) rise in temperature, in line with the Clausius–Clapeyron relation. 

In many cases, this means drier soils and vegetation, making vegetation more vulnerable to pests and diseases, and more prone to fire hazards. 

Water in the soil acts as a buffer, slowing down the temperature rise, so drier soil will heat up faster and further, causing land surface temperatures to rise even more and amplifying the impact of Urban heat island and Heat dome phenomena.

The image on the right, adapted from ESA, shows land surface temperatures as high as 65°C (149°F) in India on April 26, 2022. Note that land surface temperatures can be substantially higher than air temperatures. 

As temperatures rise, extreme weather events increase in frequency and intensity. The duration of extreme weather events can also increase, due to blocked weather patterns resulting from changes to the Jet Stream.

This contributes to shortages in food and water supplies. As long as glaciers are melting in the mountains, rivers will keep supplying some water, but the snow and ice cover is disappearing rapidly around the globe. 

The image on the right shows that food prices have risen strongly over the past few years and extreme weather events resulting from the global temperature rise have strongly contributed to the price rise. 

Further contributing to this rise is the rising demand for fertilizers that are currently all too often produced with fossil fuel, as political will to produce food in better ways remains lacking. 

Heat stress

Another issue is humidity. The more water vapor there is in the air, the harder temperature peaks are to bear.

The human body can cool itself by sweating, which has a physiological limit that is often described as a 35°C wet-bulb temperature. 

A 2020 study (by Raymond et al.) warned that this limit could be regularly exceeded with a temperature rise of less than 2.5°C (compared to pre-industrial).

Meanwhile, recent research found that in practice the limit will typically be lower and depending on circumstances could be as low as a wet-bulb temperature of 25°C.

In the video below, Paul Beckwith discusses the danger of combined high heat and humidity. 

In the video below, Guy McPherson also discusses the danger of combined high heat and humidity. 

Extinction

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.

Conclusion

This further highlights the imminence of the danger and adds further urgency to the call for immediate, comprehensive and effective action, as described in the Climate Plan.

Links

• NOAA - Global Monitoring Laboratory, Recent Daily Average CO₂ at Mauna Loa, Hawaii, U.S. 

https://gml.noaa.gov/ccgg/trends/monthly.html

• NOAA - Global Monitoring Laboratory, Methane (surface flasks) at Mauna Loa, Hawaii, U.S. 

https://gml.noaa.gov/dv/iadv/graph.php?code=MLO

• The Importance of Methane
https://arctic-news.blogspot.com/p/the-importance-of-methane-in-climate.html

• Clouds feedback and tipping point

https://arctic-news.blogspot.com/p/clouds-feedback.html

• NOAA - Globally averaged marine surface annual mean carbon dioxide data
https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_annmean_gl.txt

• NOAA - Globally averaged marine surface annual mean methane data
https://gml.noaa.gov/webdata/ccgg/trends/ch4/ch4_annmean_gl.txt

• NOAA - Mauna Loa CO2 weekly mean and historical comparisons
https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_weekly_mlo.txt

• Methane rise is accelerating

https://arctic-news.blogspot.com/2022/03/methane-rise-is-accelerating.html

• Runaway temperature rise by 2026?

https://arctic-news.blogspot.com/2022/04/runaway-temperature-rise-by-2026.html

• Extinction
https://arctic-news.blogspot.com/p/extinction.html

• Shortcomings of IPCC AR6 WGIII - Mitigation of Climate Change
https://arctic-news.blogspot.com/2022/04/shortcomings-of-ipcc-ar6-wgiii-mitigation-of-climate-change.html

• NOAA Mauna Loa CO₂ annual mean data
https://gml.noaa.gov/ccgg/trends/data.html

• NOAA globaly averaged marine surface annual mean methane data
https://gml.noaa.gov/ccgg/trends_ch4

• Is the IPCC creating false perceptions, again?

https://arctic-news.blogspot.com/2021/08/is-the-ipcc-creating-false-perceptions-again.html

• The shared socio-economic pathway (SSP) greenhouse gas concentrations and their extensions to 2500 - by Malte Meinshausen et al. 
https://gmd.copernicus.org/articles/13/3571/2020

• Clausius–Clapeyron relation

https://en.wikipedia.org/wiki/Clausius–Clapeyron_relation

• Urban heat island
https://en.wikipedia.org/wiki/Urban_heat_island

• Heat dome

https://en.wikipedia.org/wiki/Heat_dome

• ESA - Heatwave across India
https://www.esa.int/ESA_Multimedia/Images/2022/04/Heatwave_across_India

• Evaporative Demand Increase Across Lower 48 Means Less Water Supplies, Drier Vegetation, and Higher Fire Risk
https://www.drought.gov/news/evaporative-demand-increase-across-lower-48-means-less-water-supplies

• A Multidataset Assessment of Climatic Drivers and Uncertainties of Recent Trends in Evaporative Demand across the Continental United States - by Christine Albano et al.

https://journals.ametsoc.org/view/journals/hydr/23/4/JHM-D-21-0163.1.xml

• It could be unbearably hot in many places within a few years time
https://arctic-news.blogspot.com/2016/07/it-could-be-unbearably-hot-in-many-places-within-a-few-years-time.html

• The emergence of heat and humidity too severe for human tolerance - by Colin Raymond et al.
https://www.science.org/doi/10.1126/sciadv.aaw1838

• Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project) - by Daniel Vecellio et al.
https://pennstate.pure.elsevier.com/en/publications/evaluating-the-35c-wet-bulb-temperature-adaptability-threshold-fo

• Co-extinctions annihilate planetary life during extreme environmental change, by Giovanni Strona and Corey Bradshaw (2018)
https://www.nature.com/articles/s41598-018-35068-1

• Jet Stream
https://arctic-news.blogspot.com/p/jet-stream.html

• When Will We Die?
https://arctic-news.blogspot.com/2019/06/when-will-we-die.html

• Climate Plan
https://arctic-news.blogspot.com/p/climateplan.html