Climate Change and Deadly Humid Heat

by Daniel Brouse

The greatest health risk from climate change to those alive today is the increased moisture content in the air (relative humidity) combined with the increased temperatures. Climate change and rising temperatures significantly increase the humidity in the atmosphere due to the relationship between temperature and the air's capacity to hold moisture. The Clausius-Clapeyron equation describes this relationship, indicating that for every 1C (1.8F) increase in temperature, the air can hold about 7% more water vapor.

A Wet-bulb measures the temperature read by a thermometer covered in a wet cloth. As water evaporates from the cloth, evaporation cools the thermometer. This mirrors how the human body cools itself with sweat. The higher the heat and humidity (heat index), the harder it is for sweat to evaporate. The study Adaptability Limit to Climate Change Due to Heat Stress found that a wet-bulb temperature of 35C (95F) at 100% humidity, or 115F at 50% humidity, would be the upper limit of safety, beyond which the human body can no longer cool itself by evaporating sweat from the surface of the body to maintain a stable body core temperature.

When the "wet bulb" temperature reaches a temperature too hot for humans to sweat, it is referred to as the "wet-bulb temperature threshold" or simply the "wet-bulb limit." This limit represents the maximum temperature at which evaporation from a wet surface, such as the skin, can effectively cool the human body through sweating. Beyond this threshold, the body's ability to regulate its internal temperature becomes severely compromised, posing a significant risk to human health and well-being.

When the temperature exceeds the wet-bulb temperature threshold, the human body faces significant risks of heat-related illnesses and potentially fatal heatstroke. Under such conditions, the body's ability to cool itself through perspiration is severely compromised, leading to a rapid rise in core body temperature. As a result, individuals may experience symptoms such as dizziness, fatigue, rapid heartbeat, nausea, and confusion. Without intervention, heat exhaustion can progress to heatstroke, a life-threatening condition characterized by a core body temperature above 40°C (104℉), accompanied by neurological symptoms such as seizures, delirium, and loss of consciousness. Heatstroke requires immediate medical attention and can be fatal if left untreated.

Exposure to temperatures exceeding the wet-bulb threshold poses a grave risk to human health and highlights the urgent need for effective measures to mitigate and adapt to the impacts of climate change, which can exacerbate extreme heat events and increase the frequency of conditions exceeding this critical threshold.

The United Nations DRR says:
For every degree Celsius in warming, the water-holding capacity of the atmosphere increases by about 7%. Record-high sea temperatures ensure there is more moisture (in the form of water vapor) in the atmosphere, by an estimated 5-15% compared to before the 1970s, when global temperature rise began in earnest.

Deadly humid heat affects billions including the US Midwest this century. "It's very disturbing," study co-author Matthew Huber of Purdue University. "It's going to send a lot of people to emergency medical care." The study Greatly enhanced risk to humans as a consequence of empirically determined lower moist heat stress tolerance was conducted by Purdue and George Mason University and published August 15, 2023.

These results indicate that a significant portion of the world's population will experience -- for the first time in human history -- prolonged exposures to uncompensable extreme moist heat. Humans will struggle to adapt to these conditions in a warmer world as they will present widespread challenges across many aspects of food-energy-water security, human health, and economic development including in the world's most populous and most vulnerable regions.

At 3C (5.4F) of yearly average warming, more than 1.5 billion people will suffer. In the summer of 2023, the Earth experienced about a month of warming at 3C above pre-industrial levels. Both 2022 and 2023 saw a record number of heat related deaths. More than 61,000 Europeans died from extreme heat in the summer of 2022.

The average temperature for Brazil had been above the historical average from July through October of 2023. Rio de Janeiro recorded 42.5C on November 12, 2023 (a record for November) and high humidity on the 14th meant that it felt like 58.5C, municipal authorities said. The combination of heat and humidity is the greatest climate change risk to human health. On November 17, "a young Brazilian fan of US singer Taylor Swift died in Rio de Janeiro after falling ill inside the sweltering stadium where the superstar's concert was held, amid a record-breaking heatwave across large swathes of Brazil," as reported by ABC. "The show took place on the same day that Rio recorded its highest-ever heat index reading, which combines temperature and humidity, at 59.3 degrees Celsius (139 degrees Fahrenheit)."

The Water Vapor Feedback Problem
Water vapor is a powerful greenhouse gas. Since the 1970s, its rise likely increased global heating by an amount comparable to that from rising carbon dioxide. We are now seeing the consequences. In the current climate, for average all-sky conditions, water vapour is estimated to account for 50% of the total greenhouse effect, carbon dioxide 19%, ozone 4% and other gases 3%. Clouds make up about a quarter of the greenhouse effect.

The main greenhouse gases -- carbon dioxide, methane, nitrous oxide and ozone -- don't condense and precipitate. Water vapor does, which means its lifetime in the atmosphere is much shorter, by orders of magnitude, compared to other greenhouse gases. On average, water vapor only lasts nine days. The increased intensity of precipitation often results in violent rain.

Sidd said, "The biggest feedback loop is water vapor. Humans put CO2 in the air. CO2 is a greenhouse gas, so the earth gets warmer. Warmer air can hold more water vapor soaking up more water vapor from the oceans. Water vapor is a greenhouse gas, so it gets even warmer... rinse (sorry!) and repeat. Another interesting thing is that the precipitation (rain, snow, sleet) intensity is increasing."

Due to climate change humans will experience greater loss and damage to life and quality of life from air pollution, decreasing supply of potable water, extreme weather events, and disease. The greatest short term climate change risk to human health is deadly humid heat (wet-bulb temperature).

* Our climate model uses chaos theory in an attempt to adequately account for humans and forecasts a global average temperature increase of 9 degrees Celsius above pre-industrial levels. Everybody has the responsibility not to pollute. There are plenty of things you can do to help save the planet. Stop using fossil fuels. Consume less. Love more. Here is a list of additional actions you can take.

Climate Change and Health

What you can do today. How to save the planet.

The Human Induced Climate Change Experiment

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