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.
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.
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.