The most immediate and deadly health risk from climate change is not simply heat--it's the combination of heat and humidity, known as deadly humid heat or wet-bulb temperature. This phenomenon is already threatening lives across the globe and increasingly within the United States.
As temperatures rise, so does the atmosphere's capacity to hold water vapor. The Clausius-Clapeyron equation explains this: for every 1°C (1.8°F) increase in temperature, the air can hold about 7% more moisture. This additional humidity prevents our bodies from cooling through sweat, creating dangerous and potentially fatal conditions.
A wet-bulb temperature is measured using a thermometer wrapped in a wet cloth, mimicking the body's sweat-based cooling. When the air is too humid, evaporation slows or stops, and the body can no longer cool itself. A 2022 study, Adaptability Limit to Climate Change Due to Heat Stress, found that a wet-bulb temperature of 35°C (95°F) at 100% humidity--or even 115°F at 50% humidity--is the upper limit of survivability.
Beyond this threshold, even in the shade and with water, the body begins to overheat. Symptoms include confusion, dizziness, rapid heartbeat, nausea, and ultimately, fatal heatstroke. These effects can occur within hours, and without cooling infrastructure, medical intervention, or access to safe shelter, death is a likely outcome.
The United Nations Office for Disaster Risk Reduction (UNDRR) warns that each degree Celsius of warming increases atmospheric moisture by 7%. Global sea surface temperatures are now at record highs, increasing atmospheric water vapor by 5-15% compared to pre-1970s levels.
A 2023 study by Purdue and George Mason universities, Greatly Enhanced Risk to Humans from Lower Moist Heat Stress Tolerance, projects that 1.5 billion people could be exposed to deadly heat stress at just 3°C (5.4°F) of warming. In summer 2023, the Earth experienced over a month of temperatures above this threshold. Europe saw over 61,000 heat-related deaths in 2022 alone.
In Brazil, the effects were stark: Rio de Janeiro hit a record temperature of 42.5°C (108.5°F) in November 2023. With humidity, the heat index soared to 59.3°C (138.7°F)--lethal even to healthy individuals. A young concertgoer at a Taylor Swift concert in Rio tragically died due to these conditions. This isn't an anomaly--it's a harbinger of the future.
The Water Vapor Feedback Loop
Water vapor is a potent greenhouse gas. Since the 1970s, increases in water vapor have contributed as much additional warming as rising carbon dioxide. In current climate conditions, water vapor accounts for 50% of the total greenhouse effect, followed by carbon dioxide (19%), ozone (4%), and other gases (3%). Clouds account for about 25%.
Unlike CO2 or methane, water vapor has a short atmospheric lifespan--about nine days. But its rapid feedback effect intensifies warming and precipitation. The result: more violent rain events, flash floods, and further infrastructure strain.
As Sidd Mukherjee put it: “The biggest feedback loop is water vapor. Humans release CO2. The planet warms. Warmer air holds more water vapor. Water vapor traps more heat. Rinse (sorry!) and repeat.”
The U.S. and Global Impacts
In the United States, the Midwest and Southeast are particularly vulnerable to future deadly humid heat events. Globally, the world's most populous regions--South Asia, West Africa, and parts of Central America--face growing exposure to heat levels beyond the survivable wet-bulb limit. These are areas with limited access to air conditioning, water, and emergency care.
Compound Climate Risks
Beyond wet-bulb heat, humanity will increasingly face climate-related loss of life and degradation of health due to:
The Short-Term Killer: Wet-Bulb Heat
While the long-term threats of sea-level rise and biodiversity collapse loom, the most immediate and lethal consequence of climate change is wet-bulb heat. It's already killing thousands annually and will increasingly render large parts of the Earth uninhabitable if emissions are not rapidly cut.
Take Action
Our climate model uses chaos theory to reflect nonlinear, human-driven feedback systems and forecasts a global temperature rise of up to 9°C this century. This would make much of the planet inhospitable.
Everyone has a role to play. Stop using fossil fuels. Consume less. Love more. Here is a list of actions you can take now to help mitigate the threat and adapt to our changing climate.
Burning to Stay Cool: How Our Fight Against Heat Is Fueling Climate Collapse
Crossing the Heat Threshold: Wet-Bulb Temperatures Signal a Climate Turning Point
Wet-Bulb Temperatures in the US