Climate Shift Index: Humid Heat

To compute the Humid Heat CSI, we combine two complementary methods for estimating how climate change has altered the odds of a given wet-bulb temperature occurring at a given location on a given day.

The first method uses nine climate models from Coupled Model Intercomparison Project Phase 6 (CMIP6) run both with and without historical emissions of the heat-trapping gases largely responsible for human-caused climate change. We then calculate and compare the frequencies with which daily wet-bulb temperatures occur in those two modeled climates. This gives us an estimate of how much more or less likely those wet-bulb temperatures are because of human-caused climate change.

The second method is based on daily wet-bulb temperatures calculated from ECMWF’s observation-based ERA5 dataset. In this method, we first calculate how much the wet-bulb temperature in every location on the planet has changed in response to the long-term increase in the global average temperature. Next, we use this known relationship to “remove” the contribution of human-caused climate change from the observed trend in wet-bulb temperatures. As with the model-based method described above, we then compare the frequency of a given wet-bulb temperature in the observed climate and the climate we would have experienced if not for human-caused climate change. Note that Global Forecast System (GFS) forecast simulations of wet-bulb temperatures are also incorporated into our real-time attribution system until ERA5 data become available.

The final value is calculated by averaging the model-based and observation-based methods.

Wet-bulb temperature is a measure combining temperature and humidity that represents the lowest temperature a surface can reach through evaporative cooling. For humans, it tells us how efficiently our bodies can cool themselves through sweating.

The cooling process works only as long as the surrounding air can absorb more moisture. When the air is dry, the air can easily absorb more moisture and our sweat evaporates readily. That evaporation — rather than the sweat itself — is what makes us feel cooler. As humidity increases, the air can’t absorb as much additional moisture, which limits the evaporation of sweat from our skin and the extent to which our bodies can cool themselves.

Under normal conditions, wet-bulb temperature is always lower than the actual air temperature (called the dry-bulb temperature) — the greater the gap between the two, the drier the air. When the air is fully saturated with moisture, evaporation stops entirely and the two temperatures become equal — meaning no further cooling can occur, leaving dangerous levels of heat trapped inside the body.

Wet-bulb temperature is based purely on temperature and humidity: variables that climate models simulate well and that have changes that are more directly attributable to rising greenhouse gas emissions. In contrast, wet-bulb globe temperature factors in sun angle and wind, which makes it useful for monitoring daily heat risks for athletes or outdoor workers. However, those variables are harder to tie directly to climate change, making it difficult to isolate the climate signal.

Wet-bulb temperature also has a clearer connection to human survivability — there is a threshold beyond which the body can no longer cool itself through sweating — than other metrics that combine temperature and humidity. Far more physiological research exists on survivability limits based on wet-bulb temperatures than on wet-bulb globe temperatures.

Finally, observational data for wet-bulb globe temperature is limited, and it is significantly more computationally expensive to calculate than wet-bulb temperature.

“Dangerous” humid heat corresponds to a wet-bulb temperature of 25°C (77°F) or higher. Under these conditions, many people are at risk of experiencing heat illness, particularly older adults and those without access to cooling. The risks continue to increase as the wet-bulb temperature increases.

While 25°C (77°F) might seem low, it’s important to remember wet-bulb temperature measures something different from dry air temperature. It reflects how oppressive the combination of heat and humidity feels to your body and how hard it will have to work to cool down through sweating. Research suggests that our bodies can start to struggle at lower wet-bulb temperatures depending on age, health, and how acclimated a person is to humid heat.

We chose this threshold after a research review. Prior studies have assessed a variety of critical wet-bulb temperature thresholds beyond which human core body temperatures rise uncontrollably. An original “survivability limit” of 35°C (95°F) was proposed by Sherwood and Huber (2010), but we’ve since learned that conditions where the body can no longer cool itself can actually occur at significantly lower wet-bulb temperatures. For example, Vecellio et al. (2022) studied healthy young adults in the lab and found critical wet-bulb temperatures to be as low as 24°C (75.2°F). Additionally, a recent assessment by Matthews et al. (2025) looked at ~260,000 heat-related deaths from around the world and found critical wet-bulb temperature thresholds ranging from 19-32°C (66.2-89.6°F), largely due to age.

It’s important to note that the actual humid heat survivability limit will vary depending on a person’s age, health, sun exposure and other environmental conditions, and acclimatization to local humid heat.

In the Humid Heat CSI system, there are often levels of climate change signals that far exceed what the CSI typically measures. Wet-bulb temperatures are shifting in more dramatic ways — as a result, our scientists extended the range for the Humid Heat CSI to be -100 to +100.

Extremely high wet-bulb temperatures are naturally difficult to reach. This is because there is a limit to how hot and humid the atmosphere can get; as conditions near that limit, air rises and rain falls, lowering and limiting the wet-bulb temperature.

But a warming world due to human-caused climate change raises that maximum wet-bulb temperature limit. A warmer atmosphere can hold more moisture, and in many regions, this human-caused warming is enabling wet-bulb temperatures to reach levels that would have been rare if not impossible in a pre-industrial climate.

That said, rising moisture is only part of the story. Even in humid regions where moisture is increasing, rising air temperatures still drive the majority of the wet-bulb temperature trend, typically accounting for 70–80% of the total increase. In drier continental regions, where moisture is actually decreasing with climate change, rising dry-air temperatures are entirely responsible for the increase in wet-bulb temperatures.

Therefore, we’re seeing much stronger climate signals for humid heat, and needed a scale to effectively represent that.

Great question! We have several resources that can help:

Humid Heat: A Growing Health Risk in a Warming Climate — Reporting resources on why the combination of extreme heat and humidity is dangerous and how it is becoming more common in our warming climate. Topic: Health & Climate Change — Additional information, resources, and graphics on the health impacts of climate change