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Climate Shift Index: Humid Heat

The Climate Shift Index: Humid Heat quantifies the influence of climate change on daily humid heat and related health risks around the globe. It’s grounded in peer-reviewed attribution science and was launched by Climate Central in 2026.

Understanding how climate change is driving humid heat matters because extreme heat is among the deadliest weather hazards — and excessive humidity makes it more dangerous, blocking the body's ability to cool itself and raising the risk of serious or fatal heat-related illnesses.

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How the Humid Heat CSI works #

The Climate Shift Index: Humid Heat (Humid Heat CSI) is based on wet-bulb temperature, a measure combining temperature and humidity that represents how efficiently our bodies can cool themselves through sweating.

You can explore the data from the Humid Heat CSI system in two maps.

These two maps give us different pieces of the puzzle — how climate change is influencing humid heat in general, and where related health risks are most elevated as a result. Jump to the FAQ for more details.

ATT: CSI: Humid Heat and health graphics [Updated: May 2026]

The Humid heat and health map shows locations where there’s dangerous humid heat — a level of humid heat with potential health risks — and where climate change pushed the humid heat above that dangerous threshold.

ATT: CSI: Humid Heat Climate Signals graphics [Updated: May 2026]

The Humid heat climate signal map shows how much climate change increased or decreased the likelihood of reaching any given wet-bulb temperature.

ATT: CSI: Humid Heat Tool [Updated: May 2026] — Click to visualize the impact of climate change with combing today's temperature and humidity anywhere on Earth.

How the science works #

The methods and meteorological concepts behind the Humid Heat CSI system are briefly described here. You can find our more detailed, peer-reviewed methodology in Multi-method rapid attribution shows climate change is worsening humid heat (Trudeau et al., 2026).

Computing the Humid Heat CSI

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.

Explaining wet-bulb temperature

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.

FAQ #

Why does the Humid Heat CSI system use wet-bulb temperature? How is it different from wet-bulb globe temperature?

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.

How is wet-bulb temperature different from the heat index?

In contrast to the wet-bulb temperature used by our tool, the heat index describes how hot people perceive it to be — this is why it is often referred to as a “feels-like” metric. The heat index is also based on temperature and relative humidity. It is modeled, not measured, and makes a few assumptions, including that the person at risk is standing in the shade, wearing certain clothing, and has a certain body size.

What is the dangerous humid heat threshold referenced in the Humid Heat CSI tool? Why is this a significant threshold?

“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.

How did you choose this threshold?

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.

What is the dangerous humid heat threshold in heat index terms?

A wet-bulb temperature of 25°C (77°F) — the dangerous humid heat threshold in the Humid Heat CSI — can correspond to a range of heat index values depending on the specific combination of temperature and humidity. It’s less like converting Celsius to Fahrenheit and more like asking which two numbers multiply to make 12 — there are multiple valid answers. But generally, a wet-bulb temperature of 25°C (77°F) would likely be consistent with a heat index in the “extreme caution” to “danger” range defined by the U.S. National Weather Service.

Why are the scales between the CSI and Humid Heat CSI so different?

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.

I’m seeing a lot of humid heat climate signals but no “dangerous humid heat” — why?

If you’re seeing places on the “Humid heat climate signal” map where the wet-bulb temperature has been made significantly more likely by climate change but it doesn’t show up on our “Humid heat and health” map, it just means that it represents a wet-bulb temperature below 25°C (77°F). These locations could still be experiencing humid heat that is risky to health, as health risks do not just begin at the “dangerous” threshold.

I’m seeing locations with “dangerous humid heat due to climate change” but no humid heat climate signals – how is that possible?

This is something that occurs occasionally in our data, and there’s a logical explanation for it. The warming associated with climate change can push the wet-bulb temperature in a given location into the “dangerous” zone. At the same time, however, climate change can shift the probability of wet-bulb temperatures enough that the observed wet-bulb temperature is now on the lower end of all wet-bulb temperatures that location experiences. In these cases, the wet-bulb temperatures that used to be common in a given place are now relatively cool and occur less frequently than they did in the past even as climate change has pushed wet-bulb temperatures across the “dangerous” threshold.

What do the negative “times more likely” numbers mean?

When you see areas on the “Humid heat climate signal” map shaded in gray, representing a negative value, it means those places are experiencing a combination of heat and humidity that has been made less likely due to human-caused climate change.

What does the hatching on the map represent?

The hatching on the map represents areas where, historically, differences between daily forecasted weather conditions and the weather that is actually observed are too large to confidently quantify and report the fingerprint of climate change. We label these regions as “Statistical uncertainties.” We recommend caution if and when drawing conclusions from these results and would suggest using language along the lines of “Preliminary data suggest that climate change is playing a role in driving these humid heat conditions; however, more definitive observational data is needed to draw confident conclusions.” Hatching is removed when observational data become available, typically at about a five-day delay.

Where can I learn more about humid heat risks and climate change?

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

Ready to use the Humid Heat CSI in your communications?

We have supportive explainer graphics you can use, and examples of how others have used it in their reporting.