Dangerous Humid Heat Days Have Doubled Globally Since the 1970s

A new analysis from Climate Central finds that dangerous humid heat days have more than doubled globally since the 1970s, rising from an average of 10 days per year to 23 days per year, with human-caused climate change now responsible for nearly two-thirds of all dangerous humid heat days worldwide and six times as many dangerous humid heat days annually compared with the 1970s baseline. In 2025 alone, the global average reached 23 dangerous humid heat days with 19 of those days, representing 83 percent, attributable to climate change, while the analysis found that climate change is causing more dangerous humid heat days in 69 percent of 961 global cities analysed, by an average of 46 additional days per year during the last decade. Extreme heat has claimed more than a quarter of a million lives globally since 2000, making it one of the deadliest weather-related hazards, with humid heat presenting particular danger because high humidity reduces the body's ability to cool itself through sweating, causing heat to build up internally and increasing risks of dehydration, cardiovascular and respiratory problems, heat exhaustion and heat stroke.

 

The Science of Wet-Bulb Temperature and Physiological Risk

 

Climate Central's analysis uses wet-bulb temperature, a measure combining heat and humidity to assess physiological stress on the human body, defining wet-bulb temperatures of 25 degrees Celsius or higher as dangerous humid heat conditions under which many people face elevated risk of heat-related illness. This methodology, based on new peer-reviewed research published in May 2026, represents a groundbreaking approach to quantifying the specific influence of climate change on dangerous humid heat conditions worldwide by isolating the human-caused warming contribution from natural climate variability across observations from 1970 through 2025. Kaitlyn Trudeau, Applied Climate Scientist at Climate Central, said the findings show how profoundly climate change is reshaping the world, with dangerous humid heat having gone from being an uncommon event to a defining feature of daily life in some regions, pushing conditions closer to the limits of what the human body can safely endure.

The distinction between dry heat and humid heat is commercially and medically significant because conventional temperature metrics substantially underestimate the physiological danger of humid conditions, where apparent temperatures felt by the body can exceed air temperatures by ten to twenty degrees Celsius and where the evaporative cooling mechanism that prevents fatal heat stroke becomes ineffective above specific humidity thresholds. Humidity's role in determining heat danger means that seemingly moderate temperatures in tropical and subtropical regions can create life-threatening physiological stress that would not be recognised as dangerous by populations or public health systems accustomed to using dry-bulb temperature as the primary heat risk indicator. The Climate Central analysis's focus on wet-bulb temperature rather than conventional temperature metrics provides a more accurate picture of the true scale of climate change's impact on human heat exposure globally.

 

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Disproportionate Impacts and Vulnerable Populations

 

The analysis identifies older adults, children, pregnant people, individuals with underlying health conditions and people without access to cooling as facing disproportionately greater dangers from dangerous humid heat, groups that collectively represent a substantial proportion of the global population and are concentrated in regions with the highest humid heat exposure including South and Southeast Asia, Sub-Saharan Africa and tropical Latin America. Lisa Patel, Clinical Associate Professor of Pediatrics at Stanford Children's Health and Executive Director at the Medical Society Consortium on Climate and Health, said these numbers are a wake-up call and that the consequences are already playing out in real time, citing fans fainting at World Cup matches in cities like Houston as evidence that dangerous humid heat is creating immediate public health emergencies in developed country contexts as well as in the Global South. The reference to Houston illustrates that dangerous humid heat is not exclusively a developing world or tropical phenomenon, with 65 percent of 247 US cities analysed experiencing more dangerous humid heat days due to climate change, by an average of 19 additional days per year during the past decade.

The finding that climate change is responsible for 83 percent of the 23 dangerous humid heat days recorded globally in 2025 provides one of the clearest quantitative attributions of a specific weather-related public health impact to anthropogenic climate change yet produced, moving the conversation about climate health impacts from qualitative assertions to precise statistical attribution. For public health systems, emergency services and urban planners, the Climate Central analysis provides the actionable intelligence needed to anticipate where heat-related illness will strike and which populations face the greatest risk before people require emergency medical intervention, as Patel described. The peer-reviewed scientific foundation of the analysis, combining observational data from 1970 through 2025 with climate attribution methodology, provides a credible evidence base for policy responses ranging from urban cooling infrastructure investment to occupational heat safety standards and early warning systems.

 

Implications for Corporate and Financial Climate Risk Assessment

 

The doubling of dangerous humid heat days since the 1970s and the trajectory toward further increases as global temperatures rise has direct implications for corporate physical climate risk assessment, workforce health and productivity management and supply chain resilience planning for companies operating in or sourcing from high-exposure regions. Outdoor workers in agriculture, construction, manufacturing and logistics face increasing productivity losses and health risks from dangerous humid heat, with implications for labour availability, workplace safety liability and operating cost projections that are insufficiently captured by conventional temperature-based climate risk metrics. The Climate Central analysis's city-level resolution, covering 961 global cities, provides the geographic granularity needed for asset-level physical climate risk assessment that institutional investors and corporate risk managers require for portfolio and facility-level exposure quantification.

Financial institutions with loan book or investment exposure to real estate, infrastructure and corporate operations in high humid heat exposure regions face growing climate physical risk that the Climate Central findings suggest is substantially more severe than dry-temperature-based risk assessments indicate. The EBA's new Pillar 3 ESG disclosure framework requiring European banks to disclose exposures subject to physical risk by hazard type now includes heat as a specific climate-related hazard requiring disclosure, creating a regulatory imperative for financial institutions to integrate wet-bulb temperature-based heat risk assessment into their climate risk frameworks rather than relying on conventional temperature metrics that underestimate physiological danger.

 

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Outlook for Humid Heat as a Physical Climate Risk Priority

 

The Climate Central analysis establishes dangerous humid heat as one of the most rapidly intensifying physical climate risks globally, with the pace of increase, more than doubling since the 1970s with 83 percent of current exposure attributable to climate change, suggesting that without rapid decarbonisation the trajectory toward conditions approaching or exceeding the physiological limits of human tolerance will continue to accelerate. Whether the public health, corporate risk management and financial sector communities can integrate wet-bulb temperature-based dangerous humid heat metrics into their planning and disclosure frameworks at the pace required to anticipate and manage the growing health and economic impacts will be a defining test of the climate adaptation agenda over the coming decade. Sustained scientific documentation of dangerous humid heat trends through analyses like Climate Central's provides the evidence base needed to motivate both mitigation action to slow the trajectory and adaptation investment to protect the most vulnerable populations from the dangerous humid heat days that current atmospheric carbon concentrations have already made inevitable.

 

Source: Climate Central