One of the great challenges in understanding the extraordinary consequences of global warming is our tendency to think in silos. Climate change is still often regarded as solely an environmental issue—and is generally coordinated in governments by environment ministries—rather than as a systemic crisis that affects every aspect of society. That same tendency helps to explain why the rising threat of lethal humidity, which is a combination of extreme heat and high humidity that can exceed the limits of human survival, has been overlooked until relatively recently.
Climate change is not only warming the planet but also increasing humidity, because warmer air holds more moisture. The role of climate change in amplifying extreme heat and heatwaves has been widely studied, but only recently have the implications of its combination with high humidity been well understood. High humidity significantly lowers the temperature at which severe human health impacts occur. The synergistic and accelerating effect of high temperature and high humidity is rapidly moving large, densely populated regions of the planet towards the limits of physiological survivability.
This report uses 'lethal humidity' as a focal concept but focuses primarily on the broad category of extreme humid-heat events that are increasingly approaching, and in some regions exceeding, that threshold. It focuses on the rising threat posed by extreme humid heat, both as a stand-alone climate hazard and as part of a more complex pattern of interconnected hazards intensified by climate change. It highlights the accelerating impacts of extreme humid-heat occurrences and how they'll both amplify and be amplified by other climate-related events happening simultaneously or consecutively.
The report's main objective is to show that those impacts aren't isolated: they cascade through societies, magnified by other climate hazards, such as storms and flooding.
The first section examines how humid-heat hazards have evolved in frequency and intensity and surveys the societal impacts of three historical humid heatwaves: Chicago (1995), South Asia (2015), and Brazil (2023). It identifies common themes across the three cases, focusing particularly on key cascading societal consequences.
The second section explains how rapidly climate change is causing humid-heat extremes to reach and surpass lethal humidity thresholds. It identifies six defining features of climate change that shape what societies can expect from extreme events such as humid heatwaves. It also summarises climate modelling projections of humid-heat extremes and how they may be compounded by other climate hazards emerging simultaneously.
The third section focuses on Indonesia, one of the most geopolitically important and rapidly developing countries in the Indo-Pacific, where extreme humid heat will rapidly become a defining climate risk. Over the past decade, Indonesia has risen into the top five nations globally for exposure to heatwave days. The section also illustrates how humid-heat extremes may reach lethal humidity levels in future and interact with other climate hazards, such as flooding, drought, sea-level rise and energy-system stress, creating major cascading societal disruptions in the country.
The final section summarises the main findings and identifies key policy and research priorities to address the growing likelihood that humid-heat extremes will surpass lethal humidity thresholds, with severe consequences for humanity. It notes that each new decade will expose many tens of
millions of additional people to deadly humid-heat extremes. As much as 50%–75% of the global population will be exposed by late this century.
That dire trajectory elevates lethal humidity from a high-impact risk to a systemic global threat, requiring rapid reductions in greenhouse gas emissions and decisive policy responses. The Lethal Humidity Global Council identifies four urgent and necessary shifts:
- Achieving real zero emissions through complete fossil-fuel phase-out and renewable replacement.
- Removing barriers to green industry, including fossil-fuel subsidies, before 2030.
- Matching incentives and disincentives to risk through carbon pricing.
- Using economic stimulants to drive green growth and transformation.
While those shifts are essential to limit future risk, they can't fully avert the impacts already set in motion. Accordingly, this report recommends the following additional measures to improve understanding of lethal humidity and to prepare societies for accelerating impacts, many of which are now unavoidable following decades of inadequate climate action.
