One of the great challenges in understanding the extraordinary consequences of global warming is a tendency to think in silos. Climate change is still often regarded as solely an environmental issue, rather than as a systemic crisis that affects every aspect of society. 

Climate change is not only warming the planet but also increasing humidity, because warmer air holds more moisture. 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.

The report’s main objective is to show that the impacts aren’t isolated: they cascade through societies, magnified by other climate hazards, such as storms and flooding.

Report structure

• 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). 
• 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.
• 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. 
• 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. 

Key recommendations

• Accelerate research on humid-heat impacts and human survivability.
• Expand and focus research in less developed countries.
• Investigate the economic and labour-productivity impacts of warming.
• Identify and mitigate cascading societal impacts.
• Prioritise adaptation planning for tropical coastal cities.
• Strengthen and reform heat action plans.