Humid heat is exceeding human tolerance & causing mass mortality

Critical physiological limits to human heat tolerance are drawing ever closer, highlighting the urgent need to limit further climate warming and emphasising the adaptation challenge ahead.

Writing in the journal Nature Climate Change, 21st Century Weather researchers and their collaborators at institutions around the world offered a stark reminder that the hottest boreal summer on record has driven widespread humid heat mortality across every continent of the Northern Hemisphere in 2024.

Sufficiently high combinations of air temperature and humidity (levels of ‘humid heat’) can be lethal to humans. Most heat-related mortality occurs in older adults and those with existing disease, via cardiovascular and respiratory pathways. However, thermodynamic laws mean that there are upper limits to the levels of humid heat that even the healthiest can tolerate.

The highly sophisticated ‘thermoregulatory’ strategies of the human body, primarily sweating, simply cannot prevent overheating once air temperature and humidity exceed critical thresholds. If such ‘uncompensable heat’ is sufficiently long-lasting, lethal increases in internal body temperature can result.

Around 4% of global weather stations have already recorded at least one 6-hour period of uncompensable heat. These crossing events have occurred in the tropics and subtropics, where in the most impacted regions — for example, around the Persian/Arabian Gulf, across the Indo-Gangetic Plain, and in Southeast Asia — there may already be multiple uncompensable heat events each year.

Ongoing human-caused climate warming and increasing humidity will intensify these potentially lethal humid heat risks, with rapid escalation anticipated even for relatively limited additional warming

In 2024, the annual Hajj pilgrimage to Mecca saw over 1,300 reported fatalities in an event during which temperature and humidity exposure could also be determined with reasonably high confidence, and where behavioural variations were heavily constrained. It provided rare insight and an unambiguous warning of the threat from uncompensable heat.

The researchers view the mass mortality event in Mecca, during the hottest summer on record, as a stark illustration of the lethal impacts of humid heat at levels that human physiology cannot endure. 

They urge that mitigating global warming through radical reductions in greenhouse gas emissions, especially from fossil fuel use, remains a vital pathway for blunting the deadly threat posed by rising temperature and humidity. Societal adaptation is also critical given the unavoidable escalation in heat stress within even our near-term future. 

In particular, efforts must urgently address how to provide protection from humid heat beyond human physiological tolerance. These measures must be accessible to all populations; continue to function effectively despite insecurities or disruptions in energy infrastructure caused by extreme weather events or other challenges; and, wherever practicable, must not contribute to driving up greenhouse gas concentrations. The mitigation and adaptation challenges are high, but the penalty of inaction from humid heat threatens to be far higher.

This story features excerpts from an article appearing in Nature Climate Change. To read the paper in full, go to: https://www.nature.com/articles/s41558-024-02215-8

Contributing authors Emma Rasay and Santa Barley have also addressed the issue in article for The Conversation: https://theconversation.com/more-than-1-300-hajj-pilgrims-died-this-year-when-humidity-and-heat-pushed-past-survivable-limits-its-just-the-start-245271

The researchers involved are members of the Lethal Humidity Global Council, an assembly of dedicated leaders and global experts to protect humanity against lethal temperature and humidity combinations, and the risks posed by climate change more generally.

The work was led by Tom Matthews in the Department of Geography and Centre for Integrated Research in Risk and Resilience, King’s College London. 21st Century Weather Chief Investigators Professor Nerilie Abram (ANU), Professor Sarah Perkins-Kirkpatrick (ANU) and Professor Steven Sherwood (UNSW) were among the contributors.

The work featured contributions from researchers based at:

  • Nanyang Technological University, Singapore;
  • Climate Analytics, Berlin;
  • Climate Change Research Centre, University of New South Wales, Sydney;
  • Heat and Health Research Centre and Thermal Ergonomics Laboratory, University of Sydney;
  • Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles;
  • ARC Centre of Excellence for the Weather of the 21st Century and Research School of Earth Sciences, Australian National University, Canberra;
  • Heat Resilience and Performance Centre, Yong Loo Lin School of Medicine, National University of Singapore, Singapore;
  • University of Western Australia, Perth;
  • Fortescue, Perth;
  • Fenner School of Environment and Society, Australian National University, Canberra;
  • Weather and Climate Services, Islamabad;
  • Centre for Ecology and Conservation, Exeter University, Exeter;
  • Indian Institute of Public Health, Gandhinagar;
  • Walter and Eliza Hall Institute of Medical Research (WEHI), Melbourne;
  • University of Melbourne, Australia;
  • Luskin Center for Innovation, University of California, Los Angeles;
  • Minderoo Foundation, Perth.