Annotation summary

The Coffel article projects that global increases in temperature and specific humidity will significantly intensify heat stress throughout the 21st century. Regions particularly vulnerable to dangerous heat and humidity combinations include densely populated areas such as Northeast India, Eastern China, West Africa, and the Southeast US. By 2080, the frequency of extreme wet bulb temperature events is projected to rise by a factor of 100-250 in the tropics and mid-latitudes, with potential exposure to wet bulb temperatures exceeding recent deadly heat waves increasing by a factor of five to ten. Under the high emission scenario RCP 8.5, exposure to wet bulb temperatures above 35°C, the theoretical limit for human tolerance, could exceed one million person-days per year by 2080. Conversely, limiting emissions to follow RCP 4.5 would eliminate exposure to this extreme threshold.

The study emphasizes that the human body's ability to shed heat through evaporative cooling is significantly impaired in hot and humid conditions, leading to potentially dangerous heat stress. Current infrastructure and adaptive capacity in many vulnerable regions are insufficient to cope with the projected increases in heat stress. The article also highlights that substantial population growth in these regions will exacerbate exposure to dangerous heat conditions. The research suggests that heat stress will become one of the most directly dangerous aspects of climate change, impacting human health, energy infrastructure, and outdoor activities.

The authors stress the importance of both mitigation and adaptation measures to reduce the future impacts of heat stress. Without such measures, the world is likely to face significant suffering, economic damage, and increased heat-related mortality. They call for deeper epidemiological and economic analyses to understand the full implications of rising wet bulb temperatures, and for the development of effective strategies to protect vulnerable populations.