New study tracks increasing risk from dangerous heat waves

A recent study, published last Friday, provides new evidence of increasing life-threatening heat waves, focusing on the accumulation of dangerous heat that is known to cause severe health impacts. Researchers have developed a new metric – the accumulated dangerous heat index (ADHI) – that includes the combined effects of temperature and humidity to identify summer hours that surpass a dangerous threshold. Hourly exceedances are summed over each day, month, and season across the Northern Hemisphere.

Trends in ADHI are attributed to temperature, humidity, or a combination to better understand the cause of increased life-threatening heat conditions regionally. The researchers found areas historically prone to excessive heat have experienced longer-lasting, more intense, and expanded dangerous conditions, with some regions becoming uninhabitable. They found temperature to be the dominant factor driving increased ADHI during the daytime in most regions, while increasing humidity plays a more important role at night.

“We know that extreme heat causes by far the most fatalities of any type of hazardous weather, and it is crucial that we are able to fully assess and understand its risks,” said the lead author, Dr. Jennifer Francis, Senior Scientist at Woodwell Climate Research Center. “Brutal heat waves have already wreaked havoc across the Northern Hemisphere in 2026, even though summer has barely begun. Record-smashing temperatures are blamed for thousands of deaths in Europe, while dangerous heat invaded major eastern cities of the U.S. during celebrations of the 250th anniversary of the nation’s independence.”

“Escalating extreme heat is putting communities, ecosystems, and livelihoods at risk across many areas of the globe,” added co-author Dr. Natasa Skific. “The Accumulated Dangerous Heat Index gives scientists a new and straightforward tool to warn leaders and help them prepare for life-threatening heat events.” 

Co-author and MIT Research Scientist Dr. Judah Cohen noted, “As we found in our companion study that focused on severe winter conditions, the warming Arctic also appears to be contributing to the longevity of dangerous heat waves in many areas at lower latitudes.”

The findings in this new study provide further motivation to reduce emissions of heat-trapping gases, and the regional trends will assist decision-makers and planners in preparing for a future with a better understanding of the threats from deadly heat. 

Find the full paper here.