WASHINGTON, Nov. 19, 2025 (GLOBE NEWSWIRE) -- While leaders have been focusing on avoiding breaching the +1.5°C threshold of the Paris Agreement, a sweeping new scientific analysis of the most dangerous summer weather conditions across 100 major global cities revealed that minimum nighttime temperatures have been rising up to 10 times faster than daytime average highs in many global cities during oppressively hot weather.

The study by Climate Resilience for All, Extreme Heat and the Shrinking Diurnal Range: A Global Evaluation of Oppressive Air Mass Character and Frequency, analyzed weather data over a 30-year period from 1994 to 2024, isolating the two types of weather conditions, or “air masses”, considered most dangerous for human health: “dry tropical” (DT) weather, which is hot and dry, and “moist tropical” (MT) weather which is hot and humid.

Heat exposure has traditionally been measured by exposure to daytime high temperatures and increasing “average” temperatures. This study points clearly to the urgent need for preventative and responsive actions on extreme heat to explicitly account for and address the rapidly rising threat of hotter nights.

Global, regional, and city level data visualizations can be accessed via this link.

The analysis found:

Increases in nighttime temperatures, and decreases in the gap between daytime highs and nighttime lows across much of the globe

• 83% of cities in the study are experiencing sustained, higher nighttime temperatures.
• Nighttime temperatures are rising fastest in Melbourne, Australia (dry tropical), where they increase by 1°C every 5.36 years, and Dubai, UAE (moist tropical), where they rise by 1°C every 8.81 years.
  • During moist tropical weather, Santa Maria, Upington, Seoul, Samarkand, Paris, Kuwait City, Portland, and Abadan are seeing the biggest decrease between daytime and nighttime temperatures. The number of cities seeing decreases per region breaks down as follows:
    • Africa: 13 out of 15.
    • Asia: 18 out of 22.
    • Central and South America: 10 out of 11.
    • Europe: Seven out of 12.
    • Middle East: 5 out of 5.
    • North America: 14 out of 16.
    • Oceania: Nine out of 11.
  • During dry tropical weather, Melbourne, Agadir, Seoul, Mumbai, Cairo, Luxor, Kuwait City, and Santiago are seeing the biggest decrease between daytime and nighttime temperatures. The number of cities seeing decreases per region breaks down as follows:
    • Africa: 10 out of 14.
    • Asia: 13 out of 22.
    • Central and South America: Seven out of 11.
    • Europe: Four out of six.
    • Middle East: Six out of seven.
    • North Americas: 11 out of 14
    • Oceania: Five out of nine.
    • Some of the regions show weaker differentiation, possibly because dry tropical weather types are rarely present in the cities we evaluated in those regions.
      

Increases in the frequency of extreme heat days

• Over the 30-year study period, summertime moist tropical weather patterns have increased close to or over 50 percent in Central and South America, Oceania, and Africa - and have grown by 37 percent globally.
• Dry tropical weather patterns have grown by 13 percent over the same period, with the largest increase in Australia, which had a 29 percent rise.
  

“Before this analysis, we did not know how rapidly nighttime heat has been rising within the most dangerous air masses,” said Larry Kalkstein, climatologist, Chief Heat Science Advisor at Climate Resilience for All, and the study's lead author. “It is critical for us to understand how the heat of summer—that sends people to the emergency room—is shifting, and what we are overlooking when we talk about it.”

“We want this analysis to mobilize city and health leaders to urgently broaden their view of what is a 24-hour heat crisis. This research uncovers a critical blind spot in our understanding of extreme heat,” said Kathy Baughman McLeod, CEO of Climate Resilience for All.

High nighttime temperatures prevent the human body from cooling down, increasing risks of heat exhaustion, dehydration, and cardiovascular stress. When sleep is disrupted by heat, the body loses its ability to recover from daytime exposure, heightening the danger of illness and death—especially for older adults, women, and those living in poorly ventilated housing.

Heat warning systems are focused on high daytime temperatures and currently minimize the impact of overnight temperatures. The study offers guidance and urges health officials and policymakers to integrate these changing patterns into their work and to ramp up regionally targeted heat warning systems that account for the growing probability of multi-day, high-intensity events that offer little nocturnal relief.

About Climate Resilience for All

Climate Resilience for All is a global adaptation NGO dedicated to protecting the health, income, and dignity of women on the frontlines of extreme heat.

Contacts:

Geraldine Henrich-Koenis, geraldine@climateresilience.org

Kelechukwu Iruoma, kelechukwu@climateresilience.org