From the COP30 Local Leaders Forum, a critical gathering of global mayors focused on climate action, a sweeping new scientific analysis was released of the most severe summer weather conditions across 100 major global cities.
It revealed that while leaders have been focusing on avoiding breaching the +1.5°C threshold of the Paris Agreement, in many global cities, minimum nighttime temperatures have been rising much more rapidly during oppressively hot weather situations than daytime average highs, up to 12 times faster in some cities.
The study by Climate Resilience for All, Extreme Heat and the Shrinking Diurnal Range: A Global Evaluation of Oppressive Air Mass Character and Frequency, analysed weather data over 30 years from 1994 to 2024, isolating the two types of weather conditions “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 clearly highlights the urgent need for preventive and responsive actions on extreme heat, including early warnings, public health measures, and adaptation planning, to explicitly account for and address the rapidly rising threat of hotter nights.
This analysis is the first to examine how humidity, daytime, and nighttime temperatures are changing within dangerous summer air masses; how their frequency is shifting; and how these trends vary across climate regions, making a significant contribution to the science and helping inform urban heat and health interventions for practitioners everywhere.
The analysis found that increases in nighttime temperatures and decreases in the gap between daytime highs and nighttime lows across much of the globe.
Eighty-three per cent of cities in the study are experiencing sustained, higher nighttime temperatures. In Africa, 13 out of 15 cities studied are experiencing higher nighttime temperatures in moist tropical weather, and 10 out of 14 cities are experiencing higher nighttime temperatures in dry tropical weather.
Nighttime temperatures in African cities are rising fastest in Agadir, Morocco (a dry tropical climate), where they are increasing at a rate of 1°C every 5.6 years, and in Cairo, Egypt (a moist tropical climate), where they are rising 1°C every 16.8 years.
During moist tropical weather, the cities seeing decreases between daytime and nighttime temperatures are Upington, South Africa; Bamako, Mali; Abidjan, Cote d’Ivoire; Cairo, Egypt; Agadir, Morocco; Mombasa, Kenya; Espargos, Cape Verde; Brazzaville, Congo; Dakar, Senegal; Algiers, Algeria; Cotonou, Benin; Luxor, Egypt; and Reunion, Reunion Island.
During dry tropical weather, the cities seeing decreases between daytime and nighttime temperatures are Agadir, Morocco; Cairo, Egypt; Luxor, Egypt; Espargos, Cape Verde; Cape Town, South Africa; Niamey, Chad; Dakar, Senegal; Mombasa, Kenya; Brazzaville, Congo; and Reunion, Reunion Island.
Increases in the frequency of extreme heat days
Over the 30-year study period, summertime moist tropical weather patterns have increased by more than 49 per cent in Africa and by 37 per cent globally. Cities in Africa average an increase of one moist tropical day every 2.5 years.
Dry tropical weather patterns have grown by 14 per cent over the same period, with cities averaging an increase of one dry tropical day every 3.4 years. Every day’s increase in DT and MT frequency leads to a day’s decrease in the cooler, more pleasant air masses. Global, regional, and city-level data visualisations for the cities in the study are here.
“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.”
Mayor Yvonne Aki-Sawyerr of Freetown, Sierra Leone, leading a city on the frontlines of extreme heat, said, “This study reflects the lived reality of so many cities in the global south. Most of our residents lack access to cooling, and women market traders facing extreme heat all day return to homes that are dangerously hot at night.
“As we implement our heat action plan, this study sounds the alarm on the hidden threat of nighttime temperatures and pushes us toward solutions that drive awareness and protect people.”
Kathy Baughman McLeod, CEO of Climate Resilience for All, stated that the analysis was intended to “mobilise city and health leaders to urgently broaden their view of what is a 24-hour heat crisis” as the research uncovers a critical blind spot in our understanding of extreme heat.
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 minimise 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 enhance regionally targeted heat warning systems that account for the increasing likelihood of multi-day, high-intensity events with minimal nocturnal relief.
