Half of world's population endured extra month of extreme heat due to
climate change, experts say
[May 30, 2025]
By ISABELLA O'MALLEY
Scientists say 4 billion people, about half the world’s population,
experienced at least one extra month of extreme heat because of
human-caused climate change from May 2024 to May 2025.
The extreme heat caused illness, death, crop losses, and strained energy
and health care systems, according to the analysis from World Weather
Attribution, Climate Central and the Red Cross.
“Although floods and cyclones often dominate headlines, heat is arguably
the deadliest extreme event,” the report said. Many heat-related deaths
are unreported or are mislabeled by other conditions like heart disease
or kidney failure.
The scientists used peer-reviewed methods to study how much climate
change boosted temperatures in an extreme heat event and calculated how
much more likely its occurrence was because of climate change. In almost
all countries in the world, the number of extreme heat days has at least
doubled compared with a world without climate change.
Caribbean islands were among the hardest hit by additional extreme heat
days. Puerto Rico, a territory of the United States, endured 161 days of
extreme heat. Without climate change, only 48 would have occurred.
“It makes it feel impossible to be outside,” said Charlotte Gossett
Navarro, chief director for Puerto Rico at Hispanic Federation, a
nonprofit focused on social and environmental issues in Latino
communities, who lives in the San Juan area and was not involved in the
report.
“Even something as simple as trying to have a day outdoors with family,
we weren’t able to do it because the heat was too high," she said,
reporting feeling dizzy and sick last summer.
When the power goes out, which happens frequently in Puerto Rico in part
because of decades of neglected grid maintenance and damage from
Hurricane Maria in 2017, Navarro said it is difficult to sleep. “If you
are someone relatively healthy, that is uncomfortable, it’s hard to
sleep ... but if you are someone who has a health condition, now your
life is at risk,” Gossett Navarro said.
Heat waves are silent killers, said Friederike Otto, associate professor
of climate science at Imperial College London, one of the report’s
authors. “People don’t fall dead on the street in a heat wave ... people
either die in hospitals or in poorly insulated homes and therefore are
just not seen,” he said.
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Bathers cool off in the water while others sunbathe on a Barcelona
beach, Spain, Wednesday, July 24, 2024. (AP Photo/Emilio
Morenatti,File)
 Low-income communities and
vulnerable populations, such as older adults and people with medical
conditions, suffer the most from extreme heat.
The high temperatures recorded in the extreme heat events that
occurred in Central Asia in March, South Sudan in February and in
the Mediterranean last July would have not been possible without
climate change, according to the report. At least 21 people died in
Morocco after temperatures hit 118 degrees Fahrenheit (48 degrees
Celsius) last July. People are noticing temperatures are getting
hotter but don't always know it is being driven by climate change,
said Roop Singh, head of urban and attribution at the Red Cross Red
Crescent Climate Centre, in a World Weather Attribution statement.
“We need to quickly scale our responses to heat through better early
warning systems, heat action plans, and long-term planning for heat
in urban areas to meet the rising challenge,” Singh said.
City-led initiatives to tackle extreme heat are becoming popular in
parts of South Asia, North America, Europe and Australia to
coordinate resources across governments and other agencies. One
example is a tree-planting initiative launched in Marseille, France,
to create more shaded areas.
The report says strategies to prepare for heat waves include
monitoring and reporting systems for extreme temperatures, providing
emergency health services, cooling shelters, updated building codes,
enforcing heat safety rules at work, and designing cities to be more
heat-resilient.
But without phasing out fossil fuels, heat waves will continue
becoming more severe and frequent and protective measures against
the heat will lose their effectiveness, the scientists said.
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