Half of world's population endured extra month of extreme heat due to climate change, experts say

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.

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.

The Associated Press’ climate and environmental coverage receives financial support from multiple private foundations. AP is solely responsible for all content. Find AP’s standards for working with philanthropies, a list of supporters and funded coverage areas at AP.org.

FILE - A jogger runs along a trail in McAllister park as temperatures hit record highs, Tuesday, May 13, 2025, in San Antonio. (AP Photo/Eric Gay,File)

FILE - Laborers sleep under a bridge during a hot summer day in Karachi, Pakistan, Thursday, July 25, 2024. (AP Photo/Fareed Khan,File)

FILE - A man walks on a hot summer day in Srinagar, Indian controlled Kashmir, Thursday, July 25, 2024. (AP Photo/Mukhtar Khan,File)

FILE - Tourists cool off at Trevi fountain as temperatures are reaching up to 37 Celsius degrees (98.6 Fahrenheit) in Rome, Wednesday, July 24, 2024. (AP Photo/Gregorio Borgia,File)

FILE - Heatstroke patients receive treatment at a hospital in Karachi, Pakistan, Thursday, July 25, 2024. (AP Photo/Fareed Khan,File)

FILE - People sunbathe on a promenade in Barcelona, Spain, Wednesday, July 24, 2024. Several regions of Spain are under alert for high temperatures. (AP Photo/Emilio Morenatti,File)

FILE - Bathers cool off in the water while others sunbathe on a Barcelona beach, Spain, Wednesday, July 24, 2024. (AP Photo/Emilio Morenatti,File)

DUBAI, United Arab Emirates (AP) — U.S. President Donald Trump said Iran wants to negotiate with Washington after his threat to strike the Islamic Republic over its bloody crackdown on protesters, a move coming as activists said Monday the death toll in the nationwide demonstrations rose to at least 544.

Iran had no immediate reaction to the news, which came after the foreign minister of Oman — long an interlocutor between Washington and Tehran — traveled to Iran this weekend. It also remains unclear just what Iran could promise, particularly as Trump has set strict demands over its nuclear program and its ballistic missile arsenal, which Tehran insists is crucial for its national defense.

Meanwhile Monday, Iran called for pro-government demonstrators to head to the streets in support of the theocracy, a show of force after days of protests directly challenging the rule of 86-year-old Supreme Leader Ayatollah Ali Khamenei. Iranian state television aired chants from the crowd, who shouted “Death to America!” and “Death to Israel!”

Trump and his national security team have been weighing a range of potential responses against Iran including cyberattacks and direct strikes by the U.S. or Israel, according to two people familiar with internal White House discussions who were not authorized to comment publicly and spoke on condition of anonymity.

“The military is looking at it, and we’re looking at some very strong options,” Trump told reporters on Air Force One on Sunday night. Asked about Iran’s threats of retaliation, he said: “If they do that, we will hit them at levels that they’ve never been hit before.”

Trump said that his administration was in talks to set up a meeting with Tehran, but cautioned that he may have to act first as reports of the death toll in Iran mount and the government continues to arrest protesters.

“I think they’re tired of being beat up by the United States,” Trump said. “Iran wants to negotiate.”

He added: “The meeting is being set up, but we may have to act because of what’s happening before the meeting. But a meeting is being set up. Iran called, they want to negotiate.”

Iran through country's parliamentary speaker warned Sunday that the U.S. military and Israel would be “legitimate targets” if America uses force to protect demonstrators.

More than 10,600 people also have been detained over the two weeks of protests, said the U.S.-based Human Rights Activists News Agency, which has been accurate in previous unrest in recent years and gave the death toll. It relies on supporters in Iran crosschecking information. It said 496 of the dead were protesters and 48 were with security forces.

With the internet down in Iran and phone lines cut off, gauging the demonstrations from abroad has grown more difficult. The Associated Press has been unable to independently assess the toll. Iran’s government has not offered overall casualty figures.

Those abroad fear the information blackout is emboldening hard-liners within Iran’s security services to launch a bloody crackdown. Protesters flooded the streets in the country’s capital and its second-largest city on Saturday night into Sunday morning. Online videos purported to show more demonstrations Sunday night into Monday, with a Tehran official acknowledging them in state media.

In Tehran, a witness told the AP that the streets of the capital empty at the sunset call to prayers each night. By the Isha, or nighttime prayer, the streets are deserted.

Part of that stems from the fear of getting caught in the crackdown. Police sent the public a text message that warned: “Given the presence of terrorist groups and armed individuals in some gatherings last night and their plans to cause death, and the firm decision to not tolerate any appeasement and to deal decisively with the rioters, families are strongly advised to take care of their youth and teenagers.”

Another text, which claimed to come from the intelligence arm of the paramilitary Revolutionary Guard, also directly warned people not to take part in demonstrations.

“Dear parents, in view of the enemy’s plan to increase the level of naked violence and the decision to kill people, ... refrain from being on the streets and gathering in places involved in violence, and inform your children about the consequences of cooperating with terrorist mercenaries, which is an example of treason against the country,” the text warned.

The witness spoke to the AP on condition of anonymity due to the ongoing crackdown.

The demonstrations began Dec. 28 over the collapse of the Iranian rial currency, which trades at over 1.4 million to $1, as the country’s economy is squeezed by international sanctions in part levied over its nuclear program. The protests intensified and grew into calls directly challenging Iran’s theocracy.

Nikhinson reported from aboard Air Force One.

In this frame grab from video obtained by the AP outside Iran, a masked demonstrator holds a picture of Iran's Crown Prince Reza Pahlavi during a protest in Tehran, Iran, Friday, January. 9, 2026. (UGC via AP)

In this frame grab from footage circulating on social media from Iran shows protesters taking to the streets despite an intensifying crackdown as the Islamic Republic remains cut off from the rest of the world in Tehran, Iran, Friday, Jan. 9, 2026.(UGC via AP)

In this frame grab from footage circulating on social media from Iran showed protesters once again taking to the streets of Tehran despite an intensifying crackdown as the Islamic Republic remains cut off from the rest of the world in Tehran, Iran, Saturday Jan. 10, 2026. (UGC via AP)