SAN JUAN, Puerto Rico: More than 4 billion people worldwide endured at least one extra month of extreme heat between May 2024 and May 2025—an impact made significantly worse by human-driven climate change, according to a new report by World Weather Attribution, Climate Central, and the Red Cross.

The analysis reveals that compared to a world without global warming, climate change has at least doubled the number of extreme heat days in nearly every country. The consequences range from health crises and deaths to pressure on power grids and healthcare systems.

"Although floods and cyclones often dominate headlines, heat is arguably the deadliest extreme event," the report notes, highlighting how heat-related deaths are often undercounted or misclassified as other conditions like heart disease or kidney failure.

The researchers used peer-reviewed methods to estimate how much more likely extreme heat events were due to climate change. The Caribbean, especially Puerto Rico, was among the most brutal hit, enduring 161 days of extreme heat—compared to an expected 48 without climate change.

"It makes it feel impossible to be outside," said Charlotte Gossett Navarro, chief director for Puerto Rico at the Hispanic Federation. "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 reported feeling dizzy and unwell during last summer's hottest days.

Navarro added that frequent power outages in Puerto Rico, partly due to aging infrastructure and Hurricane Maria's lasting damage, made it hard even to sleep. "If you are someone relatively healthy, that is uncomfortable ... but if you are someone who has a health condition, now your life is at risk."

"Heat waves are silent killers," said Friederike Otto, associate professor of climate science at Imperial College London. "People don't fall dead on the street ... they die in hospitals or poorly insulated homes and therefore are just not seen."

The report found that deadly temperatures in Morocco, Central Asia, South Sudan, and the Mediterranean would not have occurred without climate change. To respond, Roop Singh of the Red Cross called for improved early warning systems, heat action plans, and city redesigns.

But scientists warn that without reducing fossil fuel use, heat waves will only grow worse, and current safety strategies will eventually be overwhelmed.