The world is on track to add nearly two months of dangerous superhot days each year by the end of the century, with poorer small nations hit far more often than the biggest carbon-polluting countries, a new study finds.

But efforts to curb emissions of heat-trapping gases that started 10 years ago with the Paris climate agreement have had a significant effect. Without them Earth would be heading to an additional 114 days a year of those deadly extra hot days, according to the same study.

The international collection of climate scientists World Weather Attribution and the U.S.-based Climate Central teamed up to use computer simulations to calculate just how much of a difference the landmark accord has made in terms of one of the biggest climate effects on people: heat waves.

The report—which is not yet peer-reviewed but uses established techniques for climate attribution—calculated how many superhot days the world and more than 200 countries got in 2015, how many Earth gets now and what’s projected in two future scenarios.

One scenario is if countries fulfill their promises to curb emissions and by the year 2100 the world warms 2.6°C above preindustrial times. That adds 57 superhot days to what Earth gets now, according to the study. The other scenario is the 4°C of warming that the world had been on track to hit before the Paris agreement. The study found that would double the number of additional hot days.

Pain and Suffering Coming

“There will be pain and suffering because of climate change,” said Climate Central Vice President for Science Kristina Dahl, a report co-author. “But if you look at this difference between 4 degrees C of warming and 2.6 degrees C of warming, that reflects the last 10 years and the ambitions that people have put forth. And to me, that’s encouraging.”

Nearly 200 nations gather next month in Brazil for international climate negotiations. Numerous groups of scientists, analysts and advocates are compiling reports that show the same mixed bag: The 2015 Paris agreement has made strides in the fight against climate change but it’s too little and too slow.

Northern Illinois University climate scientist Victor Gensini said Thursday’s hot day findings “highlight both the success and shortfall of the Paris Agreement.”

The study defines superhot days for each location as days that are warmer than 90% of the comparable dates between 1991 and 2020. Since 2015, the world has already added 11 superhot days on average, the report said.

“That heat sends people to the emergency room. Heat kills people,” Dahl said.

The report doesn’t say how many people will be affected by the additional dangerously hot days, but co-author Friederike Otto of Imperial College London said that “it will definitely be tens of thousands or millions, not less.” She noted that thousands die in heat waves each year already.

Imagine Recent Heat Waves But Worse

The study calculated that the weeklong southern Europe heat wave in 2023 is now 70% more likely and 0.6°C warmer than it would have been 10 years ago when the Paris agreement was signed. And if the world’s climate-fighting efforts don’t increase, a similar heat wave at the end of the century could be 3°C hotter, the report estimated.

A heat wave similar to last year’s Southwestern United States and Mexico heat wave could be 1.7°C hotter by the end of the century under the current carbon pollution trajectory, the report said.

Other groups are also finding more than hundreds of thousands of deaths from recent heat waves in peer-reviewed research with much of it because of human-caused climate change, said University of Washington public health and climate scientist Kristie Ebi, who wasn’t part of the report.

More than anything, the data shows how unfair the effects of climate change seem, even under the less extreme of the two scenarios. The scientists broke down how many extra superhot days are expected for each country by the end of the century under that scenario.

Country Data Shows High Heat Inequality

The 10 countries that will see the biggest increases in those dangerous heat days are nearly all small and dependent on the ocean, including the Solomon Islands, Samoa, Panama and Indonesia. Panama, for example, can expect 149 extra superhot days. Altogether the top 10 of those countries produced only 1% of the heat-trapping gases now in the air but will get nearly 13% of the additional superhot days.

But top carbon polluting countries, the United States, China and India are predicted to get only between 23 and 30 extra superhot days. They are responsible for 42% of the carbon dioxide in the air, but are getting less than 1% of the additional superhot days.

“This report beautifully and tangibly quantifies what we’ve been saying for decades. The impacts of global warming are going to disproportionally affect developing nations that historically haven’t emitted significant quantities of greenhouse gases,” said University of Victoria climate scientist Andrew Weaver, who wasn’t part of the study team. “Global warming is driving yet another wedge between have and have not nations; this will ultimately sow seeds of further geopolitical instability.”

Hawaii and Florida are the U.S. states that will see the biggest increase in superhot days by the end of the century under the current carbon pollution trajectory, while Idaho will see the smallest jump, the report found.

While the report makes sense, Potsdam Climate Institute Director Johan Rockstrom, who wasn’t part of the research, said people shouldn’t be relieved that we are no longer on the 4-degree warming pre-Paris trajectory because the current track “would still imply a disastrous future for billions of humans on Earth.”

This Associated Press story was published Oct. 16, 2025 by The Canadian Press.