However, the research also highlights the significant positive impact of global efforts to reduce heat-trapping gas emissions, initiated a decade ago with the landmark Paris climate agreement. Without these interventions, the Earth would be facing an alarming 114 extra superhot days annually.

This comprehensive analysis, a collaboration between the international climate scientists at World Weather Attribution and the US-based Climate Central, employed sophisticated computer simulations. They aimed to precisely quantify the difference the accord has made in mitigating one of climate change's most severe human impacts: extreme heatwaves.

While the report is currently awaiting peer review, its findings are based on well-established climate attribution methodologies. It meticulously calculated the number of superhot days experienced globally and across more than 200 nations in 2015, current figures and projections under two distinct future scenarios.

One scenario is if countries fulfill their promises to curb emissions and by the year 2100 the world warms 2.6 Celsius (4.7 Fahrenheit) above preindustrial times. That adds 57 superhot days to what Earth gets now, according to the study. The other scenario is the 4 C (7.2 F) 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.

"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."

The study defines superhot days for each location as days that are warmer than 90 per cent 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.

Thursday's study calculated that the weeklong southern Europe heat wave in 2023 is now 70 per cent more likely and 0.6 C (1.1 F) 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 (5.4 F) hotter, the report estimated.

A heat wave similar to last year's Southwestern United States and Mexico heat wave could be 1.7 degrees Celsius (3.1 degrees Fahrenheit) 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 Thursday's 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.

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 pervcent of the heat-trapping gases now in the air but will get nearly 13 per cent 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 pervcent of the carbon dioxide in the air, but are getting less than 1 pervcent 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."

World on track to add nearly two months of superhot days each year, study finds