The world is on pace to add 57 extremely hot days a year, but the study suggests it could be worse

The world is on track to add nearly two months of extremely hot days each year by the end of the century, with smaller, poorer nations more often hurt than larger carbon polluters, a study published Thursday found.

But efforts to reduce greenhouse gas emissions, which began 10 years ago with the Paris climate agreement, have had a significant impact. The same study found that without it, the Earth would be headed toward an additional 114 days a year of those deadly hot days.

The international group of climate scientists World Weather Attribution and US-based Climate Central have teamed up to use computer simulations to calculate how much difference the landmark agreement made in terms of one of the biggest climate impacts on people: heat waves.

The report — which has not yet been peer-reviewed but uses well-established climate attribution techniques — calculated the number of extremely hot days the world and more than 200 countries experienced in 2015, how many Earths have now and what is expected in two future scenarios.

One scenario is that countries follow through on their promises to reduce emissions, and by 2100 the world will warm 2.6 degrees Celsius (4.7 Fahrenheit) above pre-industrial levels. This adds 57 extremely hot days to what Earth gets now, according to the study. The other scenario is a rise in temperatures of four degrees Celsius (7.2 degrees Fahrenheit), which the world was on track to achieve before the Paris Agreement. The study found that this would double the number of additional hot days.

Pain and suffering are coming

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

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

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

The report did not say how many people would be affected by additional dangerously hot days, but co-author Frederick Otto of Imperial College London said “it will certainly be tens of thousands or millions, not less.” She pointed out that thousands already die in heat waves every year.

Imagine the recent heatwaves but worse

Thursday’s study calculated that a week-long southern European heatwave in 2023 is now 70% more likely and 0.6 degrees Celsius (1.1 degrees Fahrenheit) warmer than it was 10 years ago when the Paris Agreement was signed. The report estimated that if the world’s climate control efforts were not increased, a similar heatwave at the end of the century could be 3 degrees Celsius (5.4 Fahrenheit) hotter.

A heat wave similar to last year’s in the southwestern United States and Mexico could increase by 1.7 degrees Celsius (3.1 degrees Fahrenheit) by the end of the century under the current carbon pollution trajectory, the report said.

Other groups have also found more than hundreds of thousands of deaths from recent heat waves in peer-reviewed research, most of which are due to human-caused climate change, said Christy Eby, a public health and climate scientist at the University of Washington, who was not part of Thursday’s report.

Most important of all, the data show how unjust the effects of climate change appear to be, even under the two less extreme scenarios. The scientists analyzed the number of extremely hot days expected for each country by the end of the century under this scenario.

Data for the country shows wide variation in temperatures

The 10 countries that will see the biggest increases in those dangerously hot days are almost all small and ocean-dependent, including the Solomon Islands, Samoa, Panama and Indonesia. Panama, for example, can expect an additional 149 extremely hot days. Overall, the top 10 countries produced just 1% of the heat-trapping gases now in the air, but they will get nearly 13% more extremely hot days.

But the most carbon-polluting countries, such as the United States, China and India, are expected to see only an additional 23 to 30 extremely hot days. They account for 42% of the carbon dioxide in the air, but get less than 1% from extra hot days.

“This report beautifully and concretely identifies what we have been saying for decades: that the effects of global warming will disproportionately affect developing countries that have not historically emit significant amounts of greenhouse gases,” said Andrew Weaver, a climate scientist at the University of Victoria, who was not part of the study team. “Global warming is driving another wedge between the have-haves and the have-nots; this will ultimately sow the seeds of further geopolitical instability.”

The report found that Hawaii and Florida are the two US states that will see the largest increase in extremely hot days by the end of the century under the current carbon pollution trajectory, while Idaho will see the smallest jump.

While the report makes sense, the director of the Potsdam Climate Institute, Johan Rockström, who was not part of the research, said people should not take comfort that we are no longer on a path of 4 degrees of warming before Paris because the current path “still means a catastrophic future for billions of people on Earth.”

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