Some of the extreme temperatures recorded in the Southwestern United States, southern Europe and northern Mexico at the beginning of the month would have been “virtually impossible” without the influence of human-caused climate change, according to research made public Tuesday.

During the first half of July hundreds of millions of people in North America, Europe and Asia sweltered under intense heat waves. A heat wave in China was made 50 times as likely by climate change, the researchers said.

World Weather Attribution, an international group of scientists who measure how much climate change influences extreme weather events, focused on the worst heat so far during the northern hemisphere summer. In the United States, temperatures in Phoenix have reached 110 degrees Fahrenheit, roughly 43 Celsius, or higher for more than 20 days in a row. Many places in southern Europe are experiencing record-breaking, triple-digit temperatures. A remote township in Xinjiang, China, hit 126 degrees, breaking the national record.

“Without climate change, we wouldn’t see this at all,” said Friederike Otto, a senior lecturer in climate science at Imperial College London and co-founder of World Weather Attribution. “Or it would be so rare that it basically would not be happening.”

But in a climate changed by fossil fuel emissions, heat waves of this magnitude “are not rare events,” she said.

Before the industrial revolution, the North American and European heat waves were virtually impossible, according to the researchers’ statistical analysis. China’s heat wave would only have happened about once every 250 years.

If the composition of the atmosphere remained at today’s levels, the United States and Mexico could expect heat waves like the one this July about once every 15 years. In southern Europe, there would be a 1 in 10 chance each year of a similar event. In China there’s a 1 in 5 chance each year of a reoccurrence.

But because humans are continuing to burn fossil fuels and put extra greenhouse gases into the atmosphere, the odds will continue to tip in extreme heat’s favor: even if we stop, temperatures will not cool again, they will just stop rising.

“The heat waves we are seeing now, we definitely need to live with,” Dr. Otto said.

As temperatures have climbed in Europe, Greece has faced a rash of wildfires that have forced the largest evacuations in the country’s history. The blistering heat has made firefighting efforts more challenging, officials said. More frequent and more intense wildfires in the Mediterranean can also be linked to climate change, according to a recent study.

“We have rising risks from heat,” said Julie Arrighi, director of the Red Cross and Red Crescent Climate Centre and one of the researchers with World Weather Attribution. “It is deadly.” She emphasized the need to adapt cities and critical infrastructure to extreme heat, but also to reduce greenhouse gas emissions at the same time.

Many local and national governments, especially in Europe, have created heat action plans that include things like public cooling centers, and advance warning and coordination between social services and hospitals.

But even where these programs exist they are imperfect, and for now, the human cost of extreme temperatures remains high. The death toll from this month’s heat won’t be clear for some time, but more than 100 people have already died this summer in Mexico of heat-related causes, according to the national health secretary. Last summer, approximately 61,000 people died across Europe because of heat waves, according to another recent study.

World Weather Attribution’s heat wave study was not peer-reviewed, but the findings are based on standardized methods published in 2020. The group uses more than a dozen climate models to compare observed temperatures from the real world with modeled projections of the planet without human-caused climate change.

“This methodology is very standard in the field,” said Andrew Pershing, vice president for science at the nonprofit group Climate Central. He was not involved in the Tuesday study but has collaborated with World Weather Attribution in the past.

The sheer heat much of the planet is currently experiencing is “shocking” in a historical context, Dr. Pershing said, but added that the findings of climate change’s role are “not surprising.”

The first two weeks of July were probably Earth’s hottest on human record, according to an analysis by the European Union’s Copernicus Climate Change Service. The National Oceanic and Atmospheric Administration forecasts more unusually hot temperatures across most of the United States in August.



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