In a stark new finding, a study shows that six straight months of anomalously mild conditions in large parts of northern Siberia so far this year, along with an Arctic temperature record of 100.4 degrees (38 Celsius) that occurred in June, would have been virtually impossible without human-induced global warming.

The study, released Wednesday by the World Weather Attribution project, was produced through a collaboration between climate researchers from multiple institutions in France, Germany, the Netherlands, Russia, Switzerland and the United Kingdom.

The researchers found that the prolonged January-to-June heat, which has led to a record spike in wildfires across the Siberian Arctic, was made at least 600 times as likely by human-caused climate change. This led them to conclude such an event would be nearly impossible in the absence of global warming.

The analysis shows that the six months of much above-average temperatures in the region would only occur less than about once in 80,000 years without human-caused climate change.

The scientists used an increasingly well-established technique known as climate detection and attribution, which is analogous to climate change detective work, to determine whether and by how much global warming influenced the odds of an extreme climate or weather event.

In this case, the researchers used statistical methods and dozens of computer models to examine six months of above-average temperatures seen during the January through June period in much of Siberia, as well as the tentative Arctic temperature record of 100.4 degrees (38 Celsius) that occurred in Verkhoyansk, north of the Arctic Circle on June 20.

Even in the current climate, the study shows, the six months of unusually mild conditions stand out, with the event expected to occur less than once every 130 years.

The study found that for the large region examined, temperatures during the past six months would have been at least 3.6 degrees (2 Celsius) cooler had the same weather pattern established itself in the January to June period in 1900 compared to 2020.

And in Verkhoyansk, maximum June temperatures have increased by at least 1.8 degrees (1 Celsius) relative to 1900.

Past studies from this group have consistently found that climate change makes heat waves more likely and more intense than they would be in the absence of human-caused global warming. But they have not reached such strong conclusions before.

For example, one study of a European heat wave last year found that global warming probably quintupled the odds of the extreme event, yet the new study finds that Siberia’s January to June heat streak might only happen less than approximately once every 80,000 years in the absence of global warming.

“From what we’ve done, it’s the strongest signal that we have seen,” said Friedericke Otto, acting director of Oxford’s Environmental Change Institute and co-leader of the World Weather Attribution initiative, during a conference call with reporters.

The temperatures in Siberia during the past six months would have been “effectively impossible without human influence,” said Andrew Ciavarella, a lead author of the research and senior detection and attribution scientist at the U.K. Met Office.

Ciavarella said there are greater uncertainties about how much global warming increased the odds of the record temperature at Verkhoyansk, but that the hottest June temperatures at that location have warmed by as much as 1.8 to 3.6 degrees (1 to 2 Celsius), making the Arctic record temperature much more likely to occur today.

“In the natural world, this event would have been nearly impossible,” he said.

The new research is not the first to conclude that an extreme event would have been virtually impossible absent human-caused climate change. So, too, did another study on a deadly 2018 heat wave in Japan, and a study published last year on a spate of heat extremes around the Northern Hemisphere.

To arrive at the study’s conclusions, researchers ran about 70 computer models with thousands of simulated years to capture the climate’s evolution both with and without the addition of greenhouse gases into the atmosphere. They also examined observational records in the broad region and in Verkhoyansk.

The study found that temperatures in the region have been warming at about three times the rate of the rest of the world, and looking forward to 2050, between another 0.9 to 9 degrees (0.5 to 5 Celsius) of warming is anticipated.

While the study itself was done as a rapid analysis without peer review, the methods the scientists used were based on peer-reviewed research and the authors said they probably will submit the work to an academic journal.

Michael Wehner, a senior scientist at Lawrence Berkeley National Laboratory who was not involved in the new study, said the methods the researchers used are “state of the art.” He described the findings as conservative.

“This is a very careful and comprehensive study performed by the finest in the field using established methods,” Wehner said via email. “Their finding of a very large human influence on this event is actually quite conservative. There is no doubt in my mind that this heat event was made much more severe by the human interference in the climate system.”

Katharine Hayhoe, a climate scientist at Texas Tech who was not involved in the new study, said many attribution studies find that human influences on particular extreme events are relatively small, but that’s not the case with this one.

“In this case, the answer is truly unprecedented. We are witnessing events today that would, as the authors state, be nearly impossible absent human-induced climate change,” she said in an email.

“The reality is that we humans are conducting an unprecedented experiment with the only planet we have. It was already unprecedented in terms of the amount of carbon we are pumping into the atmosphere every year, and now it is becoming unprecedented in terms of the impacts that we as humans are witnessing first-hand.”

Siberian heat impacts are widespread and ongoing

The prolonged, unusually mild temperatures in Siberia are having clear impacts on ecosystems, human settlements and even the climate itself. Arctic wildfires sparked unusually early this year because of hot, dry conditions in Siberia, with particularly fierce blazes occurring in Russia’s Sakha Republic. Such fires add to global warming by emitting carbon dioxide and soot, and they can also destabilize permafrost, releasing ancient stores of carbon dioxide and methane.

Arctic infrastructure is also threatened by warming temperatures, as was demonstrated by the damaging oil spill in Norilsk, which authorities have blamed on melting permafrost. While Siberia is not heavily populated, heat waves are among the deadliest weather events in much of the world, and they can exacerbate preexisting conditions while affecting people both mentally and physically.

“As emissions continue to rise we need to think about building resilience to extreme heat all over the world, even in Arctic communities — which would have seemed nonsensical not very long ago,” Otto said in a statement.