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Hot history: Tree rings show that last northern summer was the warmest since year 1


A new study has found that the hot summer of 2023 was the hottest in the Northern Hemisphere in more than 2,000 years.

When temperatures rose last year, many weather agencies said it was the hottest month, summer and year on record. But these records only go back to 1850 at best, because they are based on thermometers. Now scholars can go back to the first year of the modern Western calendar, when the Bible says Jesus of Nazareth walked the earth, but they have found no northern summer hotter than the summer of last year.

A study published Tuesday in the journal Nature used a well-established method and recorded more than 10,000 tree rings to calculate summer temperatures for each year since the first. No year has come close to the extreme heat of last summer, said lead author Jan Esper. A climate geographer at Gothenburg Research College in Germany.

Esper said that before humans started pumping greenhouse gases into the atmosphere by burning coal, oil and natural gas, the hottest year was 246. That was the beginning of the medieval period of history, when Roman Emperor Philip the Arab fought the Germans along the Danube River.

Esper’s paper showed that in the Northern Hemisphere, the summer of 2023 was up to 2.1°F (1.2°C) warmer than the summer of 246. In fact, 25 of the past 28 years have been hotter than early medieval summers, like he said. Study by co-author Max Torbinson.

“This gives us a good idea of ​​how dangerous 2023 will be,” Esper told The Associated Press.

The team used thousands of trees in 15 different locations in the Northern Hemisphere, north of the tropics, where there was enough data to get a good number going back to the first year, Esper said. He said there wasn’t enough data on trees in the Southern Hemisphere to publish, but sparse data showed something similar.

Scientists look at trees’ annual growth rings and “we can match them up almost like a puzzle in time so we can pinpoint annual dates for each ring,” Torbinson said.

Why stop looking at the first year, when other temperature reconstructions go back more than 20,000 years, asked University of Pennsylvania climate scientist Michael Mann, who was not part of the study but more than a quarter-century ago published the famous graph. For a hockey stick. Temperatures appear to have risen since the industrial era. He said that simply relying on tree rings is “vastly less reliable” than looking at all kinds of alternative data, including ice samples, coral reefs and more.

Esper said his new study uses only tree data because it is accurate enough to give temperature estimates summer after summer, which cannot be done using coral reefs, ice samples and other proxy factors. Tree rings have a higher resolution, he said.

“Last summer’s global temperature records were so stunning — breaking the previous record by 0.5°C in September and 0.4°C in October — that it is no surprise that it was the warmest during the past two thousand years. Zeke Housefather, who was not part of the study. “This summer is likely to be the warmest in 120,000 years, although we can’t be completely sure,” he said, because accurate one-year data doesn’t go back that far.

Because high-resolution annual data doesn’t go back that far, Esper said it’s a mistake for scientists and the media to describe it as the hottest in 120,000 years. He said that two thousand years is enough.

Esper also said that the pre-industrial period from 1850 to 1900 that scientists — particularly the Intergovernmental Panel on Climate Change — use as the base period before temperatures rose may have been slightly cooler than instrumental records show. Instruments at that time were often in the hot sun rather than protected as they are now, and tree rings continue to show that they were about 0.4 degrees (0.2 C) cooler than thermometers show.

This means there is little more warming from human-caused climate change than most scientists calculate, an issue that researchers have debated over the past few years.

Looking at temperature records, especially the last 150 years, Esper noted that while they are generally increasing, they tend to do so in slow rises and then giant steps, like what happened last year. He said these steps are often linked to the natural phenomenon of El Nino, a warming of the central Pacific Ocean that changes weather around the world and adds more heat to a changing climate.

“I don’t know when the next step will be taken, but I wouldn’t be surprised by another huge step in the next 10 to 15 years, that’s for sure,” Esper said in a press conference. “This is very disturbing.”

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This story has been corrected to refer to Jesus of Nazareth, not Jesus Christ.

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Read more AP climate coverage at http://www.apnews.com/climate-and-environment

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Follow Seth Borenstein on X at @borenbears

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AP’s climate and environment coverage receives financial support from multiple private foundations. AP is solely responsible for all content. Find AP’s standards for working with charities, a list of supporters and funded coverage areas on AP.org.



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