(NEWSnet/AP) — The summer of 2023 was the hottest in the Northern Hemisphere in more than 2,000 years, a new study found.

A study Tuesday in the journal Nature uses a well-established method and record of more than 10,000 tree rings to calculate summertime temperatures for each year since the year 1.

No year came even close to last summer’s high heat, said lead author Jan Esper, a climate geographer at the Gutenberg Research College in Germany.

The previous hottest year was the year 246, Esper said. That was the beginning of the medieval period of history, when Roman Emperor Philip the Arab fought Germans along the Danube River.

Esper’s paper showed that in the Northern Hemisphere, the summer of 2023 was as much as 2.1 degrees Fahrenheit warmer than the summer of 246. In fact, 25 of the last 28 years have been hotter than that early medieval summer, said study co-author Max Torbenson.

The team used thousands of trees in 15 different sites in the Northern Hemisphere, north of the tropics, where there was enough data to get a good figure going back to year 1, Esper said. There was not quite enough tree data in the Southern Hemisphere to publish, but the sparse data showed something similar, he said.

Scientists look at the rings of annual tree growth and “we can match them almost like a puzzle back in time so we can assign annual dates to every ring,” Torbenson said.

Esper said his new study only uses tree data because it is precise enough to give summer-by-summer temperature estimates, which can’t be done with corals, ice cores and other proxies. Tree rings are higher resolution, he said.

Because high-resolution annual data doesn’t go back that far, Esper said it’s wrong for scientists and the media to call it the hottest in 120,000 years. Two thousand years is enough, he said.

Looking at the temperature records, especially the last 150 years, Esper noticed that while they are generally increasing, they tend to do so with slow rises and then giant steps, like what happened last year. He said those steps are often associated with a natural El Nino, a warming of the central Pacific that changes weather worldwide and adds even more heat to a changing climate.

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