2023 marks hottest summer in over 2,000 years in Northern Hemisphere, study finds

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

A study in the journal Nature used 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.

Before 2023, the 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.


A child runs through cool a fountain in Battery Park in Manhattan on September 06, 2023 in New York City. New York City and much of the East Coast are expected to see temperatures in the mid 90 through at least Thursday. (Photo by Spencer Platt/Getty

"That gives us a good idea of how extreme 2023 is," Esper told The Associated Press.

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.

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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.

"The global temperature records set last summer were so gobsmacking — shattering the prior record by 0.5C in September and 0.4C in October — that it’s not surprising they would be clearly be the warmest in the past 2,000 years," said Berkeley Earth climate scientist Zeke Hausfather, who wasn't part of the study. "It's likely the warmest summer in 120,000 years, though we cannot be absolutely sure," he said, because data precise to a year doesn't go back that far.

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.

"I don't know when the next step will be taken, but I will not be surprised by another huge step in the next 10 to 15 years, that's for sure," Esper said in a news briefing. "And it's very worrying."

The Associated Press contributed to this report. This story was reported from Los Angeles.