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

The broiling summer of 2023 was the hottest in the Northern Hemisphere in more than 2,000 years, a new study found.

When the temperatures spiked last year, numerous weather agencies said it was the hottest month, summer and year on record. But those records only go back to 1850 at best because it's based on thermometers. Now scientists can go back to the modern western calendar's year one, when the Bible says Jesus Christ walked the Earth, but have found no hotter northern summer than last year's.

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.

Before humans started pumping heat-trapping gases into the atmosphere by burning coal, oil and natural gas, 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 (1.2 degrees Celsius) 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.

“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.

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.

Why stop the look back at year 1, when other temperature reconstructions go back more than 20,000 years, asked University of Pennsylvannia climate scientist Michael Mann, who wasn't part of the study but more than a quarter century ago published the famous hockey stick graph showing rising temperatures since the Industrial Age. He said just relying on tree rings is “considerably less reliable” than looking at all sorts of proxy data, including ice cores, corals and more.

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.

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.

Esper also said the pre-industrial period of 1850 to 1900 that scientists — especially the Intergovernmental Panel on Climate Change — use for the base period before warming may be a bit cooler than the instrumental records show. The instruments back then were more often in the hot sun instead of shielded like they are now, and tree rings continue to show that it was about 0.4 degrees (0.2 degrees Celsius) cooler than thermometers show.

That means there's been a bit more warming from human-caused climate change than most scientists calculate, an issue being hashed out by researchers over the last few years.

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.”

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