Study: Tree Rings Show Summer of 2023 Was Very Hot

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

When the temperatures increased last year, many weather agencies said it was the hottest month, summer and year on record. But those records only go back to 1850 because they are based on thermometers - devices used for measuring temperature.

Now, however, scientists can look back to year one of the modern western calendar. And researchers have found no hotter northern summer than that of last year.

A study in the scientific publication 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 the study's lead writer Jan Esper of the Gutenberg Research College in Germany.

Before modern times, the hottest summer was in the year 246, Esper said.

Esper's paper showed that in the Northern Hemisphere, the summer of 2023 was as much as 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-writer 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 places in the Northern Hemisphere where there was enough data to get a good measure going back to year 1, Esper said. There was not quite enough tree data in the Southern Hemisphere to publish, but even the limited data showed something similar, he said.

Scientists looked at the rings of yearly tree growth to then give yearly dates to every ring, Torbenson suggested.

But outside experts had questions about the study. University of Pennsylvania climate scientist Michael Mann, who was not part of the study, wondered why researchers stopped their look back at year 1. Mann suggested other temperature estimates go back more than 20,000 years. He said just using tree rings is "considerably less reliable" than looking at all sorts of other data, including ice cores, corals and more.

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

The worldwide temperature records set last summer were so high "that it's not surprising they would ...clearly be the warmest in the past 2,000 years," said Berkeley Earth climate scientist Zeke Hausfather. Hausfather was not part of the study.

"It's likely the warmest summer in 120,000 years, though we cannot be absolutely sure," he added, because data exact to a year does not go back that far.

Because high-resolution yearly data does not go back that far, Esper said it is 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 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 sheltered from it, as they are now. And tree rings continue to show that it was about 0.2 degrees Celsius cooler than thermometers show.

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

I'm John Russell.

Seth Borenstein reported on this story for the Associated Press. John Russell adapted if for VOA Learning English.

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calendar – n. an arrangement of time into days, weeks, months, and years

calculate – v. to determine by mathematical processes

reliable – adj. giving the same result on many trials; dependable

coral – n. a kind of animal (a polyp) that forms reefs

core – n. a part (such as a thin cylinder of material) removed from the interior of a mass especially to determine composition