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Climate: Last northern summer was the warmest in more than 2,000 years, tree rings show

The scorching summer of 2023 was the hottest in the northern hemisphere in more than 2,000 years, according to a new study.

When temperatures soared last year, many weather agencies said it was the hottest month, summer and year on record. But these records only date back to 1850 at best because they are based on thermometers. Scientists can now go back to Year 1 of the modern Western calendar, when the Bible says Jesus of Nazareth walked the Earth, but have not found a warmer northern summer than last year.

A study Tuesday in the Nature magazine uses a well-established method and a record of more than 10,000 tree rings to calculate summer temperatures for each year since year 1. No year has been this close to last summer’s intense heat, a said lead author Jan Esper, a climate geographer at 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 246, Esper said. It was the beginning of the medieval period of history, when the Romans Emperor Philip the Arab fought the Germans along the Danube.

Esper’s paper showed that in the Northern Hemisphere, the summer of 2023 was 2.1 degrees Fahrenheit (1.2 degrees Celsius) warmer than the summer of 246. In fact, 25 of the last 28 years have been warmer than that early medieval summer, said study co-author Max Torbenson.

“It gives us a good idea of ​​how extreme 2023 is going to be,” 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 number going back to the first year, Esper said. There wasn’t enough data on trees in the Southern Hemisphere to publish, but the scant data showed something similar, he said.

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

Why stop looking back at the first year, when other temperature reconstructions go back more than 20,000 years, asked University of Pennsylvania climatologist Michael Mann, who was not involved in the study but published the famous hockey stick graphic more than a quarter of a century ago. showing the rise in temperatures since the industrial era. He said relying on tree rings alone is “considerably less reliable” than looking at all kinds of proxy data, including ice cores, corals and more.

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

“The global temperature records set last summer were so staggering – beating the previous record by 0.5°C in September and 0.4°C in October – that it is no surprise that they are clearly warmest in the last 2,000 years,” said the Berkeley Earth climatologist. Zeke Hausfather, who was not part of the study. “This is probably the hottest summer in 120,000 years, although we can’t be absolutely sure,” he said, because precise one-year data 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 this the hottest temperature in 120,000 years. Two thousand years are enough, he said.

Esper also said that the pre-industrial period from 1850 to 1900 that scientists – particularly the Intergovernmental Panel on Climate Change – use as a reference period before warming could be a bit colder than the data show. instrumental recordings. At the time, instruments were more often exposed to the hot sun instead of being protected as they are today, and tree rings continue to show that it was around 0.4 degrees (0.2 degrees Celsius) cooler than what the thermometers show.

That means there has been somewhat more warming due to human-caused climate change than most scientists estimate, an issue raised by researchers in recent years.

Looking at temperature records, particularly those from the past 150 years, Esper noticed that while they generally increase, they tend to do so with slow increases and then leaps and bounds, like what happened Last year. He said these measurements are often associated with a natural El Nino, a warming of the central Pacific which changes the weather around the world and adds even more heat to climate change.

“I don’t know when the next step will be taken, but I won’t be surprised by another huge step in the next 10 to 15 years, that’s for sure,” Esper said during a press briefing. “And that’s very worrying.”


This story has been corrected to refer to Jesus of Nazareth, rather than Jesus Christ.


Read more of AP’s climate coverage at


Follow Seth Borenstein on @borenbears


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