by The summer of 2023 was the hottest in the Northern Hemisphere for more than 2,000 years, a new study has found.
When temperatures rose last year, many weather agencies said it was the hottest month, summer and year on record. But those records do not go back to 1850 because it is based on thermometers. Now scientists can go back to year 1 of the modern western calendar, when the Bible says Jesus Christ the Earth walked, but no summer was found warmer in the north than last year.
Tuesday’s study in the journal Nature uses an established method and a record of more than 10,000 tree rings to calculate summer temperatures for every year since year 1. No year even came close to last summer’s high heat, said the 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, 246 was the warmest year, Esper said. That was the beginning of medieval history, when the Roman Emperor Philip the Arab fought the Germans along the Danube River.
Esper’s paper showed that the summer of 2023 in the Northern Hemisphere was as much as 2.1 degrees Fahrenheit (1.2 degrees Celsius) warmer than the summer of 246. In fact 25 of the past 28 years were warmer than that early medieval summer, it 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 1 year, Esper said. There wasn’t enough tree data in the Southern Hemisphere to publish, but the sparse data showed something similar, he said.
The 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 each ring,” Torbenson said.
Why stop the look back at 1 year, when other temperature reconstructions go back more than 20,000 years, asked University of Pennsylvania climate scientist Michael Mann, who was part of the study for just over a quarter a hundred years ago the famous hockey stick graph published showing rising temperatures from the Industrial Age. He said relying on tree rings is “much 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 provide summer-by-summer temperature estimates, which cannot be done with corals, ice cores and other proxies. Tree rings are a higher resolution, he said.
“The global temperature records set last summer were so shocking – breaking the previous record by 0.5C in September and 0.4C in October – that it is no surprise that they would be the hottest over the past 2,000 years,” said the Berkeley Earth climate scientist. Zeke Hausfather, who was not part of the study. “It’s probably the hottest summer in 120,000 years, although we can’t be absolutely sure,” he said, because exact year-by-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 it the warmest in 120,000 years. Two thousand years is enough, he said.
Esper also said that the pre-industrial period of 1850 to 1900 used by scientists – particularly the Intergovernmental Panel on Climate Change – for the base period before warming may be slightly cooler than the instrumental records show. The instruments back then were more often in the hot sun instead of shielded as they are now, and tree rings continue to show that it was about 0.4 degrees (0.2 degrees Celsius) colder than thermometers indicate.
That means slightly more warming has come from human-caused climate change than most scientists calculate, a question researchers have been wrestling with for the past few years.
Looking at the temperature records, especially over the last 150 years, Esper noted that while they are generally increasing, they tend to do so with slow increases and then huge steps, like what happened last year. He said those phases are often associated with a natural El Nino, a warming of the central Pacific Ocean that changes weather around the world and adds even more heat to a changing climate.
“I don’t know when the next step will be taken, but I wouldn’t 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 a cause for concern.”
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