Scientists using climate information gleaned from tree ring data found that the summer of 2023 was the hottest for the Northern Hemisphere in the past 2,000 years.
"When you look at the long sweep of history, you can see just how dramatic recent global warming is," Cambridge Department of Geography professor Ulf Büntgen, a study co-author, said in a release accompanying the report.
Scientists from the University of Cambridge and the Johannes Gutenberg University Mainz, in Germany, published their results Tuesday in the journal Nature.
The world's leading meteorological agencies had already determined that 2023 was the hottest year on record, but that finding rests on instrument measurements that only go back to about 1850.
To extend their view of climatic history, scientists use a variety of proxy measures such as ice core samples. However, many of those measures do not provide sufficient resolution to allow a comparison to a single year's temperatures, Johannes Gutenberg University professor Jan Esper, another co-author, said in a briefing with reporters before the study's release.
"We stick with the tree rings because they are annual data," Esper said. The annually resolved climate data from tree rings enables a comparison that goes back two millennia to show just how extremely hot 2023 was.
The study also concludes that the Northern Hemisphere has already passed the temperature threshold established by the 2015 Paris Agreement on climate change. In that landmark international pact, nations agreed to limit warming to no more than 2 degrees Celsius (or 3.6 degrees Fahrenheit) above preindustrial levels and, ideally, to hold warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit).
The authors wondered if they could improve our view of just what the preindustrial temperatures were. They used large-scale tree ring data sets to recalibrate the baseline of preindustrial temperatures and determined that it was cooler than previously thought. Using that baseline, the authors said, the north of the globe surpassed the Paris limits in the summer of 2023.
Esper said he was not surprised by the results—he's worked in the field for 30 years—but he does find them worrying.
"It is very, very concerning," Esper said, adding that he worries for the future for his children. "The longer we wait to act, the more difficult it will be."
Other climate scientists not involved in the study had a mix of praise and criticism for the work.
James Hansen, director of the Climate Science, Awareness and Solutions Program at Columbia University's Earth Institute, said he agreed with the thrust of the paper that warming relative to preindustrial times is likely understated.
"My opinion is that, for all practical purposes, the world has reached the +1.5C global warming level relative to preindustrial time," Hansen told Newsweek via email.
Hansen is widely respected for his research and his influential testimony to Congress on global warming when he directed the NASA Goddard Institute for Space Studies.
The current director of the NASA Goddard Institute, Gavin Schmidt, called the study a good attempt at putting 2023 in a longer context.
"No surprise, 2023 really stands out even over two millennia," Schmidt said via email. He cautioned, however, that because the historical record the researchers used is only calibrated to northern summer conditions, the results should not be used for global comparisons of the kind the Paris Agreement is based on.
University of Pennsylvania Presidential Distinguished Professor Michael Mann was more critical of the study.
"It doesn't really add to the already-established evidence that recent warming is unprecedented not just in 2,000 years but in at least 20,000 years," Mann said via email to Newsweek, citing previous work published in Nature.
Mann and his colleagues published work in the late 1990s putting recent warming in the context of deep climate history. The "hockey stick" graph shows that global temperatures over the past 1,000 years were largely flat—the "handle" of the stick—but rose sharply in recent decades—the stick's "blade."
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