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 1, when the Bible says Jesus of Nazareth 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.
Even allowing for natural climate variations over hundreds of years, 2023 was still the hottest summer since the height of the Roman Empire, exceeding the extremes of natural climate variability by half a degree Celsius.
“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” said co-author Professor Ulf Büntgen, from Cambridge’s Department of Geography. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”
The results, reported in the journal Nature, also demonstrate that in the Northern Hemisphere, the 2015 Paris Agreement to limit warming to 1.5C above pre-industrial levels has already been breached.
Early instrumental temperature records, from 1850-1900, are sparse and inconsistent. The researchers compared early instrumental data with a large-scale tree ring dataset and found the 19th century temperature baseline used to contextualise global warming is several tenths of a degree Celsius colder than previously thought. By re-calibrating this baseline, the researchers calculated that summer 2023 conditions in the Northern Hemisphere were 2.07C warmer than mean summer temperatures between 1850 and 1900.
“Many of the conversations we have around global warming are tied to a baseline temperature from the mid-19th century, but why is this the baseline? What is normal, in the context of a constantly-changing climate, when we’ve only got 150 years of meteorological measurements?” said Büntgen. “Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context.”
Tree rings can provide that context, since they contain annually-resolved and absolutely-dated information about past summer temperatures. Using tree-ring chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements.
The available tree-ring data reveals that most of the cooler periods over the past 2000 years, such as the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century, followed large-sulphur-rich volcanic eruptions. These eruptions spew huge amounts of aerosols into the stratosphere, triggering rapid surface cooling. The coldest summer of the past two thousand years, in 536 CE, followed one such eruption, and was 3.93C colder than the summer of 2023.
Most of the warmer periods covered by the tree ring data can be attributed to the El Niño climate pattern, or El Niño-Southern Oscillation (ENSO). El Niño affects weather worldwide due to weakened trade winds in the Pacific Ocean and often results in warmer summers in the Northern Hemisphere. While El Niño events were first noted by fisherman in the 17th century, they can be observed in the tree ring data much further back in time.
However, over the past 60 years, global warming caused by greenhouse gas emissions are causing El Niño events to become stronger, resulting in hotter summers. The current El Niño event is expected to continue into early summer 2024, making it likely that this summer will break temperature records once again.
“It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought,” said Professor Jan Esper, the lead author of the study from the Johannes Gutenberg University Mainz in Germany. “When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”