Tree rings reveal summer 2023 was the hottest in 2 millennia

Last year's summer was the hottest in 2,000 years, ancient tree rings reveal.

Researchers already knew that 2023 was one for the books, with average temperatures soaring past anything recorded since 1850. But there are no measurements stretching further back than that date, and even the available data is patchy, according to a study published Tuesday (May 14) in the journal Nature. So, to determine whether 2023 was an exceptionally hot year relative to the millennia that preceded it, the study authors turned to records kept by nature.

Trees provide a snapshot of past climates, because they are sensitive to changes in rainfall and temperature. This information is crystalized in their growth rings, which grow wider in warm, wet years than they do in cold, dry years. The scientists examined available tree-ring data dating back to the height of the Roman Empire and concluded that 2023 really was a standout, even when accounting for natural variations in climate over time. 

"When you look at the long sweep of history, you can see just how dramatic recent global warming is," co-author Ulf Büntgen, a professor of environmental systems analysis at the University of Cambridge in the U.K., said in a statement. The data indicated that "2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically," he said.

Temperatures recorded during the summer of 2023 exceeded those of the coldest summer in the past 2,000 years, in A.D. 536, by 7 degrees Fahrenheit (3.9 degrees Celsius). That relatively cool summer followed a volcanic eruption that dumped huge amounts of sunlight-blocking sulfur particles into the stratosphere, which triggered global cooling, according to the study.

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Büntgen and his colleagues also compared the tree-ring data with written temperature records from the 19th century. Climate change is evaluated against a baseline average temperature that prevailed before the Industrial Revolution, and it turns out that temperatures around 1850 were slightly colder than previously thought, the researchers found.

When they recalibrated the baseline temperature to reflect this, the researchers concluded that, in the Northern Hemisphere, the threshold set by the Paris Agreement to limit warming to 1.5 C (2.2 F) above pre-industrial levels has already been breached. 

With the recalibration, the researchers also estimated that the Northern Hemisphere summer of 2023 was an average 3.7 F (2 C) warmer than all the summers between 1900 and 1950. After 2023, the next hottest summer on record was 2016, according to the National Oceanic and Atmospheric Administration (NOAA).

"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," lead author Jan Esper, a professor of climate geography at the Johannes Gutenberg University Mainz in Germany, said in the statement. 

El Niño conditions could last into early summer 2024, meaning the coming months may break last year's record, according to the study. Climate scientists forecast El Niño could quickly flip into the opposite atmospheric pattern of La Niña, but the switch probably won't diminish this summer's heat because the effects of La Niña would take time to kick in.

One limitation of the new study is that the results may only apply to the Northern Hemisphere, the authors noted, since that's where they sourced the tree-ring data. Data for the same period is sparse in the Southern Hemisphere, and the trees there may respond differently to fluctuations in the climate due to a large portion of that hemisphere being covered by oceans.