2023 was the hottest summer in the Northern Hemisphere in more than 2000 years, according to a new analysis of tree rings dating back to the year 1.

Scientists at the University of Cambridge and the Johannes Gutenberg University Mainz analyzed more than 10,000 tree rings to determine temperature levels dating back two millennia. They found the average temperature from June to August 2023 was 2.2 degrees Celsius warmer than the average temperature in the same period from the year 1 to 1890, even when accounting for broad climate variations.

The research was published in the journal Nature on Tuesday.

When you look at the long sweep of history, you can see just how dramatic recent global warming is, said professor Ulf Bntgen, from Cambridges Department of Geography. 2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.

The data relied on tree rings from 15 different regions across the Northern Hemisphere. Esper told The Associated Press there was not enough data to publish similar findings about heat in the Southern Hemisphere, but there were similarities in the limited tree rings they were able to examine.