Summer 2023 Was the Northern Hemisphere’s Hottest in 2,000 Years, Study Finds

The summer of 2023 was exceptionally hot. Scientists have already established that it was the warmest Northern Hemisphere summer since around 1850, when people started systematically measuring and recording temperatures.

Now, researchers say it was the hottest in 2,000 years, according to a new study published in the journal Nature that compares 2023 with a longer temperature record across most of the Northern Hemisphere. The study goes back before the advent of thermometers and weather stations, to the year A.D. 1, using evidence from tree rings.

“That gives us the full picture of natural climate variability,” said Jan Esper, a climatologist at Johannes Gutenberg University in Mainz, Germany and lead author of the paper.

Extra greenhouse gases in the atmosphere from the burning of fossil fuels are responsible for most of the recent increases in Earth’s temperature, but other factors — including El Niño, an undersea volcanic eruption and a reduction in sulfur dioxide aerosol pollution from container ships — may have contributed to the extremity of the heat last year.

The average temperature from June through August 2023 was 2.20 degrees Celsius warmer than the average summer temperature between the years 1 and 1890, according to the researchers’ tree ring data.

And last summer was 2.07 degrees Celsius warmer than the average summer temperature between 1850 and 1900, the years typically considered the base line for the period before human-caused climate change.