Summer 2023 was Northern Hemisphere’s hottest in 2,000 years, study finds
Editor’s Note:
Allister Aalders is on vacation and will return next week.
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 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 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.
The new study suggests that Earth’s natural temperature was cooler than this base line.
“This period is really not well covered with instruments,” Esper said, adding that “the tree rings can do really, really well. So we can use
subsntitudte this as a and even as a corrective.”
Trees grow wider each year in a distinct pattern of light-colored rings in spring and early summer, and darker rings in late summer and fall. Each pair of rings represents one year, and differences between the rings offer clues about changing environmental conditions.
This study compared temperatures in 2023 with a previously published reconstruction of temperatures over the past 2,000 years. More than a dozen groups collaborated to create this reconstruction.
Not everyone agrees that tree rings offer a more accurate picture of past temperatures than historical records do.
“It’s still an active area of research,” said Robert Rohde, lead scientist at Berkeley Earth. Rohde wasn’t directly involved in the new study, but his organization’s data was used. “This is not the first paper to come out suggesting that there’s a warm bias in the early instrumental period, by any means. But I don’t think it’s really resolved.”