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.

The new study suggests that Earth’s natural temperature was cooler than this base line, which is frequently used by scientists and policymakers when discussing climate goals, such as limiting global warming to 1.5 degrees Celsius above the preindustrial era.

“This period is really not well covered with instruments,” Dr. Esper said, adding that “the tree rings can do really, really well. So we can use this as a substitute 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 scientists clues about changing environmental conditions. For example, trees tend to grow more and form wider rings during warm, wet years.

This study compared temperatures in 2023 to a previously published reconstruction of temperatures over the past 2,000 years. More than a dozen research groups collaborated to create this reconstruction, using data from about 10,000 trees across nine regions of the Northern Hemisphere between 30 and 90 degrees latitude, or everywhere above the tropics. Some data came from drilling very thin cores from living trees, but most came from dead trees and historical wood samples.

Covering longer stretches of time results in more volcanic eruptions being included in the data. Big eruptions, at least on land, can cool the Earth by spraying sulfur dioxide aerosols into the atmosphere. Over the past 2,000 years, about 20 or 30 such eruptions have taken place and brought down average temperatures, Dr. Esper said.

(The recent Hunga Tonga eruption, by contrast, happened under the ocean and sprayed enormous amounts of water vapor into the atmosphere. Water vapor is a powerful greenhouse gas.)

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, the lead scientist at Berkeley Earth. Dr. 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.”

To some extent, slight differences between the stories thermometers and tree rings tell us about Earth’s past don’t matter for the present, said Zeke Hausfather, another Berkeley Earth scientist.

“It’s an academic question more than a practical question,” he said. “Reassessing temperatures in the distant past really doesn’t tell us that much about the effects of climate change today.”

Last year, those effects included a heat dome that settled over much of Mexico and the southern United States for weeks on end. Japan had its hottest summer on record. Canada suffered its worst-ever wildfire season, and parts of Europe also battled a series of destructive wildfires. 2024 is expected to be another hot year.