This is the best summary I could come up with:

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.

“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 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.

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.

(The recent Hunga Tonga eruption, by contrast, happened under the ocean and sprayed enormous amounts of water vapor into the atmosphere.

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