Ancient trees offer clues about how hot Earth is getting. Very hot, they say.

Based on an analysis of ancient tree rings that date back to the year 1, last summer was the hottest in the past 2,000 years, a new study released Tuesday suggests. Study authors described the warmth during the summer of 2023 across much of the Northern Hemisphere as "unparalleled."

“It’s true that the climate is always changing, but when you look at the long sweep of history, you can see just how dramatic recent global warming is,” said Jan Esper, the lead author of the study from the Johannes Gutenberg University in Germany. “2023 warming, caused by greenhouse gases, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought."

Of even more concern, study authors note, is that the 2015 Paris Agreement to keep warming globally to 2.7 degrees "has already been superseded at this limited spatial scale." The Paris Agreement seeks to keep warming below that level to stave off the worst impacts of human-caused climate change.

The new study was published Tuesday in the peer-reviewed British journal Nature.

'Exceptional' warmth

High heat swept across much of the globe last summer, exacerbating deadly wildfires in Canada and Hawaii and fueling intense heat waves in South America, Japan, Europe and the U.S., while likely contributing to severe rainfall in Italy, Greece and Central Europe, NASA reported.

It is well documented that summer 2023 was the planet's hottest summer since instrumental records began in the 1800s, however, the new study says tree rings push it back even further, to 2,000 years ago. How does that work?

How tree rings tell us about climate change

Tree rings are an example of "climate proxy data, as they provide indirect evidence of past climates," according to the University Corp. for Atmospheric Research. Scientists can use tree ring patterns to reconstruct regional patterns of climatic change, the nonprofit consortium added.

NOAA said trees that depend heavily on temperature in the growing season have narrow rings during cold periods, and wider rings for warm periods.

Using tree-ring chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements.

For this study, scientists used meteorological station records dating back to the mid-1800s combined with tree rings from thousands of trees across nine sites in the Northern Hemisphere, to recreate what annual temperatures looked like in the distant past.

Last summer, they found, was 4 degrees warmer than the estimated average temperatures for the years 1 to 1890, based on these tree ring proxies.

More: Hot, hotter, hottest: NOAA, NASA say Earth endured the most sizzling summer on record in 2023

Hottest in 2000 years? That may be an understatement

University of Pennsylvania meteorologist Michael Mann, who reviewed the study for USA TODAY, was not impressed with the findings.

He said that relying on one particular type of proxy data (tree rings), which has its own limitations, is considerably less reliable than employing a variety of types of proxy data (tree rings, corals, ice cores, etc.). "A study was published in Nature just two years ago showing that the recent warming of the planet is unprecedented in more than 20,000 years," Mann said.

"It doesn’t really add to the already-established evidence that recent warming is unprecedented not just in 2,000 years but in at least 20,000 years."

Paris Agreement breached?

According to the study, the results also demonstrate that in the Northern Hemisphere, the 2015 Paris Agreement to limit warming to 2.7 degrees Fahrenheit  (1.5 degrees Celsius) above preindustrial levels has "already been breached."

Mann also disputes this, noting "the claim in the paper about the 1.5 degree C mark is particularly galling."

"The authors are analyzing midlatitude summer temperatures in the Northern Hemisphere (which are warming much faster than the annual global ocean plus land average), so it’s a thoroughly improper measure of how close we are to the actual thresholds of 1.5C and 2C that have been defined in terms of global annual warming."

Big picture

Overall, the study authors said to focus on the big picture when it comes to climate change: "When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately," said Esper.

Contributing: Reuters

This article originally appeared on USA TODAY: Hottest summer in 2,000 years: Summer 2023 shattered records