Last summer, marked by deadly extreme heat It is devastating forest firesit was the hottest in at least 2,000 years, according to new research, which analyzed meteorological data and tree rings to reconstruct a detailed picture of the past.
The findings offer a clear picture of the “unparalleled” warming the world is experiencing today thanks to the burning of vast amounts of planet-warming fossil fuels, according to the study authors. to study published Tuesday in the Journal Nature. And it’s an alarming sign, as some scientists warn that 2024 is on track to be even hotter yet.
Global warming is currently monitored by comparing temperatures to the “pre-industrial era,” before humans began burning large amounts of fossil fuels, broadly defined as the period between 1850 and 1900. Under the Paris Agreement in 2015, countries agreed to restrict global warming. to 2 degrees above pre-industrial levels.
Last summer, the world temporarily surpassed this threshold, according to the report. Using data obtained from temperature instruments during this period, scientists found that the Northern Hemisphere summer in 2023 was 2.07 degrees Celsius warmer than the pre-industrial period.
But observational data from this period are scarce, uncertain and distorted. So, to get a fuller picture of how climate naturally varied before the start of the pre-industrial era, the study authors looked much further into the past.
To do this, they used detailed sets of tree ring records from thousands of trees in nine regions of the Northern Hemisphere, including North America and Scandinavia but excluding the Tropics, which lack good tree data.
Trees function as time capsules. The patterns of their rings – affected by sunlight, precipitation and temperature – provide a climate story for each year of their lives, going back centuries or even thousands of years.
This complex tree ring data allowed scientists to reconstruct the annual temperatures of Northern Hemisphere summers between the years 1 and 1849 and compare them to last summer temperatures.
They found that the summer of 2023 was hotter than any other summer during this period.
It was at least 0.5 degrees Celsius warmer than the hottest summer during this period, the year 246 – when the Roman Empire still ruled Europe and the Mayan Civilization dominated Central America.
At the other end of the scale, last summer was almost 4 degrees Celsius hotter than the coldest summer identified by the study, the year 536 – when a volcanic eruption pumped out huge amounts of planet-cooling gases.
Using this 2,000-year dataset, they calculated that the summer of 2023 was 2.2 degrees Celsius warmer than the long-term pre-industrial average, before robust networks of instruments could measure the climate.
The study follows a report published in November, which found that humanity has experienced the Hottest 12-month period in at least 125,000 years. The study, and others like it, are based on data drawn from other indicators, such as ice cores and coral reefs, which do not provide the same detailed annual evidence as tree rings.
This makes it difficult to compare individual days or even years with those in the past, said Jan Esper, lead author of the study and professor of climate geography at Johannes Gutenberg University in Germany.
It’s possible – and even likely – that last year was the hottest in at least 125,000 years, he added, but “we don’t have the data” to say for sure.
The deep dive into the annual temperatures of Northern Hemisphere summers is a “worthwhile endeavor,” said Kim Cobb, a climate scientist at Brown University who was not involved in the study.
What’s impressive, she told CNN, is that “we have enough temperature reconstructions from enough locations around the world to document the exceptional nature of a single year of large-scale temperature extremes.”
This “treasure trove of data” can be used to “improve our projections of future climate extremes,” she added.
While the study can put the extraordinary heat of the Northern Hemisphere in historical context, it cannot be applied on a global scale, Esper said. There simply isn’t enough data on tree rings from the Southern Hemisphere and the Tropics, he said.
The study’s findings are deeply concerning, Esper said. “There are potentially irreversible processes in the system and I am not afraid of myself. I’m old,” he added. “I’m worried about the kids.”
CNN’s Laura Paddison contributed to this report.
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