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Ancient trees unlock an alarming new insight into our warming world

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A traffic warden in Las Vegas, Nevada on July 12, 2023, where temperatures reached 106 degrees amid a heatwave


Last summer, marked by deadly extreme heat and devastating wildfires, was the warmest in at least 2,000 years, according to new research, which analyzed weather data and tree rings to reconstruct a detailed picture of the past.

The findings offer a stark insight into the “unparalleled” warming the world is experiencing today thanks to humans burning vast amounts of planet-heating fossil fuels, according to the authors of the study published Tuesday in the Journal Nature. And it’s an alarming signal as some scientists warn 2024 is on track to be even hotter still.

Global warming is currently tracked by comparing temperatures to the “pre-industrial era,” before humans started burning large amounts of fossil fuels, widely defined as the period between 1850 to 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 breached this threshold, according to the report. Using data taken from temperature instruments during this period, the scientists found the Northern Hemisphere summer in 2023 was 2.07 degrees Celsius warmer than the pre-industrial period.

But observational data from this period is sparse, uncertain and skews warmer. So, for a fuller picture of how the climate varied naturally 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 across nine regions of the Northern Hemisphere, including North America and Scandinavia, but excluding the Tropics which lack good tree data.

Trees act as time capsules. The patterns of their rings – affected by sunlight, rainfall and temperature – provide a climate history for each year of their lives, going back centuries or even thousands of years.

This complex tree ring data allowed the scientists to reconstruct annual temperatures for Northern Hemisphere summers between the years 1 and 1849 and compare them to last summer’s temperatures.

They found the summer of 2023 was warmer than any other summer during this period.

It was at least 0.5 degrees Celsius warmer than the warmest summer during this period, the year 246 – when the Roman Empire still ruled over Europe and the Mayan Civilization dominated Central America.

At the other end of the scale, last summer was nearly 4 degrees Celsius warmer than the coldest summer the study identified, the year 536 – when a volcanic eruption pumped out vast amounts of planet-cooling gases.

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A tourist cooling down in a fountain amid a heatwave in Barcelona, Spain, on July 19, 2023.

Using this 2,000-year data set, they calculated that the summer of 2023 was 2.2 degrees Celsius hotter than the long-term pre-industrial average, before robust networks of instruments could measure the weather.

The study follows a report published in November, which found humanity lived through the hottest 12-month period in at least 125,000 years. The study, and others like it, rely on data extracted from other proxies, such as ice cores and coral reefs, which don’t give the same detailed yearly evidence as tree rings.

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People use umbrellas and parasols to seek relief from the heat in Tokyo on July 30, 2023.

This makes it hard 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 is possible – even likely – last year was the hottest in at least 125,000 years, he added, but “we don’t have the data” to say for certain.

The deep dive into the year-by-year 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 “we have enough temperature reconstructions from enough places around the world to document the exceptional nature of a single year of large-scale temperature extremes.”

This “treasure chest of data” can be used to “sharpen our projections of future climate extremes,” she added.

While the study can place the extraordinary Northern Hemisphere heat into historical context, it cannot be applied on a global scale, Esper said. There simply isn’t enough tree ring data from the Southern Hemisphere and the Tropics, he said.

The study’s findings are deeply worrying, Esper said. “There are potential irreversible processes in the system, and I am afraid not of myself. I’m old,” he added. “I’m concerned for the kids.”

CNN’s Laura Paddison contributed to this report.

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