Ancient trees open an alarming new perspective on our warming world

CNN
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Last summer, marked by deadly extreme heat and devastating wildfires, was the hottest in at least 2,000 years, according to a new study that analyzed weather data and tree rings to piece together a detailed picture the past.

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

Global warming is currently tracked 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. As part of the Paris Agreement in 2015, countries agreed to limit global warming. 2 degrees above pre-industrial levels.

Last summer, the world temporarily exceeded this threshold, according to the report. Using data collected by temperature instruments during this period, scientists found that the summer of 2023 in the Northern Hemisphere was 2.07 degrees Celsius warmer than the pre-industrial period.

But observational data from this period are rare, uncertain and warmer. So to get a more complete 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 that lack good tree data.

Trees act like time capsules. The configurations of their rings – affected by sunlight, precipitation and temperature – provide a climatic history for each year of their lives, going back centuries or even thousands of years.

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

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

It was at least 0.5 degrees Celsius warmer than the hottest summer of that 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 nearly 4 degrees Celsius hotter than the coldest summer identified by the study, the year 536 – when a volcanic eruption pumped out large quantities of gases cooling the planet.

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 instrument networks could measure the weather.

The study follows a report released in November that found humanity experienced the hottest 12-month period in at least 125,000 years. The study, and others like it, rely on data extracted 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 days, or even years, with those of the past, said Jan Esper, lead author of the study and professor of climate geography at Johannes Gutenberg University in Germany.

It’s possible – 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.

Comprehensively studying the annual temperatures of Northern Hemisphere summers is a “worthy 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 places around the world to document the exceptional nature of a single year of extreme temperatures.” in large scale “.

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

Although the study can put the Northern Hemisphere’s extraordinary heat into historical context, it cannot be applied on a global scale, Esper said. There simply isn’t enough data on tree rings in the Southern Hemisphere and the tropics, he said.

The study’s findings are deeply disturbing, Esper said. “There are potentially irreversible processes in the system, and I am not afraid of myself. I’m old,” he added. “I worry about the children.”

CNN’s Laura Paddison contributed to this report.