A summer marked by deadly heatwaves across Asia, Europe, and North America last year turns out to have been the hottest in the Northern Hemisphere in at least 2,000 years, according to a new study published in the journal Nature.

Officially, 2023 went down in history books as the hottest on record for the planet — but those records only started in 1850. To see how drastically the climate has changed over millennia, the authors of the new paper studied ancient tree rings to gauge fluctuations in temperatures over the years.

The results show us how extreme the weather is becoming. And while temperatures have reached unprecedented peaks, they’re also a warning of what’s to come unless policymakers do more to turn down the heat.

“Personally, I’m not surprised, but I’m worried,” Jan Esper, lead author of the study and a professor of climatology at Johannes Gutenberg University, said in a briefing with reporters. “The longer we wait, the more expensive it will be and the more difficult it will be to mitigate or even stop [global warming].”

For this study, Esper and his colleagues were limited to data they could collect from the Northern Hemisphere outside of tropical regions. Most of the oldest meteorological stations, dating as far back as the mid- to late 1800s, are located in the Northern Hemisphere. And of those, 45 of 58 are in Europe. To look further back in time and across a broader area, they relied on tree rings from the wood archives of archaeologists.

The cross section of a tree can tell us about its life and the world in which it lived. Many trees add a layer of light-colored “earlywood” each spring and a layer of dark “latewood” each summer. Counting up the rings shows the tree’s age. Thicker rings might indicate a warmer year in trees that time their growing seasons with changes in temperature.

This is a treasure trove of data in cooler climates with defined seasons. But unfortunately, again, it’s found mostly in the Northern Hemisphere. There’s a dearth of this data in more arid and tropical regions in the Southern Hemisphere, where there might be fewer trees or trees that don’t share the same growing patterns.

Working with what they had, the researchers found that land temperatures in the summer of 2023 in the Northern Hemisphere were 2.2 degrees Celsius higher than average temperatures between the years 1–1890. On paper, that might look like a small difference. When it comes to life on Earth, that is a significant shift.

It’s a steeper rise in temperature than the goal set out in the landmark Paris agreement, which strives to stop global temperatures from climbing more than 1.5 to 2 degrees Celsius higher than they were before the Industrial Revolution.

Two degrees Celsius of global warming would be enough to shift 13 percent of Earth’s ecosystems to a new biome, according to the Intergovernmental Panel on Climate Change. Much of the Amazon rainforest is in danger of becoming a savannah, for example. Coral reefs would decline by 99 percent, and nearly 40 percent of the world’s population could experience severe heatwaves at least once every five years.

We saw a deadly taste of that already last year, with record-breaking heatwaves across Europe, North America, and China that would have been “extremely rare or even impossible without human-caused warming,” according to an international collaboration of researchers called World Weather Attribution. 

It was a particularly sweltering year in part because of an El Niño climate pattern that dealt a double whammy alongside climate change in 2023.  El Niño hasn’t ended yet, so that combo is already expected to make this summer another scorcher. Meeting the goals of the Paris accord would stop climate change in its tracks, however, if countries around the world can transition to clean energy by 2050.

“I am not concerned about myself because I’m too old, but I have two children and there are many other children out there. And for them [global warming is] really dangerous,” Esper said. “So we should do as much as possible as soon as possible.”