It’s one thing to say the Northern Hemisphere summer of 2023 was the hottest of the 150 years people have been making measurements. This well-documented claim is often dismissed by skeptics of global warming who point out that the Earth has a long history of temperature fluctuations.

That’s why it’s important that a new paper shows last summer was actually the hottest in the last 2,000 years — and that our current temperatures are even more of an outlier than we realized.

If all we had were the few decades of temperature readings to understand the past climate, we wouldn’t know whether our current warming was a major shift or a run-of-the-mill blip. Tree rings hold records that can go back thousands of years, giving us the perspective we need to understand what’s happening today.

In a paper published in Nature last week, scientists used tree rings not only to show long-term trends, but to plot Northern Hemisphere summer temperatures year by year for the last two millennia.

2023 was the hottest of them all. The next hottest 25 have all occurred since 1996. The next runner-up was way back in 246 CE.

The world’s understanding of global warming changed dramatically when scientists started to document long-term temperature trends using tree rings, ice cores, sediment layers and other natural temperature monitors. In 1998, scientists published the famous “hockey stick graph” covering the last 600 years. It showed that global temperatures rose and fell like gently rolling hills until the mid-20th century, when they suddenly soared.

Even seemingly small fluctuations can have a big impact on human life. Take 536 CE, dubbed “the worst year to be alive” by historian Michael McCormick. An Icelandic volcano erupted, spewing particles into the atmosphere and veiling much of Europe and Asia with a strange, dark fog. That caused cold, famines and a wave of plague that coincided with the collapse of the Eastern Roman Empire.

The new Nature paper shows the temperature that summer that year was just 1.9 C below the long-term average and 3.9 C colder than then summer of 2023.

The Intergovernmental Panel on Climate Change defines pre-industrial temperature as the average measured from 1850 to 1900. The Paris Agreement makes it a goal to keep global temperatures within 1.5 C of that pre-industrial period — a threshold we’re close to exceeding.

But measurements before 1900 were sparse, and Jan Esper of Johannes Gutenberg University in Germany, lead author on the new study, says the tree rings suggest the actual pre-industrial era was a bit cooler. From 1850 to 1900, temperatures were already about a quarter of a degree warmer than the average over the previous 2,000 years.

That means our current temperatures might be more abnormally warm than we realized. Ray Bradley, a climatologist of the University of Massachusetts, Amherst, an author on the original hockey stick paper, said that and the new paper show us how our current era fits into the bigger sweep of time.

“You often hear politicians — ignorant politicians — saying climate varies … so don’t get too excited about all this greenhouse gas we’re putting into the atmosphere,” said Bradley. But the natural records suggest it hasn’t been this warm in 2,000 years, maybe longer, “so that’s a pretty exceptional situation.”

Exceptional but not hopeless — climatologists say it’s not too late to keep global warming within a manageable range. If we listen to what nature is telling us, we can keep 536 CE as the worst year to be alive.