Around 1,500 years ago, shortly after the collapse of the Roman Empire, a baobab tree started growing in what is now Namibia. The San people would eventually name the tree Homasi, and others would call it Grootboom, after the Afrikaans words for “big tree.” As new empires rose and fell, Homasi continued growing. As humans invented paper money, printing presses, cars, and computers, Homasi sprouted new twigs, branches, and even stems, becoming a five-trunked behemoth with a height of 32 meters and a girth to match.

And then, in 2004, it collapsed.

The tree’s demise was sudden and unexpected. In March, at the end of the rainy season, Homasi was in full bloom. But by late June, its health had suddenly deteriorated. One by one, its stems broke off from the gargantuan trunk and toppled. The last of them fell on New Year’s Day, 2005, ending 15 centuries of life.

Common throughout sub-Saharan Africa, the African baobab is one of the biggest flowering plants in the world, and reputedly one of the longest-lived. It’s also known as the upside-down tree, because its bare branches look like roots, or as the monkey bread tree, because of its nutritious and edible fruit. It’s exceptionally long-lived, but recently, several of the oldest baobabs have been dying. Homasi, for example, was part of a grove of seven baobabs, six of which perished within a two-year period.

This isn’t an isolated event. Of the 13 oldest known baobabs in the world, four have completely died in the last dozen years, and another five are on the way, having lost their oldest stems. “These large and monumental trees, which can live for 2,000 years or more, were dying one after another,” says Adrian Patrut from Babes-Bolyai University in Romania, who has catalogued the deaths. “It’s sad that in our short lives, we are able to live through such an experience.”

Baobabs often have hollow trunks, with huge internal cavities that humans have used as shops, houses, chapels, and even prisons. When trees are hollow, it’s usually because the wood inside them has died. But baobab hollows were never filled; instead, these trees periodically produce new stems in the way that other trees sprout new branches. It’s the stems, fused together in a ring, that form the hollow space. That’s why the cavity is lined with bark, and shrinks with age.

Patrut discovered this unique architecture by visiting almost all the biggest and oldest baobabs in the world, and carbon-dating their various stems. He and his team would bore tiny holes into the wood and extract millimeter-wide chips, which they sent to the U.S. to be analyzed. And through this work, they also learned exactly how old these mythical trees can get.

European explorers claimed that baobabs could live for up to 5,000 years, but their true lifespan has been hard to calculate. They don’t necessarily lay down new growth rings every year, and even when they do, those rings are often too faint to count. Carbon-dating is the only way to accurately work out their age.

The oldest tree that Patrut’s team studied—the Panke baobab from Zimbabwe—was more than 2,500 years old when it died in 2011. Two other trees—Dorslandboom in Namibia and Glencoe in South Africa—are also more than 2,000 years old; their largest and oldest stems have collapsed, but parts of them are still alive. The same can’t be said for the Platland tree, which was arguably the biggest and most visited baobab. In 2016 and 2017, all five of its stems split apart and fell.

No one can say if baobabs have died off in this way in centuries past; these trees decay very quickly, and leave few traces behind. “But when around 70 percent of your 1,500 to 2,000-year-old trees died within 12 years, it certainly is not normal,” says Erika Wise from the University of North Carolina, Chapel Hill. “It is difficult to come up with a culprit other than climate change.”

Patrut agrees. Although he originally suspected that Homasi was killed by a disease, none of the fallen trees have shown signs of infection, and the pattern of their deaths doesn’t fit with a spreading contagion. Many of them were found in national parks, which absolves agricultural practices and other local human activities. Instead, Patrut blames “an unprecedented combination of temperature increase and drought in southern Africa, over the past 10 to 15 years.”

Annual rains have become more unpredictable, much to the misfortune of the largest and oldest baobabs. To stay erect, these trees must absorb between 70 and 80 percent of their volume in water. “If they don’t have enough rain when they flush their leaves or produce their flowers, they die,” says Patrut. That was what happened to the Chapman tree of Botswana—perhaps the most historically famous baobab in the world. The rainy season that was meant to start in September 2015 didn’t begin until February 2016. By then, it was too late. The Chapman tree collapsed in January, with just 40 percent water in its stems.

It’s not just the baobabs, either. Around the world, the creaking deaths of ancient trees are testifying to the period of extraordinary environmental change that we are living through. “In Wyoming, Idaho, and Montana, I’ve come across whole forests of trees that have died since 2001,” says Wise. “While they are not as old as the baobabs, they are 400 to 500 years old. The die-off has other immediate causes, like insects, but a 500-year-old tree has experienced a lot of insect outbreaks and lived through them. Something is pushing them over the brink this time around.”