What triggered the jumps? As Broecker guessed in the late 1980s and (after some 30 years of debate) many scientists now agree: abrupt and dramatic changes in the Atlantic Meridional Circulation.
The fact that the climate could change violently had enormous implications. As more carbon was released into the atmosphere, Broecker and other scientists became increasingly concerned that the planet might not be degrading in a steady, monotonous way, as when “heat rises.” They worried that humans were pushing the climate toward a giant leap. “Our climate system has shown that it can do very strange things,” he wrote in 1997. “We are entering dangerous territory and provoking a grumpy beast.” What remained to be answered was a very important question: Could a leap be predicted?
In the 1990s, Ditlevsen found climate change, as always, a bit boring, but this—this was exciting. He began analyzing the ice-core record for warning signs of an impending jump. He looked for patterns that preceded those 25 cataclysms: signals in oxygen-18 content, say, or calcium. Anything that reliably preceded an abrupt change. But the clues, if they existed at all, were easy to miss. Finding them was ultimately a problem of statistics: what is a real signal, what is mere noise. Sometimes, Ditlevsen enlisted his father, a professor of mathematics and engineering at another Danish university. (The father-son pair co-wrote a 2009 paper on rapid climate change.) In all those years, Ditlevsen never found an early warning sign in the ice-core data.
But elsewhere on the planet, scientists were accumulating evidence that specific parts of the climate system were approaching dangerous thresholds and major transitions of their own: the melting of the Greenland ice sheets (7 meters of sea-level rise) and the Antarctic ice sheets (another 60 meters), the death of the Amazon rainforest (incalculable loss of biodiversity), the catastrophic disruption of the monsoons (droughts affecting billions of people).
The Intergovernmental Panel on Climate Change (IPCC), made up of some 200 arbiters of the climate canon, devoted ever more pages of its reports to this kind of risk. And scientists agreed on the language they used to describe what they observed. They called the thresholds “tipping points.”
The turning points are Absolutely everywhere. Throw water on a fire and the flames will shrink but recover. Throw enough water on it and you will cross a threshold and put it out. Tip a chair over and it will wobble before settling back on its four legs. Push harder and it will topple over. Birth is a turning point. So is death.
Once a system has been pushed to its tipping point, all brakes have been removed. There is no way out. As a 500-page article puts it: report As recently noted, climate tipping points “represent some of the most serious threats facing humanity.” Crossing one of them, the report continues, “will severely damage our planet’s life support systems and threaten the stability of our societies.”
In 2019, the European Union launched a project on climate tipping points, involving around fifty scientists from fifteen countries. The aim was to assess the short-term risk of, for example, a shutdown of the AMOC or the Amazon turning into a savannah. Ditlevsen joined the project as a leader. His partner was Niklas Boers, a climate physicist at the Technical University of Munich in Germany.
In his PhD days, Boers had been studying pure mathematics before dropping out: “I don’t want to say it didn’t make sense, but it didn’t interest me,” he says. However, climate had a lot at stake. “The whole climate system is so complex that it’s where the beauty of mathematics, of probability theory, of dynamical systems and of complexity theory can really come through.” He had been investigating early warning signals in a variety of data sets and decided to investigate the AMOC.
Much as we have a natural walking speed, the AMOC has a preferred flow rate. It is measured in Sverdrups, named for the Norwegian oceanographer Harald Sverdrup, who in the first half of the 20th century modernized the study of the oceans with a wide-ranging textbook and curriculum. The flow rate varies by location, but today, at a latitude of 26 degrees north, the flow rate is 17 Sverdrups, or 17 million cubic meters per second. The Sverdrups can swing up or down, but over time the flow rate returns to that preferred flow rate. However, when a system approaches a tipping point, the flow rate is reduced. character The fluctuations change. With the AMOC, we may see the flow struggling more and more to regain its equilibrium. The flow may drift farther and farther away from the comfortable baseline. And the system may take longer to return to its routine state. These characteristics—the greater the meandering, the slower the return to the original baseline—are an obsession of mathematicians who study tipping points. If we plot the data for a system that is about to tip, we will see that the data points first follow a nice, predictable path; then the path becomes unstable, and then it veers off into wide, sharp swings. The system is becoming less stable and taking longer to recover. We can almost feel sorry for it. We can sense a kind of sickness.