What caused this surge? As Blocker speculated in the late 1980s, and as most scientists now agree (after more than 30 years of debate), it was a sudden and dramatic change in the Atlantic Meridional Overturning circulation.
That the climate could change radically had big implications. As more carbon was pumped into the atmosphere, Blocker and other scientists grew increasingly worried that it was only making the planet worse in a monotonous way: it was getting hotter. They worried that humans were changing the climate too rapidly. “Our climate system has proven capable of doing some very strange things,” he wrote in 1997. “We are entering dangerous territory, provoking a cranky beast.” The all-important question that remained was, could abrupt change be predicted?
In the 1990s, Ditlefsen was a bit bored by run-of-the-mill climate change, but this was exciting. He started analyzing ice-core records to look for warning signs of coming abrupt changes. He was looking for patterns that preceded the 25 cataclysms, like a hint of oxygen-18 content or calcium. Anything that would certainly precede an abrupt change. But the hints, if there were any at all, were easy to miss. Finding them was ultimately a question of statistics: what was a real signal, and what was just noise. Ditlefsen sometimes enlisted the help of his father, a professor of mathematics and engineering at another Danish university (the father and son co-authored a paper on rapid climate change in 2009). All the while, Ditlefsen never found any early warning signs in the ice-core data.
But elsewhere on Earth, scientists were accumulating evidence that certain parts of the climate system were approaching dangerous limits and undergoing major changes of their own: the melting of the Greenland ice sheet (seven metres of sea level rise) and the Antarctic ice sheet (another 60 metres), the disappearance of the Amazon rainforest (an immeasurable loss of biodiversity), and catastrophic disruptions to monsoons (droughts affecting billions of people).
The International Panel on Climate Change, a group of some 200 leading adjudicators on climate change, devotes many pages to these kinds of risks in its report, and scientists are converging on a word to describe what they’re seeing: they call the thresholds “tipping points.”
The turning point It happens everywhere. If you pour water on a fire, the flames get smaller but then bounce back. If you pour enough water on it, you can put out the fire by stepping over a threshold. If you tip a chair over, it wobbles and then bounces back up on all four legs. If you push it harder, it falls over. Birth is a turning point. So is death.
Pushing the system past tipping points means all the brakes are removed. There is no way out. A recently released 500-page report states that climate tipping points are “some of the most serious threats facing humanity.” The report goes on to say that passing these points would “severely damage the planet’s life-support systems and threaten societal stability.”
In 2019, the European Union launched a project on climate tipping points, bringing together around 50 scientists from 15 countries. The main goal is to assess near-term risks, such as a shutdown of the AMOC or the Amazon becoming a savanna. Ditlefsen became the project’s leader, partnering with Niklas Boers, a climate physicist at the Technical University of Munich in Germany.
As a PhD student, Boers had been pursuing a degree in pure mathematics but dropped out. “I don’t want to say it was pointless, but it wasn’t interesting,” he says. But climate was a really important issue. “The whole climate system is so complicated that it’s really where the power of mathematics, probability theory, dynamical systems, and complexity theory comes into play.” He had been looking at early warning signs in various datasets and decided to take a closer look at the AMOC.
Like a human’s natural walking speed, the AMOC has a preferred flow, measured in Sverdrup, named after Norwegian oceanographer Harald Sverdrup, who modernized ocean studies in the first half of the 20th century with his extensive textbooks and curricula. Flow varies by location, but currently at 26 degrees north latitude, the flow is 17 Sverdrup, or 17 million cubic meters per second. Sverdrup can fluctuate up and down, but over time the flow will return to its preferred flow. However, as the system approaches a tipping point, character Changes in fluctuations. In the AMOC, you might see flows have an increasingly hard time returning to equilibrium. They might move further and further away from a comfortable baseline. And the system might take longer to return to normal. These features (the more meandering, the slower it will return to normal) are the obsession of mathematicians who study tipping points. If you plot data for a system about to tip over, the data points will follow a clean, predictable path at first, then the path will become unstable, and then there will be large, violent fluctuations. The system will become unstable and take a long time to recover. You almost feel sympathy. You can feel a kind of sickness.
