Not so long ago, it was science fiction. Now, it’s hard science—and that should frighten us all. The latest reports from the prestigious and sober Intergovernmental Panel on Climate Change (IPCC) make increasingly hair-raising reading, suggesting that the planet is approaching possible moments of irreversible damage in a fashion and at a speed that had not been anticipated.
Scientists have long worried that climate change will not continue to advance in a “linear” fashion, with the planet getting a little bit hotter most years. Instead, they fear, humanity could someday experience “non-linear” climate shifts (also known as “singularities” or “tipping points”) after which there would be sudden and irreversible change of a catastrophic nature. This was the premise of the 2004 climate-disaster film The Day After Tomorrow. In that movie—most notable for its vivid scenes of a frozen-over New York City—melting polar ice causes a disruption in the North Atlantic Current, which in turn triggers a series of catastrophic storms and disasters. At the time of its release, many knowledgeable scientists derided the film’s premise, insisting that the confluence of events it portrayed was unlikely or simply impossible.
Fast forward 11 years and the prospect of such calamitous tipping points in the North Atlantic or elsewhere no longer looks improbable. In fact, climate scientists have begun to note early indicators of possible catastrophes.
Take the disruption of the North Atlantic Current, the pivotal event in The Day After Tomorrow. Essentially an extension of the Gulf Stream, that deep-sea current carries relatively warm salty water from the South Atlantic and the Caribbean to the northern reaches of the Atlantic. In the process, it helps keep Europe warmer than it would otherwise be. Once its salty water flows into sub-Arctic areas carried by this prolific stream, it gets colder and heavier, sinks to lower depths, and starts a return trip to warmer climes in the south where the whole process begins again.
So long as this “global conveyor belt”—known to scientists as the Atlantic Meridional Overturning Circulation, or AMOC—keeps functioning, the Gulf Stream will also continue to bring warmer waters to the eastern United States and Europe. Should it be disrupted, however, the whole system might break down, in which case the Euro-Atlantic climate could turn colder and more storm-prone. Such a disruption might occur if the vast Greenland ice sheet melts in a significant way, as indeed is already beginning to happen today, pouring large quantities of salt-free fresh water into the Atlantic Ocean. Because of its lighter weight, this newly introduced water will remain close to the surface, preventing the submergence of salty water from the south and so effectively shutting down the conveyor belt. Indeed, exactly this process now seems to be underway.