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.

By all accounts, 2015 is likely to wind up as the hottest year on record, with large parts of the world suffering from severe heat waves and wildfires. Despite all this, however, a stretch of the North Atlantic below Iceland and Greenland is experiencing all-time cold temperatures, according to the National Oceanic and Atmospheric Administration. What explains this anomaly? According to scientists from the Potsdam Institute for Climate Impact Research and Pennsylvania State University, among other institutions, the most likely explanation is the arrival in the area of cold water from the Greenland ice sheet that is melting ever more rapidly thanks to climate change. Because this meltwater starts out salt-free, it has remained near the surface and so, as predicted, is slowing the northern advance of warmer water from the North Atlantic Current.

So far, the AMOC has not suffered a dramatic shutdown, but it is slowing, and scientists worry that a rapid increase in Greenland ice melt as the Arctic continues to warm will pour ever more meltwater into the North Atlantic, severely disrupting the conveyor system. That would, indeed, constitute a major tipping point, with severe consequences for Europe and eastern North America. Not only would Europe experience colder temperatures on an otherwise warmer planet, but coastal North America could witness higher sea levels than those predicted from climate change alone because the Gulf Stream tends to pull sea water away from the eastern U.S. and push it toward Europe. If it were to fail, rising sea levels could endanger cities like New York and Boston. Indeed, scientists discovered that just such a slowing of the AMOC helped produce a sea-level rise of four inches from New York to Newfoundland in 2009 and 2010.

In its 2014 report on the status of global warming, the IPCC indicated that the likelihood of the AMOC collapsing before the end of this century remains relatively low. But some studies suggest that the conveyor system is already 15%-20% below normal with Greenland’s melting still in an early stage. Once that process switches into high gear, the potential for the sort of breakdown that was once science fiction starts to look all too real.

Tipping Points on the Horizon

In a 2014 report, “Impacts, Adaptation, and Vulnerability,” Working Group II of the IPCC identified three other natural systems already showing early-warning signs of catastrophic tipping points: the Arctic, coral reefs, and the Amazonian forest. All three, the report suggested, could experience massive and irreversible changes with profound implications for human societies.

The Arctic comes in for particular scrutiny because it has experienced more warming than any other region on the planet and because the impact of climate change there is already so obvious. As the report put it, “For the Arctic region, new evidence indicates a biophysical regime shift is taking place, with cascading impacts on physical systems, ecosystems, and human livelihoods.”

This has begun with a massive melt of sea ice in the region and a resulting threat to native marine species. “For Arctic marine biota,” the report notes, “the rapid reduction of summer ice covers causes a tipping element that is now severely affecting pelagic [sub-surface] ecosystems as well as ice-dependent mammals such as seals and polar bears.” Other flora and fauna of the Arctic biome are also demonstrating stress related to climate change. For example, vast areas of tundra are being invaded by shrubs and small trees, decimating the habitats of some animal species and increasing the risk of fires.

This Arctic “regime shift” affects many other aspects of the ecosystem as well. Higher temperatures, for instance, have meant widespread thawing and melting of permafrost, the frozen soil and water that undergirds much of the Arctic landmass. In this lies another possible tipping-point danger, since frozen soils contain more than twice the carbon now present in the atmosphere. As the permafrost melts, some of this carbon is released in the form of methane, a potent greenhouse gas with many times the warming potential of carbon dioxide and other such gases. In other words, as the IPCC noted, any significant melting of Arctic permafrost will “create a potentially strong positive feedback to accelerate Arctic (and global) warming.” This, in fact, could prove to be more than a tipping point. It could be a planetary catastrophe.

Along with these biophysical effects, the warming of the Arctic is threatening the livelihoods and lifestyles of the indigenous peoples of the region. The loss of summer sea ice, for example, has endangered the marine species on which many such communities depend for food and the preservation of their cultural traditions. Meanwhile, melting permafrost and coastal erosion due to sea-level rise have threatened the very existence of their coastal villages. In September, President Obama visited Kotzebue, a village in Alaska some 30 miles above the Arctic Circle that could disappear as a result of melting permafrost, rising sea levels, and ever bigger storm surges.