The Atlantic Meridional Overturning Circulation (AMOC) — the massive ocean current that keeps northern Europe temperate — may be approaching a tipping point. After 35 years of research, physical oceanographer Stefan Rahmstorf now estimates a 50/50 chance of collapse, up from just 5% three decades ago. A study published last month predicts the AMOC will slow to half its current flow by the century's end.

Researchers have amassed roughly two decades of solid ocean data from moored buoys dating back to 2004. In the subtropics, the AMOC has already slowed at roughly 1 Sverdrup per decade (one Sverdrup equals one million cubic meters of water per second). While climate models disagree wildly on timing, studies using the most realistic simulations suggest the slowdown will persist and worsen.

The consequences of collapse would be severe: Europe could cool by up to 10°C, monsoons in Africa and Asia would weaken, and the Southern Ocean could release stored carbon, accelerating warming. Twelve thousand years ago, a similar AMOC collapse plunged Europe back into ice-age conditions. Iceland has already designated the risk a national security threat.

The science remains uncertain, with limited data and conflicting models. But as Rahmstorf notes, "We really don't want this to happen."