The Atlantic Meridional Overturning Circulation — the conveyor belt that carries warm water north and cold water south, heating Northern Europe by several degrees — can be pushed toward collapse by a single large volcanic eruption.
Vettoretti, Jochum, and colleagues at the Niels Bohr Institute combined ice core records with hundreds of climate model runs to demonstrate the mechanism. A large equatorial eruption injects sulfur and dust into the stratosphere, blocking solar radiation. The surface cools. Sea ice expands. The expanded ice alters ocean salinity patterns — fresh meltwater dilutes the North Atlantic surface, reducing the density contrast that drives the overturning circulation. If the system is already close to a tipping point, the volcanic cooling is enough to push it over.
The analogy is a balance board: when the system is far from the edge, perturbations rock it and it returns. When it's close to the edge, the same perturbation tips it. The eruption doesn't need to be extraordinary. It needs the ocean to already be vulnerable.
The ice core record supports this. Historical collapses of the AMOC — events that plunged Northern Europe into sudden cold for centuries to millennia — correlate with periods of intense volcanic activity near the equator. The eruptions were known. The collapses were known. What wasn't recognized was the causal link between them, because the mechanism (salinity dilution from eruption-driven ice expansion) operates on a timescale of years to decades, long enough to obscure the connection.
The contemporary relevance is that the AMOC may already be approaching a tipping threshold due to greenhouse warming, which is also adding fresh meltwater to the North Atlantic. If the system is near the edge, the next large tropical eruption could provide the push that decades of gradual warming couldn't deliver alone. The trigger for abrupt climate change might not be gradual at all.