Imagine a house of cards where each card supports the others. One wobbles, and the neighbors start to lean. Earth's climate system works similarly: ice sheets, ocean currents, tropical forests, all are connected. If one element crosses a threshold, it can drag the rest down with it.
Each system in the model can be in one of two states: stable or tipped. For example, an ice sheet either exists or is collapsed. The links between them were set based on expert assessments. To find out which states are most likely, the researchers ran more than 3.5 billion simulations, randomly varying initial conditions and parameters.
But the most interesting finding emerged under moderate warming of 1 to 4°C. There, the largest volume of the stability basin is occupied by states in which both ice sheets have collapsed. So the ice sheets are the key cards in the house: if they fall, the other elements may hold, but the house itself is already leaning dangerously.
In rare cases, the model showed oscillations: the Greenland ice sheet and the Atlantic circulation began to "tug" at each other, alternately collapsing and recovering. Such cycles could explain ancient climate oscillations seen in paleoclimate data.
The house of cards we are building is becoming ever more fragile. Understanding which cards hold the whole structure helps assess risks and, perhaps, shore up the foundation before the house collapses.
The model ran over 3.5 billion simulations: if one simulation were executed per second, it would take more than 110 years.
The idea of cascading climate transitions echoes Kim Stanley Robinson's novel 'The Ministry for the Future,' which describes chain reactions in the climate system.