A house of cards stands as long as every card is in place. Remove one, and the neighbors start to fall. Earth's climate works the same way: ice sheets, ocean currents, and forests are interconnected. If one part breaks, it can drag others down with it.
Each system in the model can be in one of two states: normal or broken. For example, an ice sheet either exists or has melted. The scientists ran more than 3.5 billion computer simulations, varying conditions to see what would happen under different levels of warming.
They found that if Earth warms by more than 4 degrees, nearly all systems collapse. And under modest warming, it is usually the ice sheets that break first. That makes them the most important cards in the house: if they fall, the whole climate becomes highly unstable. That is why it is so crucial not to let the planet overheat.
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.