Dramatic temperature shifts have happened in the past, driven partly by changes in a major ocean currents.
A "great ocean conveyor" helps transport heat around the globe via surface and deep-sea movements of water.
Scientists are exploring whether global warming might slow or shut it down - a scenario considered "low probability, high impact".
This could disrupt mostly wind-driven surface currents such as the Gulf Stream, which helps to bring milder weather to Northern Europe.
1 Surface currents carry warm, salty water from the tropics.
2 The water cools, its density increases and it sinks to the deep ocean.
3 The cold water flows back to the equator, driving the "ocean conveyor" which in turn contributes to the Gulf Stream that warms northern Europe.
4 As ice melts, freshwater dilutes the warm salty water from the tropics.
The water becomes less dense so does not sink as fast, weakening the "conveyor" and therefore possibly disrupting the Gulf Stream.