What is the fastest ocean current in the world?

The Gulf Stream, moving at up to about nine kilometres an hour at its core - which sounds slow and is enormous, because it is carrying something like 30 million cubic metres of water a second past Florida, more than all the world's rivers combined many times over.

It is a heat pipe. Warm water moves north, gives up its heat to the atmosphere over the North Atlantic, and that is a large part of why northwest Europe is far milder than its latitude - London sits further north than Newfoundland. The current is one limb of the Atlantic Meridional Overturning Circulation, which is driven by cold salty water sinking in the far north and pulling more warm water up behind it.

Meltwater is the concern. Fresh water from Greenland dilutes the salt, which makes the surface water less dense and less inclined to sink, which weakens the pull. Measurements suggest the circulation has slowed, though how much and how fast is contested, and estimates of when it might collapse range from this century to not at all. The consequences of a collapse would be regional rather than the abrupt ice age of the films - but they would be severe, and the uncertainty is not reassurance.