In 1769, Ben Franklin was the first to map the Gulf Stream. It’s Earth’s most famous current, moving more water than the Amazon River.

But the Gulf Stream is just one part of the global ocean conveyor, a system of currents that connects the world’s oceans.

In tropical seas, wind and tides drive warm surface currents, like the Gulf Stream.

Near the poles, cold air, evaporation, and ice formation make the seawater colder and saltier. It sinks to the bottom, and warm tropical water is pulled up to take its place.

In this way, the global ocean conveyor carries tropical heat toward the poles. And carries nutrient- and carbon-rich water from the poles to the tropics, where it feeds phytoplankton, the base of the world’s food web.

The conveyor’s stability over more than 10,000 years has helped regulate climate, weather, and fish populations, contributing to the rise of human civilization.

But since 1850, before the Industrial Age, the Gulf Stream has shown signs of slowing. It’s at its weakest in 1,000 years.

In 2009 and ’10, it moved a third less warm water than usual, causing colder winters in the Eastern U.S. and Europe.

Melting ice in the Arctic has been releasing freshwater onto the ocean surface, disrupting the flow of the cold, salty waters that drive the North Atlantic part of the ocean conveyor.

In this way, paradoxically, a warming climate can bring colder winters in the north.