All of the world's oceans have a similar pattern of currents. Surface waters warm near the equator, then flow toward the poles, where they cool and sink. The cold, dense bottom water makes its way back to restart the cycle. This pattern has particular significance in the North Atlantic, where the flow of warm surface water helps moderate the climate of Northern Europe, parts of which might otherwise resemble Greenland.
A lot of people have pondered whether the warming induced by climate change could interfere with this conveyor belt, preventing the water that nears the Arctic from cooling and sinking. Most analyses, however, suggest that this could only happen after the world had warmed enough that Europe wouldn't need the currents to moderate its temperature.
A new study, however, suggests that there's a tipping point for the Atlantic conveyor that could be reached much sooner. It only relies indirectly on warm temperatures; instead, it is driven by the melting of the Greenland Icecap. And the new research suggests we've already gone nearly halfway to the tipping point.