What would happen if the ocean currents stopped?

The Atlantic Meridional Overturning Circulation carries warm surface water north and returns cold dense water south at depth, transporting roughly a petawatt of heat toward the North Atlantic. If it slowed or stopped, north-west Europe would cool substantially — model estimates vary widely but commonly fall in the range of several degrees, with the largest effects in winter — while heat piled up in the tropics and the South Atlantic. Sea level on the eastern seaboard of North America would rise by tens of centimetres, because the current currently draws water away from that coast. Tropical rainfall belts would shift south, disrupting monsoon systems that a large fraction of the world's agriculture depends on. Paleoclimate records show abrupt North Atlantic circulation changes have happened before, with regional temperature shifts of several degrees within decades.

How confident this is

This is a reasoned extrapolation rather than an observation. The mechanism is understood, but the scale of the consequences depends on assumptions that could reasonably be argued differently.

The Earth

• What would happen if the Earth stopped spinning? Everything not bolted down would keep moving east at up to 465 metres per second, and the oceans would migrate to the poles.
• What would happen if the Sun went out? Eight minutes and twenty seconds of normality, then permanent night — and surface temperatures below freezing within a week.
• What would happen if the Earth lost the Moon? Tides would drop to about a third, nights would be dark, and Earth's axial tilt would eventually wander.
• What would happen if all the ice on Earth melted? Sea level would rise by about 70 metres, redrawing every coastline on the planet.
• What would happen if all the bees died? Serious, expensive and not civilisation-ending — most staple calories come from wind-pollinated grasses.
• What would happen if the Earth's magnetic field flipped? It has flipped hundreds of times already, most recently 780,000 years ago, with no mass extinction attached.
• What would happen if all the trees disappeared? A carbon and water-cycle catastrophe — but not suffocation, because most oxygen production is marine and the atmosphere holds a huge reserve.
• What would happen if the Earth spun twice as fast? A twelve-hour day, more violent weather, and everyone at the equator about 1.4 per cent lighter.
• What would happen if the Moon were closer to Earth? Tides scale as the inverse cube of distance — at half the distance they would be eight times larger.
• What would happen if all the volcanoes erupted at once? Sulphate aerosols would cool the planet for years, then the released carbon dioxide would warm it for millennia.
• What would happen if all the insects died? Far worse than losing bees — decomposition, pollination and the base of most terrestrial food webs would fail together.
• What would happen if the Amazon rainforest burned? It holds well over a hundred gigatonnes of carbon and generates much of its own rainfall — losing it would change climate far beyond South America.
• What would happen if all the bacteria disappeared? Complex life would end within years — nitrogen fixation, decomposition and digestion all run on bacteria.
• What would happen if the Earth stopped moving in its orbit? It would fall into the Sun in about 65 days.