What would happen if the Earth's magnetic field flipped?

Geomagnetic reversals are recorded in the magnetisation of seafloor basalt as it cools, and the record shows hundreds of them, irregularly spaced, with the last full reversal — the Brunhes–Matuyama — around 780,000 years ago. A reversal is not instantaneous: it takes something between one and ten thousand years, during which the dipole field weakens to perhaps ten per cent of its normal strength and the poles wander, sometimes with multiple poles at once. The consequences for life appear to have been small, because no mass extinction in the fossil record lines up with a reversal, and the atmosphere continues to provide the bulk of radiation shielding regardless of the field. The consequences for technology would be considerably larger than for biology: satellites, aviation at high latitudes, power grids and navigation systems all depend on a stable field and would face far more frequent geomagnetic disturbance. Aurorae would appear at low latitudes, which is the pleasant part.

How confident this is

This one is settled. The mechanism is understood and the consequences follow from measurements or arithmetic rather than from guesswork.

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 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 the ocean currents stopped? North-west Europe would cool sharply while the tropics warmed — the ocean moves as much heat poleward as the atmosphere does.
• 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.