What would happen if all the volcanoes erupted at once?

There is no mechanism that would trigger the world's roughly 1,350 potentially active volcanoes simultaneously, which is why this sits at the speculative end. The physical consequences can still be reasoned about from known eruptions. Sulphur dioxide injected into the stratosphere forms sulphate aerosol that reflects sunlight, and single large eruptions have measurably cooled the planet — Pinatubo in 1991 lowered global mean temperature by around 0.5 °C for about two years. Simultaneous global eruption would produce a far deeper and longer volcanic winter, collapsing agriculture, alongside ash fall, acid rain and grounded aviation. On the other side of that, the carbon dioxide released would remain for millennia and would drive warming afterwards, which is the pattern inferred from large igneous provinces such as the Siberian and Deccan Traps, both of which coincide with mass extinctions.

How confident this is

This is speculation. The scenario is far enough outside anything observed that the answer depends heavily on which assumptions you start from — treat it as a thought experiment, not a prediction.

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 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.