What would happen if the Amazon rainforest burned?

The Amazon basin stores on the order of 150 to 200 gigatonnes of carbon in biomass and soil, and releasing a large fraction of that would add substantially to atmospheric carbon dioxide, comparable to years of global fossil fuel emissions. The forest also generates a large share of its own rainfall: water transpired by trees is carried west and falls again, sometimes several times, in a cycle often described as flying rivers, which supplies rainfall to agricultural regions in Brazil, Paraguay and Argentina. Losing that recycling is the basis for the dieback hypothesis, in which deforestation past some threshold makes the remaining forest too dry to persist and it converts to savanna. It is also the single richest concentration of terrestrial biodiversity on the planet, holding an estimated ten per cent of known species. The oxygen framing — the "lungs of the planet" — is the weakest part of the case: mature forest consumes almost as much oxygen through respiration and decay as it produces.

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 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 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.