Planets with too much carbon dioxide could lose oceans to space

Attention super-villains: with huge quantities of carbon dioxide, you can evaporate all of Earth’s water off to space. Although it probably won’t happen here, the same process might make Earth-like planets around other stars uninhabitable.

We already knew one way to dry out Earth or a similar planet: just wait. As the sun ages, it will get about 9 per cent brighter every billion years. The increase in solar radiation will warm the Earth, making water vapour mix into the upper atmosphere.

There, water molecules will be exposed to ultraviolet rays, which will break them into hydrogen and oxygen – and then many of those lightweight hydrogen atoms will fly off into space. Over time, Earth’s oceans will dwindle, because the lost water is never replaced.

That fate comes from a warming sun, but CO₂ may offer another path to the same destination, argues Max Popp of Princeton University. He and his team modelled Earth’s climate, and found that adding large quantities of CO₂ to the atmosphere – far more even than what we’re doing now – could also heat the planet until it leaks water.