February 11, 2016

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. 

The team simulated an ocean-covered world slightly warmer than Earth. Adding CO2 lets the atmosphere retain more heat, which changes global wind patterns, pushing clouds into new areas.

Those clouds then capture even more heat, creating a feedback loop as warming continues. “You destabilise the climate,” Popp says. “As the planet warms, it takes up more energy.” 

The process only ends when the clouds are so thick that they block more heat from reaching the planet than they capture. “Once you are in this very warm climate then you stabilise again,” Popp says. By then, the world has warmed to an average temperature of about 57 ˚C – similar to the endpoint for a planet exposed to a brightening sun, and hot enough to lose water.

Luckily, it shouldn’t happen here. “You could in theory get up there,” says James Kasting at Pennsylvania State University. But “it would take a lot of CO2, and there might not be enough carbon in the entire fossil fuel reserve to do it.”

Losing water this way is more of a worry for hotter planets that are closer to their stars, Kasting says. On such planets, the carbon dioxide released by volcanoes might be enough to pull the trigger.