We’ve Got 1.5 Billion Years Left With a Habitable Earth

And then we need to get off this rock

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One day the world will end, and unless we've managed to Noah's Ark ourselves into the deep recesses of space, we'll end along with it. The sun is getting brighter—roughly 1 percent every 110 million years—and eventually this ticking increase is going to cook us out of a home.

When it really comes down to it, the sun has control over our planetary thermostat. As the temperature rises, more water is evaporated into the atmosphere. Water vapor is a strong greenhouse gas, and soon enough we've got a runaway greenhouse effect. Then, bam, 650 million years later the Earth has turned into Venus.

According to a new study, though, we may have a little more time than that. What previous estimates tended to neglect was the Earth's climate system—how the land and the air and the sea all interact, keeping each other in check. Using a more advanced climate model, two scientists, Eric Wolf and Owen Brian Toon, dug into the details of the apocalypse.

We don't need to get all the way to a Venus-level disaster, they say, for Earth to be a rather awful place to live.

“While a catastrophic runaway greenhouse would unquestionably sterilize the planet, habitability may become threatened before this ultimate tipping point is reached,” the scientists wrote in their study. “A more stringent estimate for the hot limit to planetary habitability is based on the so-called moist greenhouse climate.”

With even lesser levels of warming, the upper portions of the Earth's atmosphere will become wetter. And water in the upper atmosphere is more likely to break down and be lost to space. Eventually, the scientists say, the increasing warming will cause “Earth’s oceans to effectively evaporate away to space.”

On the one hand, this will delay the transformation of the Earth into a giant hot mess. On the other, the oceans will be evaporating.

The scientists found that the Earth would stay “habitable” until the sun's output ticked up to at least 15.5 percent higher than it is now—giving us roughly 1.5 billion years left.

But these end days would not be happy days.

First off, when it gets this hot, clouds will cease to exist. Instead, the air will be steam. Then, says Nanci Bompey, reporting on the paper for the AGU's blog:

Temperatures in areas just below the Arctic Circle would resemble today’s tropics, and there would be a lot more rain as the oceans evaporated...

"There would be twice as much rainfall everywhere, a lot more floods and things like that,” Toon said. “It will be like a really unpleasant day in the Sahara Desert, but rainy.”

With a 15.5 percent increase in solar output, the scientists say, the annual average temperature in the tropics would be 114 F. At the poles, 74 F.

But, still, good news, right?

“While such a hot climate would undoubtedly provide great challenges for humanity, Earth will remain safe from both water loss and thermal runaway limits to habitability even for a 15.5% increase in solar constant," the authors of the new study write. We'll just all be camping out at the South Pole (the North Pole will be long gone) and pretending it's the Australian outback.

One important aside: In terms of comparing the sun-induced apocalypse against modern warming, the two are really not on the same scale, at all. In this study, the authors say that a 2 percent increase in the sun's energy is equal to us doubling the atmospheric concentration of carbon dioxide. Matching a 15.5 percent increase in solar energy, then, isn't really something we could do.

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