In his book Storms of my Grandchildren, noted climate scientist James Hansen issued the following warning: "[I]f we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty."

Venus has a thick atmosphere that is 96.5 percent carbon dioxide, which keeps its surface at nearly 900°F (482°C). The planet's water boiled off to space long ago. Could that really happen on Earth, which is farther from the sun, and where the CO2 level is just now rising past 400 parts per million?

The key to the argument is a well-documented positive feedback loop. As carbon dioxide warms the planet through the greenhouse effect, more water evaporates from the ocean—which amplifies the warming, because water vapor is a greenhouse gas too. That positive feedback is happening now. Hansen argues that fossil-fuel burning could cause the process to run out of control, vaporizing the entire ocean and sterilizing the planet.

Respected as Hansen is, the argument hasn't convinced climate scientists who specialize in the evolution of planetary atmospheres. During the Paleocene-Eocene Thermal Maximum (PETM), 56 million years ago, a huge natural spike in CO2 sent temperatures on Earth soaring—but life went on and the ocean remained intact.

"I think you can say we're still safe against the Venus syndrome," says Raymond Pierrehumbert of the University of Chicago. "If we were going to run away, we'd probably have done it during the PETM."

In the past few years, however, physicists have been training supercomputers on the lowly water molecule, calculating its properties from first principles—and finding that it absorbs more radiation at more wavelengths than they'd realized before. In a paper published this week in Nature Geosciences, those calculations have rippled into a simple climate model. The paper's conclusion contains this slightly unsettling sentence: "The runaway greenhouse may be much easier to initiate than previously thought."

National Geographic asked the lead author, Colin Goldblatt of the University of Victoria in British Columbia, to explain.

In an earlier paper, published just last year, you wrote that "it is unlikely to be possible, even in principle, to trigger a runaway greenhouse."

Yeah—and I was wrong! I was plain wrong then.

What do you say now?

It used to be thought that a runaway greenhouse was not theoretically possible for Earth with its present amount of sunlight. We've shown that, to the contrary, it is theoretically possible. That doesn't mean it's going to happen—but it's theoretically possible.

What changed?

The models we had were underestimating the amount of radiation that would be absorbed in a water-vapor-rich atmosphere.

How does that connect to the runaway greenhouse?

Going back to absolute basics—the surface of the Earth emits radiation, and some of that radiation gets absorbed in the atmosphere by gases like CO2 and water vapor. This means less radiation can get out to space than if there were no greenhouse atmosphere. Or conversely, to get the same amount of radiation out to space to balance the energy you're getting from the sun, the surface needs to be hotter. That's what's happening now: Because we're making the greenhouse effect stronger, the Earth is heating up so it will come back into balance.

Now, if you put enough water vapor in the atmosphere, any radiation from the surface will get absorbed before it gets out to space—all of it, everything. Only the upper part of the atmosphere can emit radiation to space. So it turns out there's a fixed amount of radiation you can emit to space once you have enough water vapor.

It's like if you take a layer of tinted glass—one layer, you'll be able to see through. But if you stack up 10, 20, or 100 layers, you can't see through it.

So the runaway greenhouse effect happens when the amount of incoming solar radiation exceeds this fixed limit?

Exactly. It happens when you absorb more sunlight than you can emit thermal radiation. And what I've shown here, which is new, is that the limit on how much radiation Earth can get out to space is smaller than we previously thought. And the amount of sunlight that will be absorbed in a water-vapor-rich atmosphere is bigger than we previously thought. So the implication for the Earth now is that it is possible to absorb more sunlight than you could emit to space from a water-vapor-rich atmosphere.

But your model does not consider the moderating effect of clouds.

That's correct. You start off with the simplest model you can, and then you build in complexity. We've calculated the maximum amount of sunlight Earth will absorb and the maximum amount of thermal radiation it will emit. So the next step will be to do some modeling with clouds in, which will probably modify the answers.

Clouds reflect sunlight, but if you put them high enough in the atmosphere, they'll also have a greenhouse effect. On Earth today, the reflection effect dominates—clouds overall have a cooling effect.

What does your work say about Hansen's warning?

What my results show is that if you put about ten times as much carbon dioxide in the atmosphere as you would get from burning all the coal, oil, and gas—about 30,000 parts per million—then you could cause a runaway greenhouse today. So burning all the fossil fuels won't give us a runaway greenhouse. However, the consequences will still be dire. It won't sterilize the planet, but it might topple Western civilization. There are no theoretical obstacles to that.

What does Venus teach us?

Because Venus is nearer the sun, it gets more energy from the sun than we do—it's like standing nearer the campfire. We think Venus experienced this runaway greenhouse early in its history. Venus's past is Earth's future.

The sun increases its luminosity slowly with time. At the beginning of the solar system, the sun was only 70 percent as bright as it is now. It's going to keep getting brighter. Given that the runaway greenhouse happens when there's more solar radiation absorbed than we can emit thermal radiation, it's just going to happen.

When?

In somewhere between half a billion and a billion years.

At the end of your 2012 paper, you suggested we might forestall that by moving Earth's orbit farther from the sun.

I put that in as a little joke—as a little nod to Don Korycansky, an astronomer. When Don first proposed that you could just move the Earth out with gravity assists from asteroids, he ended up on the Daily Show talking about it.

As a species we are technologically adolescent at the moment. If we get through adolescence, if we get through the next couple of hundred years alive, as a mature species who is not screwing up the planet that we live on, and then if you're talking about on timescales of hundreds of millions of years—how are we going to keep our planet alive? Then I think that's the kind of thing you might start to think about.