The NASA climatologist outlines how and when the accumulation of greenhouse gases will make Earth uninhabitable for our species—and why human life cannot be transferred to a different planet.

Question: What will life be like if carbon emissions continue to grow?

James Hansen: Well, if we allow emissions to continue at a high rate, in this century we're going to see ice sheets begin to disintegrate. And one of the things I write about in my book is the effect that will have on storms, because as Greenland begins to release more fresh water, cold fresh water, and Antarctica does, what it does is cool the North Atlantic Ocean and the southern ocean, and that increases the temperature gradient between low latitudes and middle and high latitudes. And that will increase the strength of storms that are driven by horizontal temperature gradients. So our children can look forward to increasing storms. And with a rising sea level that is going to lead potentially to a very chaotic situation, because once you have hundreds of cities in the situation analogous to what happened in New Orleans, then we've got an economic situation that's just out of control globally.

In the long run, if that really happened, as I point out in the book, over centuries, we could actually get a runaway greenhouse effect, and then that's it for all the species on this planet. And as I try to point out, there's no practical way to escape from this planet; we can't even transfer one species to another planet. I discuss the monarch butterfly and just how complex it is. And for us to hope that we could transplant life from our planet to another planet is really unrealistic. [00:21:59.17]

Question: What is the runaway greenhouse effect?

James Hansen: A runaway greenhouse effect means once the planet gets warmer and warmer, then the oceans begin to evaporate. And water vapor is a very strong greenhouse gas, even more powerful than carbon dioxide. So you can get to a situation where it just -- the oceans will begin to boil, and the planet becomes so hot that the ocean ends up in the atmosphere. And that happened to Venus. That's why Venus no longer has carbon in its surface. Its atmosphere is made up basically of carbon dioxide because it had a runaway greenhouse effect. Now the earth, it can go unstable either toward a cold climate or toward a hot climate. And the earth has had a runaway snowball earth situation. This happened most recently about 700 million years ago. The earth froze all the way to the equator.

So these runaway situations can occur. We've never had a runaway greenhouse effect, because if we did, that would have been the end. Once -- that's a permanent situation. In the case of a snowball earth, when the earth becomes ice-covered, then the planet can escape from that situation because volcanoes continue to go off, but the weathering process is greatly reduced. So volcanoes put carbon dioxide into the atmosphere, and it builds up more and more until there's enough to melt the ice. But we can't push the planet off of the runaway greenhouse end. That's the end for everybody if we do that.

Question: How long would this take to occur if we stay on this path?

James Hansen: Well, you would have to -- first of all, you'd have to melt the ice sheets, and that takes a while. The Antarctic ice sheet is a couple miles thick. But with continued rapid increase in greenhouse gases, that -- you could melt the ice sheets in less than a century. And then things start to get hotter and hotter. So over a period of several centuries it would be conceivable to have a runaway greenhouse. That would also require bringing into play what we call the methane clathrates or methane hydrates. We already observe in the tundra region in Canada and Siberia that as the tundra is melting, methane, frozen methane, begins to be released. And methane is another powerful greenhouse gas. And there have been times in the earth's history when the methane hydrates on the continental shelves melted and went into the atmosphere and caused global warming of six to nine degrees Celsius, which is 10 to 18 degrees Fahrenheit. So if you add that on to the carbon dioxide warming and the water vapor warming, you could begin to push the planet into a very different state.