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I read it news that:

Hawking Gives Humans 1,000 Years to Escape Earth

How did he arrive at this figure? Whats gonna make human race extinct in 1,000 years?

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closed as off topic by Waffle's Crazy Peanut, user1504, twistor59, dmckee Apr 16 '13 at 17:57

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This isn't really a question about physics. – user1504 Apr 16 '13 at 15:09
While Hawking is a physicist and physics and astronomy give the input data on which he may be basing that opinion, this is not a physics question. – dmckee Apr 16 '13 at 17:57

From what I can gather from a quick Google search, he didn't give a reason. He just said the sentence "We won’t survive another 1,000 years without escaping our fragile planet." in a public lecture and the media picked up on it.

In a previous interview in 2001, Hawking said "I don't think the human race will survive the next thousand years, unless we spread into space. There are too many accidents that can befall life on a single planet. But I'm an optimist. We will reach out to the stars." (source: Wikiquotes) This suggests that his reason for thinking this is in terms of the probabilities of accidents.

I don't have any information about which type of accident Hawking has in mind, but they could include external threats like asteroid impacts or gamma ray bursts, as well as self-generated ones such as war or climate change.

It should be noted that Stephen Hawking is a theoretical physicist, and as far as I know doesn't do any primary research on the possible future of humanity, so he doesn't have any special authority on this subject. It's not an unreasonable thing to say at all, just don't take it as fact just because Hawking said it.

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Still, there's a good probability in his saying, "fragile" planet. I think he solely means our destruction to our own planet - which is our inability to understand the exponential function. Our destruction depends on how much we've destroyed till now ;-) – Waffle's Crazy Peanut Apr 16 '13 at 15:00

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