Martin Rees:  So let’s suppose astronomers had found evidence that there was an asteroid heading for the earth and that we calculated it might hit the earth in 2100. That’s 83 years from now.  And suppose we could say that with ten percent confidence, not with certainty.  What would the reaction of the public be?  Would we say, “Well, in 50 years’ time, we’ll have better technology to deal with it.  It may miss us anyway, so let’s do nothing now.”  I don’t think we would.  I think if that were the case, we would do our damnedest, starting now, to see what we could do to make sure that we could somehow deflect it or minimize its impact.   And here’s the analogy with climate change because in the context of our knowledge of climate change now, we can say that there is a ten percent chance that by the year 2100, climate change will have triggered irreversible tipping points, rather like the melting of Greenland’s ice, which would eventually raise sea level by about seven or eight meters and also cause huge global changes in climate.  We can’t be sure, but that’s likely.  My view is that we should already be prepared to pay the insurance premium to reduce that risk now.  We shouldn’t say, “Well, 50 years from now, people will be richer and they will have better technology. Let’s leave it to them.”  We should start now.  And I think the analogue of the asteroid indicates that it is worth paying an insurance premium now to eliminate a risk, even if it’s an uncertain risk.  We shouldn’t just say it may not happen.  We shouldn’t say that they can deal with it better in 50 years than we can now.  We should already start to see what we can do about it.