"Let me put it in a rather larger picture framework. Let's go to the longest time frame, the time frame of the life of our sun. As a star, our sun is about halfway through its life cycle. In the long run, we only have a couple of billion more years likely that we can inhabit this planet. By that time, we're going to have to be out of here before our sun dies. Now, I don't think we need to wait that long, and we certainly shouldn't wait that long. At the moment, we are not on a sustainable path."