Too many cars, too many factories, too much detergent, too much pesticides, multiplying contrails, inadequate sewage treatment plants, too little water, too much carbon dioxide - all can be traced easily to too many people.

Here's the scientific community saying, fundamentally, "If we don't change our ways, we're screwed." And they got no attention at all. Even though the Union of Concerned Scientists put out this statement which was signed by more than half of all the Nobel laureates in science and another 1,500 distinguished scientists.

There are substitutes for oil; there is no substitute for fresh water.

Stanford may be the best university in the world, but you can get all the way through here without knowing where your food came from, without being able to say where we came from, without being able to give a coherent description of why the climate is changing and why we should be concerned about it. So I started teaching a course in human evolution and the environment that's open to all Stanford students, no prerequisites.

People have to decide, first of all, how they'd like to live, and how secure they want to be from disaster. After that, scientists can help determine what would be necessary to achieve that.

Sometimes I think the Congress feels that if you only decided tomorrow to switch to wind power that in two years we'd be getting 80 percent of our electricity from wind power. It's nonsense. Normally it takes 20 to 30 years after a new technology is demonstrated and deployed before it powers even 15 or 20 percent of the grid. There's this long lag time, and we haven't even decided which directions to go.

I don't think scientists can dictate from above what we should do, because it's not a matter of scientific decision. If you want to have everybody living like a Beverly Hills millionaire, then 2 billion people might be too many. If we want to have a battery-chicken kind of world, with everybody having an absolute minimum diet, you might be able to support 10 billion.