We face a climate crisis now that is the most serious challenge our civilization has ever confronted. And the greatest country in the world [America] has to remain a part of this unprecedented global agreement to deal with it.

I flew on Air Force Two for eight years, and now I have to take off my shoes to get on an aeroplane.

Consider what kind of car you get. Buy cars and other products that have the least impact environmentally.

I think that we should ban so-called junk guns. I think we should ban assault weapons like the weapons used here [in Fort Worth], yes. I think that the kinds of weapons that have no legitimate use for hunting or the kind of weapon that a homeowner would use, I think they should be banned, yes, those kind of weapons.

I've learned the importance of changing people's minds at the grassroots level so that whoever does run will have a much better chance of encountering public opinion that reaches a critical mass and brings about a change not only in White House policies but in the Congress and in the state legislatures and all around the world.

There are people who say, "God is in complete control of everything that happens, and if the Earth is getting warmer, then maybe God intends that." Well, no. God intends for us to take responsibility for how we treat God's creation, and if we choose to use the thin shell of atmosphere surrounding our planet as an open sewer for 110-million tons of global-warming pollution every day, the consequences are attributable to us. And if you are a believer, as I am, I think God intends for us to open our eyes and take responsibility for the moral consequences of our actions.

It seems an easy choice - sacrifice the tree for a human life - until one learns that three trees must be destroyed for each patient treated. Suddenly we must confront some tough questions. How important are the medical needs of future generations?