I definitely challenge people. But hopefully, I am working harder than anybody else, and so people won't resent the fact that I want them to work hard, as well.

Greenhouse gas pollution, through its contribution to global climate change, presents a significant threat to Americans health and to the environment upon which our economy and security depends.

Even if there's controversy, I'm going to make the decision, and people are going to be happy in one instance and unhappy in the next. But that's the job I've been given and the job I'm going to embrace.

Climate change is the greatest threat of our time.

We need an international solution to the challenge of climate change, there's no question about it. But the real key is, can you actually get that - or get that solution - without the US' involvement? Can you not lead domestically, and expect the rest of the world will take action... the US needs to lead, we need to be out there.

I don't think any administration, when they come in, thinks that their job is to tell the scientists what the science looks like or to be quiet about the science. Scientists need to remain true and not allow science to be politicized. Scientists are not politicians, and no politician should consider themselves to be a scientist.

I think that one of our most difficult international challenges is clearly climate change and making sure that the Paris agreement goes forward. While I think the international community understands that President Trump is not taking the same position as President Obama.