I don't have the illusion that there's any position or role in the world with as much potential for bringing about change as that of president of the United States.

Today I challenge our nation to commit to producing 100 percent of our electricity from renewable energy and truly clean carbon-free sources within 10 years.

The center of the earth is about a million degrees.

I understand that being optimistic in this age is challenging, but I am optimistic nonetheless.

Researchers at the Naval Postgraduate School have told us that the entire Arctic ice cap may totally disappear in summer in as little as five years if nothing is done to curb emissions of greenhouse gas pollution.

Our way of life is at stake, our grandchildren are at stake, the future of civilization is at stake.

Believe in the power of your own voice.