For tonight's "Conversations with Great Minds" - Thom is joined by author, environmentalist, and activist Bill Mckibben. In 1988, he wrote "The End of Nature" - the heir to Rachel Carson's "Silent Spring" - and since then has gone on to write more than a dozen noteable books about the environment and our human impact on it. He is the co-founder and Chairman of the Board of the grassroots climate campaign 350.org, which has coordinated 15,000 rallies in 189 countries since 2009. In 2010, the Boston Globe called him "the nation's leading environmentalist" and Time magazine described him as "the world's best green journalist." And recently he was elected a fellow of the American Academy of Arts and Sciences. His latest book - "Earth: Making a Life on a Tough New Planet" - is a guide to living on our fundamentally altered planet.