Why did James Hansen retire on April 2 after 32 years as director of NASA's Goddard Institute for Space Studies? As he told the enterprising students of Columbia University's Sustainability Media Lab who captured him in the following video, "I want to devote full time to trying to help the public understand the urgency of addressing climate change."

It's not exactly the "spend more time with my family" excuse often give by retiring government officials, but his family is nonetheless the reason for this change. He's worried about preventing "climate chaos" and instead preserving the relatively stable climate of the past 10,000 years—when human civilization developed and flourished—for his five grandchildren.

That means full-time activism for the 72-year-old, and perhaps the risk of getting arrested at more coal-mine or tar-sand protests. "We can't burn all the fossil fuels without guaranteeing that young people and future generations are going to suffer the consequences of that," Hansen noted in the video of our loading of what he calls the "climate dice." "We don't make [the fossil fuel companies] pay for the climate impacts, which are already significant. The taxpayer picks up these costs or just the public. If you get asthma from air pollution, you pay the health cost, not the fossil fuel company."

His preferred solution? A cap-and-dividend system in which a price is put on any carbon-based fuel and the revenues collected are distributed to every taxpayer to offset increasing energy costs. After decades spent trying to move society as a scientist—including the first testimony to the U.S. Congress urging action on global warming in 1988—he will now focus on educating and influencing the public and policy leaders as an activist.