In this interview, outspoken climatologist James E. Hansen reflects on his 30-plus years of studying climate change and warning the public about its dangers. He talks about the most recent developments in climate science and the questions still to be answered. Hansen endorses a gradually rising fee on carbon—with all proceeds refunded to the public—as the most expedient way to stabilize Earth’s climate by the end of the century. A carbon fee would, he says, spur innovation in clean energy technologies and energy efficiency. Hansen talks about the continued constraints on scientific communication under the Obama administration and explains why the Keystone XL pipeline remains a focal point for protesters. Although recently retired as NASA’s top climate scientist, Hansen intends to continue with both his science and his activism. He explains why efforts at geoengineering may soon become inevitable. He shares his concerns about the irreversible effects of continued climate change and the growing fears among many climate experts that it may already be too late to turn things around.