Hansen has been warning the world about the dangers of global warming for nearly 40 years. His testimony at a landmark 1988 Senate hearing on the greenhouse effect helped push the looming climate crisis into the public consciousness. That has made him one of the world's most influential climate scientists. Hansen spent several decades as director of NASA's Goddard Institute for Space Studies and now, at age 82, directs the Climate Science, Awareness and Solutions program at Columbia University. In the years since his pivotal testimony, many of Hansen's fundamental scientific predictions about the Earth's climate future have come true. Greenhouse gas emissions have increased and global temperatures have continued to rise. The world's glaciers and ice sheets are melting, and sea level rise is accelerating. Hansen, however, has been disappointed by the scientific community's response to some of his more recent projections about the future of a warming Earth, which some researchers have characterized as unrealistically dire. (Author: Chelsea Harvey, E&E News )