James Hansen is a NASA scientist who issued the first widely publicized warnings about climate change. He testified at a 1988 U.S. Senate hearing that the emission of carbon dioxide from burning fossil fuels was already producing a greenhouse effect, and during the administration of George W. Bush, political appointees tried to keep him from speaking out about global warming. Hansen, 68, has become increasingly convinced that a climate crisis is upon us, warning this summer that the climate system is racing toward "tipping points" which, if passed, would lead to irreversible and catastrophic effects. He is also part of the Our Children's Trust suit in which 21 young people are suing the U.S. government for violating their constitutional rights by promoting the production of greenhouse gases through the use of fossil fuels.