The most famous words in the history of the global-warming fight came one June day in 1988, when a middle-aged NASA climatologist named James Hansen testified to a Senate committee that the planet was warming and that our carbon emissions were the cause. "It is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here," he said that day, and in doing so demonstrated the two reasons he's been so crucial.
One is the evidence: He had among the biggest computer models of the climate, and among the most extensive sets of global-temperature data; their power gave him the quiet confidence to speak and to keep speaking when he became a center of controversy.
The second reason is the way he spoke: straightforwardly, without technical jargon. If Paul Revere had said, "We can say with some confidence that some portion of the imperial forces stationed in Boston may have mobilized," the Minutemen would have taken days to arrive. It's a good thing he and Hansen both spoke plainly.
He plans to speak more plainly still. He announced his retirement from NASA on April 1st: "I've served my country for many years at NASA, but leaving the federal payroll will make it easier for me to take on all governments, including our own, that are dropping the ball on climate change." Expect to see him in court, and expect him to make more history.