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James Hansen has been publicly speaking about climate change since 1988. The NASA climatologist testified to Congress that year and he’s been testifying ever since to crowds large and small, most recently to a small gathering of religious leaders outside the White House last week. The grandfatherly scientist has the long face of a man used to seeing bad news in the numbers and speaks with the thick, even cadence of the northern Midwest, where he grew up, a trait that also helps ensure that his sometimes convoluted science gets across.

This cautious man has also been arrested multiple times.

His acts of civil disobedience started in 2009, and he was first arrested in 2011 for protesting the development of Canada’s tar sands and, especially, the Keystone XL pipeline proposal that would serve to open the spigot for such oil even wider. “To avoid passing tipping points, such as initiation of the collapse of the West Antarctic Ice Sheet, we need to limit the climate forcing severely. It’s still possible to do that, if we phase down carbon emissions rapidly, but that means moving expeditiously to clean energies of the future,” he explains. “Moving to tar sands, one of the dirtiest, most carbon-intensive fuels on the planet, is a step in exactly the opposite direction, indicating either that governments don’t understand the situation or that they just don’t give a damn.”

He adds: “People who care should draw the line.”

Hansen is not alone in caring. In addition to a groundswell of opposition to the 2,700-kilometer-long Keystone pipeline, 17 of his fellow climate scientists joined him in signing a letter urging Pres. Barack Obama to reject the project last week. Simply put, building the pipeline-and enabling more tar sands production-runs “counter to both national and planetary interests,” the researchers wrote. “The year of review that you asked for on the project made it clear exactly how pressing the climate issue really is.” Obama seemed to agree in his second inaugural address this week, noting “we will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations.”