Yet what people hear from Hansen today is not so much his science but his broad, unscientific views on, say, the evils of oil companies. In 2008, he wrote a paper, the thesis of which was that runaway climate change would occur when carbon in the atmosphere reached 350 parts per million — a point it had already exceeded — unless it were quickly reduced. There are many climate change experts who disagree with this judgment — who believe that the 350 number is arbitrary and even meaningless. Yet an entire movement, 350.org, has been built around Hansen’s line in the sand.

Meanwhile, he has a department to run. For a midlevel scientist at the Goddard Institute, what signal is Hansen sending when he takes the day off to get arrested at the White House? Do his colleagues feel unfettered in their own work? There is, in fact, enormous resentment toward Hansen inside NASA, where many officials feel that their solid, analytical work on climate science is being lost in what many of them describe as “the Hansen sideshow.” His activism is not really doing any favors for the science his own subordinates are producing.

Finally, and most important, Hansen has placed all his credibility on one battle: the fight to persuade President Obama to block the Keystone XL pipeline. It is the wrong place for him to make a stand. Even in the unlikely event the pipeline is stopped, the tar sands oil will still be extracted and shipped.