Bill McKibben is a household name in climate justice circles - known not only for his extensive writing on global warming but for his environmental campaign group 350.org.
During what have now been almost two weeks of sessions and negotiations at COP21, he has graced numerous stages and podiums, talking, challenging, educating and on one occasion calling on his audience to really tune into the lyrics of Radiohead frontman Thom Yorke and carry their essence deeper into society. He has, in short, had a busy time of it in Paris.
But as author of The End of Nature, which is widely regarded as the first book on climate change for a popular audience, he is no stranger to hard work. Between its publication in 1989 and now, carbon emissions have risen exponentially, far outstepping the pace of any grassroots effort to counter their sweeping and drastic effects.
That imbalance, however, has not stopped Bill McKibben from doing everything a single human being can to wake the world to the insanity of its own behavior.
He has, in the grand scheme of things, been highly effective, and prizes such as the Ghandi Peace Award, which he received in 2013 underscore that. But the tough nature of the negotiations here in Paris are testimony to the fact that the climate struggle is far from over. When asked if it lacks leadership, McKibben says there is certainly no Martin Luther King-like figure blazing a trail. But that is not per se a bad thing.
"The way I think of it is as a fossil fuel resistance, like the French Resistance during the war - spread out, kind of sprawling, and local. That's good, because the fossil fuel industry is spread out, sprawling in every place, and you need to be able to stand up to it everywhere."
He has been good at getting people doing just that. More than good actually. The weekend before this COP got underway, his organization played a decisive role in getting 785,000 people to take part in climate marches in 175 countries around the world. And in Paris, where the terrorist attacks of November 13th led to the cancellation of what was set to be the biggest protest of all, 20,000 people laid out their shoes in a symbolic call for climate justice.
McKibben says such actions are vital to keeping up the pressure. Not just now, as the talks enters their crunch hours, but in the days, months and years to come.
"The most important thing an individual can do is not be an individual quite so much - come together and join these large movements that perhaps are capable of changing the systems and structures that are causing the problem."
Ultimately that means decarbonization, which he says "means keeping most of the coal and oil and gas that we know about underground and not finding any more. Stop looking for that, stop digging it up."
Like so many in Paris, McKibben's message is as clear as the oceans, rivers and skies we all want to enjoy in their unpolluted form. "The science has been clear for twenty years. We've won the argument, now we have to figure out how to win the fight."