Climate change is an existential threat, so are civil disobedience and direct action the only way to save the planet? And is a global carbon tax the best tool to do the job?

Justin Rowlatt speaks to protestors from the new and militant environmentalist movement Extinction Rebellion as they occupy the UK's Department of Energy building in protest at their government's alleged failure to tackle global warming. He also speaks to Ben Stewart of the 49-year-old campaign group Greenpeace, who have themselves been targeted by their new rivals for not being radical enough.

But what policy change should they be calling for? Professor Bill Nordhaus of Yale University received this year's Nobel Prize for Economics for his work on economic models for how government's might go about taxing carbon dioxide emissions. But why does he think that so few governments are implementing it?

Producer: Laurence Knight

(Picture: Extinction Rebellion activists occupying the UK Department of Energy in London; Credit: Roger Harrabin/BBC)