This year, one of the last fugitives of the Earth Liberation Front pleaded guilty to arson — at a moment when climate activists are again flirting with radical ideas. The plea comes at a moment when the story of the Earth Liberation Front seems more relevant than ever. After decades in which America’s environmental movement confined its activities largely to rallies, marches and other lawful forms of protest, frustrated activists have begun taking a more confrontational approach. Recently, some climate activists have begun to openly contemplate the possibility — in their eyes, the necessity — of directly sabotaging the infrastructure of the carbon economy. Foremost among them is the academic Andreas Malm, whose recent book, “How to Blow Up a Pipeline,” calls for smashing the tools of fossil-fuel extraction as a last-ditch means of averting ecological collapse.