PARIS, France—A group of radicals gathered on the periphery of the Paris climate talks Wednesday to issue a manifesto. “A transformation of the world’s entire economic system is essential,” their missive began in typically grandiose fashion. “Our economies are hard-wired to fossil fuels. To overcome this carbon entanglement, countries need to implement strong climate policies, including strengthening carbon pricing and … .”

Wait a second, I mixed up my notes. That was today’s joint press release from the Organization for Economic Cooperation and Development, the International Energy Agency, the Nuclear Energy Agency, and the International Transport Forum, four of the stodgiest policy groups around. It was issued from the heart of the United Nations Climate Change Conference and Twenty-First Conference of the Parties, known semi-affectionately on the inside as “COP21.”

The radicals were at another event, far outside the well-guarded hangar walls of the Le Bourget airport complex. With them were moderates, labor leaders, community advocates, progressive politicians, and a guy in a flannel shirt who described himself during the question-and-answer session as a “possibilitator.” Like the OECD and its partners, the group at the Salle Olympe de Gouges, a multipurpose theater less than a mile from the site of two of the November 13 suicide bombings, called for total transformation to stop climate change from wiping out much of the habitable world.

Unlike those groups, however, the event’s organizers have no faith that any sort of significant transformation will be possible in the accord being hammered out now. So they offered a plan of their own.

The headliner at the sold-out event (tickets were free but limited; attendees lined up for more than half an hour to get in) was Naomi Klein, the Canadian journalist and author whose 2014 book, This Changes Everything: Capitalism vs. the Climate, has become a rallying point for a large swath of the environmental movement. Colleagues and collaborators joined her onstage to launch their own vision for the future in the form of a pamphlet called “The Leap Manifesto.”