Jeffrey D. Sachs is the director of the Earth Institute at Columbia University. He is also an economist and a special advisor to the UN Secretary-General Ban K-moon. Five years ago, in Copenhagen, member states failed to sign an agreement on limiting climate change. Jeffrey Sachs tells Marc Perelman that the UN Climate Change Conference in Paris next year is "the last chance for the multilateral approach" and that failure to reach an agreement in Paris would be a "disaster".