As we look beyond Paris, Bloomberg Philanthropies will work with all the major players — nations, cities, businesses, and civic organizations — on a coordinated campaign to cut greenhouse gas emissions immediately. The Paris Agreement requires national progress beginning in 2020, but that won’t do. The challenge is too urgent. The dangers of delay are too severe. And the momentum that Paris has provided is too valuable to squander. The work of becoming a real team — not just one that exists on paper — must begin now.