Will China and India offer up ambitious climate action plans this year? The fate of the Earth might be determined by the answer. Unless those large and fast-growing economies curb their emissions, we can’t avert catastrophic climate change. Their willingness to make serious commitments will be necessary to build enough momentum for an international deal at the UN Climate Change Conference in Paris this December.

The success of Paris will not just be decided during the two-week negotiating marathon. Much of what happens there will be determined by what has happened up to that point. The central challenge is building mutual trust...