For this Earth Day it is especially important we focus on the world's top priorities. In September, all 193 governments will meet at the UN to set targets for the world for the next 15 years in what is explicitly labeled "Sustainable Development Goals." So, it is worth looking at what we should do first. At my think tank, the Copenhagen Consensus, we've asked 60 teams of top economists including several Nobel laureates to evaluate the economic, social and environmental costs and benefits of the targets, so we can pick the best ones. Here is what we should do in environment.