The most important number in environmental economics that most people have never heard of is called the Social Cost of Carbon (SCC). The SCC in plain language is the economic damage a single ton of Carbon Dioxide (or its equivalent) emitted at a certain point in time causes over its life time. And CO2 is the Methuselah of greenhouse gases. Once it’s up there, it causes warming for hundreds of years. Read more of Professor Auffhammer's EI blog post.