In the December 2015 Paris Agreement, nations of the world agreed to reduce their net greenhouse gas emissions to zero by the second half of the century. For developed countries, accomplishing that goal requires a reduction of carbon dioxide emissions by more than 80 percent from 1990 levels by 2050. As ambitious and even unachievable as that goal may seem, some developed countries have already made considerable progress in conceptualizing and even adopting legal approaches for achieving decarbonization. This paper describes the approaches being taken in two major developed country jurisdictions — California and Germany — and suggest lessons from that experience that could be useful to the United States.