Meeting internationally discussed targets for limiting atmospheric greenhouse gas concentrations and associated increases in global average temperatures will require a major departure from business as usual in how the world uses and produces energy. This report recommends a U.S. policy goal stated in terms of a budget for cumulative greenhouse gas emissions over the period 2012‑2050. With only so much to “spend” during this period, the nation should act now to: (1) take advantage of key near-term opportunities to limit greenhouse gas emissions (e.g., through energy efficiency and low carbon energy sources), and to create new and better emission reduction opportunities for the longer term (e.g., invest in research and development); (2) create a national policy framework within which actors at all levels can work toward a common goal; and (3) develop policy mechanisms durable enough to persist for decades but flexible enough to adapt to new information and understanding.