The political accord struck by world leaders at the United Nations negotiations in Copenhagen in December 2009 allows participating countries to express their greenhouse gas commitments in a variety of ways. For example, developed countries promised different percent emissions reductions relative to different base years by 2020. China and India committed to reducing their emissions per unit of gross domestic product (GDP) relative to 2005 by 40 and 20 percent respectively. Such flexibility promotes consensus by allowing each country to use its preferred commitment formulation. However, the disparate approaches and widely varying baseline trends across different economies complicate comparing the likely emissions reductions and economic efforts required to achieve the commitments. This paper provides such a comparison by analyzing the Copenhagen targets using the G-Cubed model of the global economy. We begin by formulating a no-policy baseline projection for major world economies. We then model the Copenhagen Accord’s economy-wide commitments, with a focus on fossil-fuel-related CO2. We show how different formulations make the same targets appear quite different in stringency, and we estimate and compare the likely economic and environmental performance of major emitters’ Copenhagen targets. The analysis also explores the spillover effects of emission reductions efforts on countries that did not adopt economy-wide emissions targets at Copenhagen.