uting roughly 3.3 % of global emissions, shipping is a significant and rapidly growing source of carbon dioxide in the atmosphere.1 In the absence of ambitious new mitigation policies, recent emissions scenarios suggest that emissions from marine vessels may grow by a factor of 2 or 3 over current levels by 2050 because of sustained growth in maritime shipping.2 But shipping is also one of the most energy-efficient means of transportation, currently facilitating over 80% of world trade; as international trade flows continue to expand, moving