Updated emissions from ocean shipping

Abstract

Marine vessel inventories demonstrate that ship emissions cannot be neglected in assessing environmental impacts of air pollution, although significant uncertainty in these inventories remains. We address this uncertainty by employing a bottom‐up estimate of fuel consumption and vessel activity for internationally registered fleets, including cargo vessels, other commercial vessels, and military vessels. We identify model bias in previous work, which assumed internationally registered ships primarily consume international marine fuels. Updated results suggest fuel consumption is ∼289 million metric tons per year, more than twice the quantity reported as international fuel. According to our analysis, fuel used by internationally registered fleets is apparently allocated to both international and domestic fuel statistics; this implies either that ships operate along domestic routes much of the time or that marine fuel sales to these ships may be misassigned. If the former is true, then allocation of emissions to international shipping routes may underestimate near‐coastal emissions from ships. Our updated inventories increases previous ship emissions inventories for all pollutants; for example, global NO x emissions (∼6.87 Tg N) are more than doubled. This work also produces detailed sensitivity analyses of inputs to these estimates, identifying uncertainty in vessel duty‐cycle as critical to overall emissions estimates. We discuss implications for assessing ship emissions impacts.