Liquefied natural gas (LNG) used as a bunker fuel has the potential to offer important reductions in atmospheric pollution—that is, air pollutants and greenhouse gas (GHG) emissions - from ships. Compared to traditional oil-derived bunker fuels such as heavy fuel oil (HFO), LNG clearly emits significantly lower quantities of sulfur oxides (SOx), nitrogen oxides (NOx), and particulate matter (PM). At the same time, it also contains up to 30 percent less carbon per unit of chemical energy (calorific value). Because of this lower carbon content, the use of LNG results in carbon dioxide (CO2) emissions at combustion that are lower than for traditional oil-derived bunker fuels usually burned in ship engines. This lower carbon content of LNG allows for a theoretical reduction in GHG emissions, yet it remains unclear whether there is a true holistic lifecycle GHG benefit of using LNG relative to oil-derived bunker fuels. The reason for this is that LNG is effectively liquefied methane, and methane is itself a highly potent GHG. Over 20-year and 100-year time horizons, methane is respectively 86 times and 36 times more potent a GHG than CO2 (IPCC 2013). Therefore, any GHG emissions from unburnt methane released to the atmosphere - called methane leakage - can diminish or even entirely offset the theoretical GHG benefit of the use of LNG. In the current literature, different GHG emissions factors for LNG (depending on the varying methane leakage assumptions applied to LNG production pathways and its use on board vessels) reflect this uncertainty. This leads to a wide range of outcomes in the literature with regard to the GHG benefits from the use of LNG - or disbenefits, if the emissions of methane are assumed to be high. To test the consequences of different scenarios of LNG use, the GHG benefits or disbenefits are not presumed either way. Instead, the consequences of a foreseeable range of methane leakage, GHG emissions, and machinery efficiencies across the lifecycle are analyzed to place bounds on the size of the GHG benefits or disbenefits. These are then discussed in the context of the maritime transport sector's climate targets.