The study estimates a whopping 150 percent rise in methane emissions over the period under review. The suspected culprit would be an increase in LNG powered vessels. But the numbers show a mere 28 percent increase in the use of LNG as bunker fuel. The explanation is simply that in a successful effort to optimise LNG engine with respect to NOx this has led to increase methane slip as a consequence.
Due to new methodologies, this study can distinguish domestic shipping from international emissions on a voyage basis. Access to more accurate data in looking at the split between domestic and international emissions also shows that the estimated share of domestic shipping emissions has doubled from 15% to 30%.
The 150% increase in methane emissions over the period is significantly greater than the uptake of LNG as a bunker fuel, which rose by only 28%. This could be explained by the optimisation of LNG engines for lower NOx emissions which may translate into higher methane emissions.‘Methane is not yet regulated by the IMO. Methane has a much much stronger global warming potential but is shortlived in the atmosphere compared to carbon dioxide. Several industry organisations and IMO-members have already made submissions to include methane in subsequent Energy Efficiency Design Index (EEDI) revisions.
The new study found that carbon intensity has improved between 2012 and 2018 for international shipping as a whole, and for most ship types. The study notes that ship emissions are projected to increase from about 90% of 2008 emissions in 2018 to 90-130% of 2008 emissions by 2050. The COVID-19 pandemic is also expected to result in a small but temporary dip in emissions over the next few years.
The study uncovers a significant change in the bunker fuel mix. Marine diesel oil (MDO) and LNG consumption grew by 6 and 0.9% respectively, and thereby replacing the percentage of heavy fuel oil consumption accordingly. The use of methanol as bunker fuel has only experienced a slight increase from 130,000 tonnes to 160,000 tonnes of total consumption on voyage-based international routes.
Average ship sizes of containership, cruise ship and oil tankers have increased, as well as the average installed engine power. Average shipʹs fuel consumption has increased over the period, but at a lower rate than the increase in average installed power. This decoupling’ is likely due to the general and continued trend for slow steaming. If and when ships pick up speed again there is an increased risk that the decoupling will be lost.
SOx and PM emissions increased over the period despite of an overall reduction in HFO use and an increase in MDO and LNG use (largely due to the introduction of emission control areas with their restrictions on sulphur content). The explanation for this is that the average sulphur content increase in HFO over the period exceeded the sulphur content reduction associated with the change in fuel use.
The study will be considered by the next Maritime Environment Protection Committee (MEPC) under the International Maritime Organisation (IMO) later this year.