Lower speed – less emissions?

Ships in the Mediterranean Sea have reduced their speed by more than 30 per cent since 2008, which has led to a 45 per cent cut in average ship NOx emission factors.

Lowering the speed of ships – also known as slow steaming – became more widespread from 2008 because shipping companies needed to cut costs by saving on fuel, as fuel prices had doubled over just a few years, and to reduce overcapacity, as transport demand fell due to the global economic recession. In practice, slow steaming means reducing the speed of a vessel from about 20–25 knots to 16–19 knots. As a result, fuel use and accompanied fuel costs for ship voyages are cut significantly. In addition, emissions of air pollutants, such as sulphur dioxide (SO₂) and nitrogen oxides (NOx), and of the greenhouse gas carbon dioxide (CO₂) decrease sharply with reduced ship speed.

Using satellite measurements, a group of researchers have analysed changes in European ship NOx emissions between 2005 and 2012, looking at four European shipping lanes in the Mediterranean, Bay of Biscay, North Sea, and Baltic Sea.

Their results show that European ship NOx emissions increased by about 15 per cent from 2005 to 2008. This increase was followed by a reduction of some 12 per cent in 2009 – a result of the global economic downturn in 2008–2009, after which emissions remained relatively stable from 2009 to 2012.
The impact of slow steaming was estimated for shipping in the Mediterranean Sea. Observations of ship passages through the Suez Canal and satellite-derived ship densities suggest that ships in the Mediterranean have reduced their speed by more than 30 per cent since 2008. Ship average NOx emissions increased from 2005 to 2008, then decreased by 46 per cent from 2008 to 2009, to stay relatively constant, at 30 per cent below the 2005 levels, in the years from 2009 onwards.

The lower ship NOx emission factors from 2009 coincide with the implementation of slow steaming in 2008–2009. The fact that average emission factors per ship have remained low since 2009 indicates that slow steaming has remained a common practice in the Mediterranean Sea.
According to the study, the implementation of slow steaming in 2009 contributed to offsetting the 2005–2007 increase in ship NOx emissions, but the relative contribution of the shipping sector to total European NOx emissions still increased from 11 per cent in 2005 to 14 per cent in 2012.
So in spite of the implementation of slow steaming, the emission estimates suggest that one in seven of all NOx molecules emitted in Europe in 2012 originated from the shipping sector, up from one in nine in 2005.

The study concludes that the growing share of the shipping contribution to overall European NOx emissions suggests a need for the shipping sector to implement additional emission abatement measures.

Christer Ågren
Study: “Ships going slow in reducing their NOx emissions: changes in 2005–2012 ship exhaust inferred from satellite measurements over Europe” (July 2015). By K F Boersma, G C MVinken and J Tournadre. Published in Environmental Research Letters 10 (2015) 074007. Link: http://iopscience.iop.org/1748-9326/10/7/074007

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