NOx emissions – NO and NO2 – lead to tropospheric ozone (O3) production and aerosol formation, both of which deteriorate air quality and influence climate change. Currently, ships are thought to account for 15% of total anthropogenic NOx emissions, but there are considerable uncertainties in measuring these emissions and the figure could be much higher or lower. Until now, ship emissions have usually been quantified by sporadic aircraft campaigns, or more commonly using "bottom-up methods": calculating individual ship emissions based on the fuel consumed, the engine type, the speed, loading and route taken.

To improve these estimates Folkert Boersma from Wageningen University in the Netherlands and his colleagues took and analysed satellite measurements of nitrogen dioxide (NO2) between 2005 and 2012 over shipping routes across European Seas, including the Mediterranean Sea, the Atlantic Ocean between Spain and the English Channel, the North Sea and the Baltic Sea.

"Because the satellite measurements are robust and cover otherwise unobserved areas, this enables us to provide regionally-specific estimates of how nitrogen oxide emissions have been changing in time, and how they compare to bottom-up inventories," said Boersma.

On average, European ship emissions of NOx increased by around 15% between 2005 and 2008, the satellite measurements revealed. Then, in 2009, there was a 12% fall in emissions, partially caused by the global economic downturn of 2008–2009, with emissions remaining at those levels until 2012.

The steady rise in emissions from 2005 to 2008 closely matches the increase in shipping traffic at that time, but the fall in 2009 is not so simple. By looking at the volume of cargo passing through the Suez canal, Boersma and his colleagues showed that the drop in trade only partially explains the observed fall in emissions. Closer inspection of Mediterranean shipping traffic revealed that the remainder of the fall in emissions was likely due to a desire to save fuel.

"We could see the implementation of the practice of 'slow steaming' – the fact that ships sail at a slower speed to reduce fuel costs, which had a beneficial effect on NOx emissions," said Boersma, whose findings are published in Environmental Research Letters (ERL).

Over the same time period, emissions of NOx on land have been decreasing. "This means that in 2005 only one in nine of the NOx molecules emitted in Europe was originating from ships, but in 2012 this number had increased to one in seven," said Boersma. "With a lot of ship traffic close to the European coasts, this means that the shipping sector has become responsible for a larger share of the overall pollution burden."

Across the globe, particularly along the increasingly busy tropical shipping routes, the figures are only likely to become worse. Over the coming decades ship emissions are expected to increase, because of economic growth, and trade in the developing world.

"There are initiatives, such as sulphur and nitrogen emission control areas, that are being negotiated by seafaring states to make certain seas cleaner, but progress is still slow," said Boersma. "If countries could agree on these, a lot of progress could be made." In the meantime, as this study shows, encouraging ships to "go slow" could be the best way to help keep a lid on emissions.

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