When Saving Lives Kills: The Strange Case of Maritime Ship Emissions

Ships large enough to carry cargo across the world’s oceans have big engines. In fact, they have really big engines. The ones that power the monster container ships that are the linchpin of global trade stand over four stories high and stretch for most of the length of a basketball court. They weigh over two thousand tons and produce in excess of 100,000 horsepower.

Fuel consumption of these engines is not measured in gallons but in tons (around 380 tons per day when steaming on the high seas). The fuel they burn is not the refined hydrocarbons that goes into cars but a viscous ‘bunker fuel’ that is more or less a leftover product from the oil refining process.

Bunker fuel is laden with sulphur and other pollutants (about 2,000 as much sulphur as the diesel fuel used in cars). With around 90,000 cargo ships plying the world’s oceans at any one time, moving 90% (by volume) of the world’s goods, it is no surprise that emissions from maritime vessels have a serious impact both on the global climate and on public health.

NOAA's National Ocean Service

First, their impact on climate. Both the carbon dioxide and the nitrous oxides emitted by these engines play a significant role in climate change, contributing as much to global warming as the aviation sector but with much less public awareness. Projected growth in global trade means that by 2050 shipping could be responsible for 10% of global greenhouse gas emissions.

Next, sulphur emissions. The sulphur and other particulates that ship engines put into the air, particularly around port cities, is a major public health concern. It is estimated that the world’s fifteen biggest container ships together emit as much sulphur as 760 million cars per year. This leads to various forms of respiratory illness. Emissions directly attributable to marine vessels cause 5,000 premature deaths from air pollution every month. This is why the International Maritime Organization passed a rule in 2008 requiring that by 2020 sulphur emissions from marine fuel use should be reduced by 90%.

It is here that things start to get complicated. Although the emissions that ship engines emit undoubtedly lead to climate warming and respiratory illness, the sulphur emissions from these same ships also lead to global cooling. This is because sulphates emitted into the atmosphere can react with water vapor to form sulphuric acid droplets. These droplets reflect sunlight away from the ocean surface resulting in a significant cooling effect. The same sulphates, even without becoming acid droplets, can also spur the formation of clouds. The clouds prevent the ocean from absorbing a meaningful portion of the sun’s warmth. So while burning bunker fuel has a warming effect through its greenhouse gas emissions, it also has an even bigger cooling effect through its sulphur emissions. Ships, perhaps surprisingly, cool the climate.

Ship tracks NASA
Satellite photo of clouds created by ships sailing in the Pacific off Alaska

This makes for a complicated picture already, but it is worth noting that the story does not end there. Burning dirty bunker fuels also releases black carbon and other particulate matter into the sky. Dark surfaces absorb more heat than light surfaces and so black carbon suspended in the air (or falling onto nearby ice) leads to warmer air (or ice) than if there was no black carbon. This means that ships are responsible for annother climate warming effect through their emissions of black carbon.

But hang on, this is not yet the last word.  Another element that comes out of the ships’ exhausts is iron. Most of this iron drifts back down to sea behind the ship as iron oxides where, in soluble form, it becomes a nutrient for microscopic marine organisms called phytoplankton. As they photosynthesize, these phytoplankton suck carbon out of the atmosphere. Studies show that deposits of soluble iron from marine transportation could comprise up to 60% of the soluble iron in nutrient deficient northern Pacific and Atlantic latitudes by 2100. This artificial fertilization effect could result in huge amounts of additional carbon being drawn out of the atmosphere.

Ships, it seems, are intricately involved in the complicated web of climate change in which humanity is ensnared. First they warm the climate through greenhouse gas emissions. Next they cool it through sulphur emissions. Then they warm it again through black carbon and other particulates. Finally they cool it again by fertilizing the ocean behind them with soluble iron.

If climate change is your issue, you will obviously want to know what the net effect of this ‘warming-cooling-warming-cooling’ cycle is. Many studies suggest that the net effect is almost certainly a cooling one. But if air pollution and public health is your issue, you will be far more concerned with the 60,000 premature deaths per year that ship emissions cause and you will be looking for ways to reduce these deaths. Of course, by reducing the respiratory deaths by bringing down the sulphur, you are almost certainly increasing future deaths through the effects of climate warming.

Balancing these sorts of dilemmas is one of the growing challenges of our time. A couple of years back, I took an initial stab at exploring the ethics of the shipping emissions dilemma in an article in the journal Climatic Change.

To be honest, it is unlikely there is an ethical formula that can provide clear cut answers to these questions. There are too many uncertainties in the science and too many unknowns in the politics, the economics, and the public will to provide mathematically precise answers to these difficult challenges.

What ship emissions do show us is that humanity is now heavily implicated in planetary processes at both the local and the global scale. This is what many people mean by the idea of the Anthropocene. We are inevitably tied into the earth’s future in ways that we are only just beginning to understand.

Notice, however, that the mess we are in with shipping emissions is entirely accidental. Those who designed the system for moving massive amounts of goods across the world’s oceans never intended to melt glaciers, to raise sea levels, to cause asthma and strokes, or to farm phytoplankton. They simply wanted to increase standards of living and create for themselves some wealth in the process.

But now that we know how these unintended effects stack up, we can claim no such innocence. Even decisions about ship emissions come with a much greater weight of responsibility.

This is one of the qualities of the synthetic age. From here on, the world’s shape is one we must choose deliberately.

Global impacts like climate change can no longer be a giant, and potentially devastating, “…..oops!”


Container ship photo by NOAA’s National Ocean Service
Ship track photo is a NASA image by Jeff Schmaltz, MODIS Rapid Response Team


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