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Study: CO2 Output from Shipping On Same Order as That of Aviation, Could Double by 2050

Transport-related annual emissions of CO2, NOx, SO2 and PM10 and the fuel consumption in Tg estimated for the year 2000 (rescaled from Eyring et al., J. Geophys. Res., 2005a). Click to enlarge.

Current carbon dioxide emissions from the shipping industry likely exceeds those from aviation, and could double by 2050, according to recent studies by the Institut für Physik der Atmosphäre (IPA) of the Deutsches Zentrum für Luft- und Raumfahrt (DLR) and by the College of Marine and Earth Studies of the University of Delaware.

In addition, marine emissions of SO2 could also double, while NOx emissions could exceed those of road traffic in the same time span.

The studies reveal converging estimates of current ship emissions and suggest that shipping emitted around 800 Tg CO2 and contributed around 2.7% to all anthropogenic CO2 emissions in 2000 (1 Tg = 1012 g = 1 million metric tons = 1 Mt). The same studies put aviation emissions of CO2at about 650 Tg.

Given uncertainties in all emission inventories, these figures should be considered best estimates within a bounded range of 600 to 900 Tg of CO2 per year, according to the IPA. The IPA concludes that CO2 emissions from shipping are of the same order as published CO2 estimates for aviation.

For comparison, aviation and road transport contributed around 2.2% and 14%, respectively. Other comparisons suggest that shipping accounts for around 15% of all global anthropogenic nitrogen oxides (NOx) emissions and for around 8% of sulfur dioxide (SO2) emissions. The relatively high contribution is a result of marine engines operating at high temperatures and pressures without effective NOx emission reduction technologies and because of the high average sulfur content (2.4%-2.7%) in marine fuels.

Recent studies from DLR have shown that fuel consumption from ocean-going ships has increased by a factor of 4.3 from 1950 to 2000, reaching around 280 Tg today.

Future scenarios demonstrate that significant reductions are needed to offset increased emissions due to growth in seaborne trade and cargo energy intensity. If no aggressive emission reduction strategies are introduced, CO2 and SO2 emissions from ships could double present-day values by 2050, and NOx emissions could exceed present-day global road transport.

An International Maritime Organization (IMO) study of greenhouse gas emissions estimates that emissions from the global fleet would increase dramatically in the next 20 years as globalization leads to increased demand for bigger, faster ships, according to a report in the Guardian. Without action the IMO predicts that by 2020, emissions from ships would increase up to 72%.

Global comparisons of emission totals from different transport modes describe only part of the picture, IPA cautions. For example, the related passenger and freight transported volumes will need to be considered in addition. Also, the distribution of shipping activity follows major trade routes, such that ship emissions near coastal areas affect regional air quality, environment, and public health.




Ships transport around 90% of global trade so it's not altogether surprising the emissions are high.

There was a very interesting article in New Scientist about technologies to reduce carbon emissions from ships:



I was just going to comment on that. Some of these new sail systems can reduce fuel use 30% and pay for themselves in less than a year.


Some cruiser ships's speed is 21 to 22 knots.
Perhaps this speed is too high for these ships to be pulled by kites.


I'd suspect many shippers will use these sails to increase the speed. With all the time consuming security precautions already in place, or looming on the horizon, they will want to get to ports faster than ever. Time is money, hence the longer the ship is tied up at the dock, or waiting to be cleared, the less money the vessel is making.

Rafael Seidl

The fraction of global emissions produced by shipping is fairly meaningless, what you need is an environmental impact assessment of those emissions. CO2 from ships presumably has much the same effect as dilute CO2 from any other source. For NOx and SOx, the impact is probably very different - at least far from shore.

In terms of propulsion efficiency, the primary trend for container ships appears to be ever-increasing size of both hulls and propellers. Panama is widening the locks on its canal to enable these larger ships to pass through. The next hurdle after that are the Malacca Straits, where the water is "only" 25m deep in some places. No doubt they'll dynamite those if need be.

Large marine diesels feature thermodynamic efficiencies of up to 52%, better than any other single-stage engine type. Two-stroke designs featuring actuated exhaust valves can be as many as five stories tall and run at ~100RPM, slow enough to drive the propeller without a gear box.

That still leaves about half of the primary energy going up in smoke or coolant heat. Turbocompounding is used on smaller four-stroke engines that feature a transmission of some sort anyhow.

Part-load and Dynamic Performance of a Medium-speed Four-stroke Engine with Two-stage Turbo Charging and Power Transfer System


Of far more import ong range is what happens when bunker fuel runs out.... Its used on these ships because its CHEAP and nothing else I know is as cheap save coal.

Bill Young

Rod Adams at Atomicinsights .com suggests nuclear propulsion for large maritime vessels. He proposes a pebble bed reactor which would be less expensive to fabricate and operate than the PWR type reactors used by the various navies of the world.

A nuclear powered cargo ship can afford to go faster than an oil fired one. This would permit higher revenue to offset the added capital cost of the nuclear powered ship. Fuel consumption increases rapidly with speed and the fuel costs for a nuclear ship are minimal.

The NSS Savannah was not economically viable when fuel costs were about $2/barrel but those days are long gone.

The CO2 emissions with nuclear power are negligible. The NOx and SO2 emissions are non-existant.


Nuke powered cargo ships would put dirty bomb material in every backwater port in the world. I doubt you would ever get much support for creating thousands of mobile, unsecure, nuclear reactors.


In open ocean SO2 and NOx emissions do not really matter. They got washed down by precipitation quite fast, and their impact on the ocean is virtually zero. In coastal waters and in internal seas, like Baltic or Mediterranean, it is totally different matter. Their pollution contribute (and will contribute on increasing manner) to acid rain and smog-forming over the land quite significantly.

SO2 problem could be addressed quite easily. Approaching the coast, merchant vessel could switch to low-sulfur diesel fuel. It is not significant price or technological burden, but troublesome to implement due to legal problems.

NOx is much more difficult to curb. The only technologically available option as of today is massive intake air humidification. Still, NOx emissions from mammoth marine two strokes will be very high.

As for CO2 from shipping… The best way to address this issue is to relax and look sideways.

Warren Heath

The simple solution to CO2 from shipping is to burn methanol. Low emissions and can easily be made CO2 neutral, can be burned at higher efficiency than diesel, and ocean spills of methanol are environmentally benign, unlike diesel fuel.

Iceland alone, has enough geothermal power to produce all the methanol needed for ship propulsion 10 times over. Bring waste or biomass liquid CO2 there by tanker, bring back methanol in the same tanker.

Rafael Seidl

Warren -

you're forgetting one thing: refineries need to sell *all* of their products to *someone*. The amount of heavy oil and tar contained in a barrel of oil varies greatly by reservoir, as does the sulfur content.

Nevertheless, given the volume of oil consumed, you inevitably end up with a fairly large amount of bunker oil. Hydrocracking and removing the sulfur from this would be very expensive indeed, substantially raising prices for all other consumers of petroleum products - ind those transported by ocean-going ships.


I am obviously missing something, but why does Aviation produce substantially less NOx(almost 1/10th)and almost zero PM compared to Marine for approximately the same fuel consumed?

Assuming that gas turbine engines dominate the Aviation contribution, they currently operate with non-premixed combustion in the same way as Diesel engines. It is well known that current gas turbine research is working on reducing NOx, PM and improving efficiency by employing premixed lean combustion. Going by the results published here they need not bother.

I am no expert on state of the art gas turbines for aviation so maybe there is a plausible explanation.

Rafael Seidl

Daydreamer -

while there are a few ships that run on gas turbines - mostly military vessels - most rely on diesel engines because they are much more efficient, especially in part load.

The reason for the high thermodynamic efficiency of diesels is that peak process temperatures are very high, up to 2500K. This is possible because the combustion chambers in ICEs reciprocate and are only exposed to these very high temperatures very briefly, via radiative and convective heat transfer. The surfaces are cooled by the fresh charge plus engine coolant plus oil jets against the underside of the piston crown/cavities within it. Unfortunately, high temps mean high NOx production rates, mostly NO that is further oxidized to toxic NO2 under the influence of sunlight once it escapes into the environment.

By contrast, gas turbines combust fuel continuously. Large implemnetations, such as those on aircraft, feature turbine vane with internal channels that deliver a small mass flux of compressed fresh charge to the tip of their leading edges. This relatively cool air acts as a shroud that protects the vanes against melting. Even so, feasible peak process temperatures are limited by the available materials to well below 1800K. Very small turbines (e.g. turbochargers) cannot be equipped with internal channels, so their peak operating temperatures are limited to just 1300-1400K depending on the material used.

Lower temps mean a lot less NOx is produced - the relationship is roughly exponential with temperature above 1500K. However, with aircraft engines even these small amounts present a problem, partly because the fraction of N20 (laughing gas, high GWP) is higher and partly because the emissions take place in the stratosphere where they linger longer.


Methanol is likelyseveral dozen times the cost of bunker fuel. Thus it cant be used to replace it. In fact most fuels cant be used to replace bunker fuel its simply too cheap.



I was not questioning the better efficiency of Diesel engines compared to gas turbines.

The question is the producton of NOx or as correctly assumed NO. Gas Turbines feature a compressor, a combustion chamber and a turbine. In the combustion chamber there are no vanes so what you say is not relevant. In any case we are talking about the reaction at the flame front wich in the case of non-premixed combustion is locally very high. I can understand that a certain amount of EGR is achieved inside the combsution chamber but other than that I see no remarkable reduction in NO in a gas turbine verses a Diesel.

It makes no difference if the combustion is continuous or intermittent. Thermal NO production is the dominant reaction and it is depends on temperature. Why should gas tubine engines produce 1/10th the amount of NO compared to Diesel engines? If you can explain that with respect to the combustion process I will be satisfied.


What if you just slow the ships down 10 - 20% ?
What is the equation for power vs speed - is it V^3 ?

Also, Rafael, how come you know so much (I am not being smart here) - could you post a brief resume ?

Rafael Seidl

daydreamer -

it would lead too far to go into more detail on this thread. Just consider that it's actually both temperature and the time spent there that matter, as well as the pressure at the time. Peak pressures in modern gas turbines - which operate according the the Rankine cycle - are on the order of 35 bar. In a diesel engine, they can be over 200 bar, so the likelihood of a nitrogen molecule encountering an oxygen radical is much higher. You might want to Google "Zeldovich mechanism" and "prompt NO".

Mahonj -

I'm a student of Mechanical Engineering at the TU Vienna, Austria. My specialty is in engine and drivetrain design, including the associated environmental aspects. I'll be starting on my master's thesis soon.

Wrt to your questions:
- slower speed: time = money in shipping. The speed container ships are designed to travel at is based on the optimum trade-off of speed vs. fuel cost. Note that the bow and aft wakes of a ship will interfere with one another. For a given hull length (i.e. Froude number), there are speed regions where they cancel out and others where they reinforce each other. Fuel economy is much better when they cancel out, so 20% slower might actually not yield as great an improvement as you would hope.

Power requirement is indeed proportional to the third power of vehicle speed, modulated by a bunch of factors including the effective water mass that has to be dragged along (see above).

Mike in TX

CO2 has nothing to do with "global Warming". This is a witch hunt of the modern day. If you chart the sun's activity and the world's temp you can see the connection.
Humans lived in much warmer conditions during the middle ages and even warmer about 5k yrs ago. Polar bears survived 3k yrs of much warmer temps.

Look it up. Do some home work. Stop following the heard!!!



Gas turbines operate on a Brayton cycle and not a steam based Rankine cycle. For gas turbines thermal or Zel'dovich NO production is dominant and not pressure dependent but on temperature and time. Prompt or Fenimore mechanism may take place at lower temperatures and may be relevant in gas turbines. Perhaps you may care to look at CHEMKIN for an understanding of emissions formation. In any case I don't need to "google" my knowledge of fundamental combustion.

Warren Heath

Wintermane, actually methanol costs $1 per gallon, even taking its 50% energy content of fuel oil, it still would compare favorably cost wise, and that is only until the Iran war starts, and then you can expect those fuel oil prices to double at least.

Rafael, you seem to be saying, yes emissions by ship engines is a problem, but solving the problem would involve a change from the status quo, so we don't want to do that.

I'm assuming that at some point in time, governments will want to address the emissions & environmental problems of shipping, by moving from a rapidly depleting and environmentally destructive oil product, for which methanol is an obvious solution. The problem of the Crude Oil fractions is likely to be opposite to what you mention. The increased proportion of alcohols replacing lighter distillates implies that there should be proportional reduced consumption of the heavier distillates as well.


Some modern cruise ships have technology to reduce the NOx emissions.
I had the opportunity to travel in Royal Caribbean's "Navigator of the Seas" 3 months ago.
There is no visible smoke going out the exhaust chimneys.
This ship is powered by six 12V46C medium speed engines
(Wärtsilä) each engine is capable of 16,900 HP at 500 RPM.
For the emission controls of these engines see:


Some of the clean technologies of some Wärtsilä engines are:
-- common rail fuel injection
-- direct water injection
-- selective catalytic reduction

aussie paul

It can only be a good thing when shipping is forced to use more expensive clean fuels such as natural gas, biodiesel and ethanol.
Bulky ores are shipped around the world instead of being concentrated where they are mined.
Foods and manufactured goods are shipped from countries with very low wages to countries halfway round the world who already produce those products locally, causing farming and manufacturing communities to collapse.
Manufactured goods are shipped halfway across the planet because it is easier and cheaper than setting up a factory in the recieving country.
The true cost of all this unecessary to and fro-ing is not realized in the price of the cargos but in the health of the planet.



In diesel engine initial temperature of compressed air is much higher than in gas turbine. So, combustion of fuel vapor layer around the fuel droplet will end up with higher temperature. And it is not possible to reduce somehow significantly firing temperature in diesel, because it is needed for ignition of injected fuel.

Second, diesel fuel has higher C/H ratio than jet fuel, and hence temperature of combustion front is higher.

Some reduction of NOx from diesel is accomplished by modification of combustion process. One of the most widespread tricks is to inject small amount of fuel to initiate combustion, and inject the rest later to have major combustion event after TDC, when piston is already descending and decreases temperature of the gases quite rapidly. It is also means less violent detonation (“ignition delay”), so compression ratio could be slightly raised to compensate for reduction in thermal efficiency without risk to destroy the engine.

Another practice is to dilute fresh air with 3-atom gases having high specific heat to adsorb thermal energy of combustion with less temperature increase, like CO2 (EGR) or water vapor (intake air humidification).


Passenger vessels often use medium speed 4-stroke engines coupled with transmission or in diesel generator / electric motor configuration; the main reason is to reduce noise and vibration, annoying passengers. Some cruise vessels even use exceptionally quite steam turbines. These engines use marine diesel fuel, which is much cleaner than bunker oil used in two-strokes, do not produce visual smoke, and could employ exhaust gas aftertreatment, like SCR. Marine diesel fuel costs twice as much as bunker oil, but cruise and passenger vessels could afford it because fuel cost represents small percent of their expenses.

Big merchant vessels are practically universally powered by monstrous low-RPM two stroke diesels, which are capable to burn extremely dirty bunker oil. Exhaust gases of such fuel contain high concentration of ash, sulfur oxides, soot, and NOx, so smoke is visible and aftertreatment devises fail in no time.

For merchant fleet fuel accounts for 60% of expenses, so switch from bunker oil to marine diesel fuel will increase cost of transportation by 60%. And any way, bunker oil is simply waste after oil refining, and it should be combusted any way. Two stroke marine diesel emitting most their pollution harmlessly in mid-ocean, which is much better than combust it in steam boilers of electric power stations.

P.S. Wartsila discontinued use of their proprietary direct water injection technology about 3 years ago. It caused some increased wear problems.

P.P.S. Aussie, planet is not a living creature and does not have “health”.


Thank you for this interesting explanation.

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