Air cargo has a bad reputation as the freight mode with by far the worst CO2 footprint. It turns out that this negative reputation may be highly exaggerated and air cargo is less of an environmental hazard than often thought.
CO2 emissions for the various modes of freight are usually measured in terms of grams of CO2 emitted per ton-kilometer of freight – that is per ton of freight and per kilometer (km) of distance.
Air freight’s bad reputation is based on the fact that long haul flights produce an estimated 602 grams of CO2 emissions per ton-kilometer. That’s a high number. It means that on a long haul route of 10,000 kilometers, such as a flight on the major trans-Pacific trade route from Shanghai to LA, each ton of cargo is causing 6 tons of CO2 emissions into the atmosphere!
Air cargo’s footprint of 602 grams (g) per ton-km, compares with 62g for road, 22g for rail, and an impressively slim 8g per ton-km for long haul container shipping.
However that number of 602g per ton-km of air cargo is misleading. Many airlines don’t own freighters and fly cargo exclusively on the lower deck, with about half of air cargo traveling on the lower deck of passenger aircraft. If your freight is less than 160cm (64”) high, there is a good chance it’s flying on the lower deck.
Now, as their name suggests, passenger aircraft primarily serve passengers. No airline buys or flies a passenger aircraft primarily for the cargo, as cargo represents just 9% of their revenue. So those passenger aircraft are flying anyway, with or without the cargo.
That makes all the difference. When cargo flies on a passenger flight, its footprint is limited to the marginal extra fuel burned to cover the weight of the cargo. That footprint is a full order of magnitude smaller!
To be specific, when you include the weight of the aircraft itself, the average fuel burned by an aircraft is 200g of fuel per ton-km of load. On the other hand, if the airplane is flying anyway, the marginal fuel burned for an extra kg of load is just 20-30g per ton-km. That brings the carbon footprint of lower-deck cargo down to 60-90 grams per ton-km, comparable to the 62g of road.
Aircraft have one final advantage – they tend to fly like crows, in a straight line, whereas trucks will zigzag along highways and drive 10-30% further than the direct distance. With this extra factor it’s clear that the marginal carbon footprint of lower-deck air is no worse than road.
To be sure, the freight industry is still a significant emitter of CO2, and air emits a lot more than ocean or rail, but it’s good to know that lower-deck air cargo is probably 7-10x less emissive than is usually thought, and is comparable to trucking. Environmentally conscious shippers may like to take note of the massive difference in the marginal carbon footprint of lower deck vs. main deck air cargo and make their choices and calculate their footprint accordingly.