How does the climate impact of an LNG-powered cruise ship compare to flying?

Question

Taking the Iona as an example, it is claimed that

"The cruise ship will have a reduced carbon footprint with zero sulphur and nitrogen emissions."

Sulphur and nitrogen, while pollutants, are not greenhouse gases, and I'm asking specifically about the impact on the broader climate.

This article argues that flying is "three times greener than cruising".

Carnival claim LNG offers a 25% reduction in CO2

However, this article discusses the benefits of LNG in shipping and seems to suggest the greenhouse gas emissions of LNG are only 6% lower than heavy fuel oil on average.

My interpretation is that the benefits of LNG are only slight (from a climate perspective) and therefore flying is still lower impact than cruising. But I don't know if these claims are on a like-for-like basis.

Are there any studies that do a direct comparison of modern aircraft to modern LNG-powered ships?

N.B. I'm aware it is hard to compare the end-to-end climate impact of a holiday, considering the cruise ship has features and services built-in, which would be provided separately by a destination that you fly to (and therefore not included in the emission cost of flying).

Answer 1

tl;dr: Cruise emissions are lower than flight emissions even before switching to LNG, which might have unintended consequences due to leaks in the supply infrastructure.

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Cruise emissions

Carnival Cruise Line published an independently verified sustainability performance summary in 2018. According to this report, their ships release 251 grams CO2e per ALB-km. ALB-km is a measure like passenger-kilometer (pax-km), but in this case refers to "available lower berth" (source), which typically has capacity for two passengers.

If we generously assume a 20% improvement from the switch to LNG, emissions would drop to 200 g CO2e/ALB-km, or 100 g CO2e/pax-km.

Flight emissions

Emissions for air travel is more complicated, because it depends on the distance and type of aircraft, among other factors. There are at least two different calculators that take these factors into account.

• The ICAO carbon emissions calculator from the United Nations International Civil Aviation Organization.
• The flight emissions calculator from the myClimate Foundation, a Swiss non-profit. In contrast to the ICAO, this calculator includes an estimate of the non-CO2 impacts of flying, such as high-altitude NOx and contrails. The specific impact of these effects is still being worked out, but this calculator includes a conservative estimate.

Cozumel, Mexico, is a popular Caribbean cruise destination. There are also regularly scheduled direct flights from Miami, USA, so it makes for a good comparison. It's a round trip flight of 1,792 km.

• ICAO calculator: 266 kg CO2, or 148 g CO2/pax-km
• myClimate calculator: 0.400 mt CO2e, or 222 g CO2e/pax-km

Conclusion

At 126 g CO2e/pax-km, a conventional cruise ship is already better than the lower estimate of 148 g for flying, and has about half the impact of the higher estimate of 222 g. Thus, any improvement in cruise ship efficiency from a switch to LNG, while welcome, is superfluous -- it was already better than flying.

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Leaks in natural gas infrastructure

But there's a catch -- recent studies indicate leaks in natural gas infrastructure have been underestimated for years, and methane (the chief component of natural gas) as a greenhouse gas is 34 times more potent than CO2 on a 100-year timescale.

So switching from diesel to LNG (which is made from natural gas) might actually be worse from a life-cycle emissions perspective.

Answer 2

This is going to be unpopular, because I can remember things that mankind has forgotten. A long time ago people were saying that the shading from contrails might cause another ice age. Now the word contrails has been almost forgotten and a recent weather program was calling them something else entirely and intimating that they are natural. Contrails cause global dimming, i.e. they prevent the sunlight from penetrating to Earth, i.e. this makes the Earth cooler. Not much obviously, but people do not realise what a precarious balance Earth's climate is in. Therefore by this reasoning,if the choice is between a cruise and a flight, the flight is greener.