Climate Action —Air Travel

I’ve written a post showing you how individual choices affect global warming. You should read that instead. This is a dry, technical supporting post showing you where the numbers come from. If you read it, please read it after my post on methods. For reasons given there, I’ll be relying a lot on a book by Mike Berners-Lee for emission intensities of actions, although every figure will be cross-checked against other sources.

Air travel

Berners-Lee estimates 3.5 tons of emissions for a return flight in economy class from London to Hong Kong (19,300km). That works out at 181 grams of emissions per passenger-kilometre.

This estimate falls inside the range of 80–218 grams given by the IPCC for medium to long haul flights. The reason it’s towards the high side of the range is likely that Berners-Lee applies a factor of 1.9x to adjust for a well known effect whereby high-altitude emissions have a larger effect on global warming.

The UK’s official GHG conversion factors give another useful data point. They estimate 103 grams per passenger-km for long-haul flights; once the 1.9x factor is applied this again seems very consistent. Short-haul flights are 79 grams and domestic flights 145 grams. The figures for different types of flights are close enough that I don’t feel it’s useful to split out categories.

Having cross-checked it, I’m happy with 181 grams of emissions per passenger-kilometre in economy.

Berners-Lee estimates that in the UK personal flights account for about 8% of 13 tons, or 1.04 tons of emissions a year. Using the conversion factor from the methods article, cutting out such a flight each year would reduce the 2100 temperature on the Little Planet by 0.11 °C.

Now to cross-check that. Estimating how far the average person in the UK/US flies is hard, because international flights are not included in any country’s emission statistics. This 2013 study estimates that the average UK household emits 1.13 tons of CO₂ from flights a year. (That’s the mean; the median is 0! Only 41% of households fly.) ONS gives the average household size as 2.4, so that’s about 0.47 tons per person per year. Applying Berners-Lee’s 1.9x correction factor, we get 0.89 tons per person per year. Flights have been increasing rapidly since 2013, so that seems quite consistent with Berners-Lee’s estimate.

International flights are not included in emissions figures for most countries. Source.

Combining our figures of 1.04 tons of emissions and 181 grams of emissions per passenger-kilometre, we find that the average person in the UK flies 5,750km a year.

That’s a pretty misleading statistic, though; as mentioned above, only 41% of UK households fly at all in any given year. Assuming that about 41% of people fly, we can split that 5,750km and say that in any given year:

  • About 59% of people in the UK don’t fly at all.
  • About 41% of people in the UK fly on average 14,000km a year.

The flight distance from London to Miami is about 7,100km, or 14,200km for a return flight. Close enough to 14,000km that I’ll use it in the main article. Combining 14,200km with Berners-Lee’s 181g/km estimate, that return flight (in economy class) emits 2.6 tons.* Applying the conversion factor from the methods article, we find that cutting out such a flight each year reduces the 2100 temperature on the Little Planet by 0.27 °C.

Source.

As you can see in the table above, Business class generates about 222.1/76.6 = 2.9x the emissions of Economy. So a business class return trip from London to Miami emits 7.5 tons; cutting it out reduces 2100 warming on the Little Planet by 0.79°C.

Credit.

The Energy Information Administration’s Annual Energy Outlook 2020 (AEO2020) says that air travel accounts for 1,020 billion passenger miles in 2020.

The AEO2020 isn’t clear on whether or not it includes international flights, but the St Louis Fed’s economic data (above) shows seasonally adjusted ‘U.S. Air Carrier Domestic and International, Scheduled Passenger Flights’ were heading towards 90 billion passenger miles a month before COVID-19. That’s 1,080 billion passenger miles, so I’m confident that the AEO2020 figure a) includes international flights and b) isn’t skewed by COVID-19.

Converting to SI units, the AEO2020 figure becomes 1,640 billion passenger km. A US Census webpage gives 2020 population at 330 million. Dividing, we find the average person in the US flies about 5,000 km a year, which is surprisingly close to the UK figure of 5,750 km a year.

As with the UK, that figure is misleading because many people don’t fly. This 2018 survey estimates that 48% of the US adult population flew in 2018. The 2016 version of the survey notes that nearly every adult who flew made some personal flights. Given that, I’m just about comfortable extrapolating the 48% figure to the whole US population. Using that, we find:

  • About 52% of people in the US don’t fly at all.
  • About 48% of people in the US fly on average 10,400km a year.

Conveniently, the flight distance from London to New York is 5,500km, or 11,000km for a return flight, which is close enough to 10,400km to use in the main article. Using the same methods as before, that return flight (in economy class) emits 2.0 tons; cutting it out each year would reduce the Little Planet’s 2100 temperature by 0.21°C. In business class, a flight would emit 5.7 tons; cutting it out each year reduces the Little Planet’s 2100 temperature by 0.60°C.

Standard economic theory provides a good case for letting people carbon offset their flights. The problem is that in practice it doesn’t work. I looked through the four largest US Airlines; two of them, Delta and United, let you offset your flights via their partners Conservation Coast and Conservation International. If you click on those links, you’ll see they’re running projects verified by Verra’s VCS program, which is the largest carbon offsetting program in the world.

… That’s great?

No.

Verra’s VCS program is an empty facade. ProPublica published an excellent expose about it, which I wrote about here. (I was pretty annoyed when I wrote that, so it’s a little intemperate.) To my mind, the most damning part is this:

[ProPublica’s Lisa Song] asked the CEO how the company could guarantee that the credits it continues to sell are actually helping to preserve trees if it might go as long as two decades without checking.

[Verra CEO] Antonioli said, “Anybody who is interested in buying such credits would want to do their due diligence and check to see what’s happening on the ground.

Which is to say, Verra’s actual certification means nothing. United and Delta are ‘greenwashing’ their emissions. By misleading environmentally conscious fliers into thinking their flights have no effect, it’s likely that their offsetting schemes are increasing the number of flights and so total emissions.

Key science, with sources. Minus bad statistics. Minus shaky methodology. Minus politicisation, left or right.