How do you think of travel distances in your head? People often think in terms of the time taken to get there rather than the actual distance. One hour (each way) is not unusual for regular commute by car and for a weekend excursion you might think three hours was reasonable. By air you can get quite a long way in three hours. Here I compare GHG emissions for various travel modes by km and by hour.
This post was inspired by some much prettier graphics from the Visual Capitalist showing emissions per km. I have come to think that metric is positively misleading.
The first chart shows the usual metrics: kg CO2e/km or CO2e/passenger.km for the public transport cases. In this chart the emissions for the car are not far off the flights. If there is more of one of you in the car, then it is fair to divide the car emissions between you; if there are four of you the emissions per person are similar to train travel.
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| Emissions per km or passenger km. Data from [1] |
Now the emissions per hour of travel. Emissions per hour are emissions/km times km/hour (see speeds below). By this metric the car is nowhere near as bad as the plane. To put this into perspective, the typical emissions per person in the UK are roughly 20 kg/day. That means one hour of long haul flight is equivalent to 8 days of normal emissions.
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| Derived from the same data as above, with speed assumptions below. |
Notes and assumptions
The air travel emissions include radiative forcing. This is the extra impact over and above the actual fuel use due to contrails and some complicated atmospheric chemistry. Carbon Brief has a good explanation if you want the details.
This table details speed assumptions.
| Mode | Miles/ hour | Notes |
|---|---|---|
| Ferry (passenger) | 28 | Based on Cairnryan (Scotland) to Belfast 2 hours 15, 62 miles |
| National rail | 60 | 1 hour Cambridge to London |
| Battery electric vehicle | 60 | |
| Bus (outside London) | 20 | London buses usually run with more passengers at a time so lower emissions per passenger |
| Ferry (car) | 28 | As the passenger ferry |
| Diesel car | 60 | |
| Short haul flight | 320 | London to Berlin 580 miles, 1 hour 50 minutes |
| Long haul flight | 500 | Typical cruising speed |
[1] UK Government GHG Conversion Factors for Company Reporting 2023
Nicola, why do you think emissions/km is misleading? You don't say.
ReplyDeleteI've wonder about the 'per passenger' emissions of say, the Harwich to Hook crossing. It is designed mainly for freight and with cars a minor weight load. The marginal increase of emmisions per extra foot passenger(& cycle) must be very small
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