Mobility Matters Extra - The most carbon-intensive transport modes
Life's complicated, what can I say?
When I started out this mini-series, my plan was simple. Come up with a Net Zero transport moonshot in a few weeks. But writing this week’s post, it gave me an insight into the complexity of the issue at hand, one that I feel warrants more detailed exploration. So, I have changed tack. Over the coming weeks, we will explore in more depth some of the inter-related matters associated with developing a moonshot, before doing the moonshot in more depth in the New Year. I hope that you can forgive my change of tact on this.
By the way, look out on Friday morning this week. You will get your first in depth data analysis email!
James
Simple questions with complex answers
If we are to set a vision of how we are to achieve Net Zero in transport by 2030, we must ask a simple question: How much carbon is emitted by the transport sector? In the UK, as of 2019, that is 120.84 Million tonnes carbon dioxide equivalent (MtCO2e), with the overwhelming majority of it cominge from passenger cars.
Shortest blog post in history, then. Easy. Just tackle cars and we will be fine.
But you and I know that the world is more complicated than this. If we are to design interventions that take account of how transport carbon emissions are emitted in this country, then we need to consider the primary reasons why transport carbon emissions do vary according to what we do and how we travel.
This is something that has long been debated in academic circles. There is some degree of commonality in policy approaches - less car travel, curbing the use of avaition, and encouraging more use of sustainable transport. And the easy policy approach is to simply say “do all of it now.” But any plan that stands a chance of success needs to understand the nuances involved so that immediate big wins can be achieved whilst the thorniest issues continue to be worked on, to the point where a good solution is identified.
Within this context, in this post I will consider several factors that influence the level of carbon emissions across transport. These are trip mode, embodied carbon, trip distance, and utilisation. What is presented below is a high level summary of the current state of the research into each.
Trip mode
So what is the most carbon-intensive mode?
This seems like something that has an easy answer to it. Its walking and cycling, clearly. Walking certainly has the top spot in terms of carbon emissions however you cut the data. It requires no additional technology other than your shoes, and so the main carbon emissions come from creating the energy that is needed to power your walking, namely your food.
Data from the UK puts the different modes in a rank order by gCO2e/km2 (grams of CO2 equivalent per passenger kilometre). Domestic flights, the black cab, and a medium-sized petrol car are among the worst. Buses fare worse for emissions compared to 2 people in a petrol car, otherwise all of the lowest emitting modes are sustainable transport modes.
I highly recommend that you review the UK Government’s methodology on conversion factors for carbon emissions from transport. Emissions are highly variable according to vehicle weight, and car size, so much so that some estimate that the growth in SUVs alone has been the second biggest cause of emissions rises. Even things like using the air conditioning or driving style can impact on carbon emissions. Accordingly, whilst it is far from perfect, the UK government has been forced to use factory-reported emissions standards in estimating carbon emissions.
Why is walking and cycling not on this chart? It’s a very simple reason. There is no good quality data on it. Whilst the above data does take account of the energy inputs required to power the vehicles (that’s why electric vehicles have some emissions against them), only one such example exists for active travel (more on this in a moment). And if you are counting breathing as an emission, well that then has to count for all other transport modes as well.
Embodied carbon
The idea behind this is simple. Before you commence on a trip, there is already a degree of embodied carbon within that trip. A lot of energy goes into creating the things we need to do a trip. Energy is needed to make cars and all of their components. Energy is needed to create the materials for roads and cycle tracks. Energy is needed to build and power buses and trains. At almost every point in the transport system, energy is needed to help you get around and that is the case even if you don’t make the trip. After all, someone had to build the laptop you are working from home on.
Guides like those produced by the Institution for Civil Engineers put this challenge very simply. Every choice we make has a carbon impact, and so it is our responsibility to make the choice with the lowest energy requirement, and accordingly the lowest carbon impact at every stage with every process involved in the creation of that trip, infrastructure, or vehicle.
You can guess how challenging this kind of process is. To do so in a perfect world requires perfect knowledge of the carbon impacts of each process, as well as overcoming issues of double-counting. Trade-offs need to be made in order to keep any such assessment usable and to make actions clearly defined. The Highways England Carbon Tool, for instance, considers embodied carbon in terms of the materials used, but third party manufacture and transport is not considered.
Civil engineering is way ahead of other transport fields in terms of estimating embodied carbon primarily because their assessment is simple. We are building a thing, and materials and processes are needed to build a thing, and therefore we can estimate the carbon involved simply. Doing this in itself could lead to a huge saving, with up to 30% of global greenhouse gas emissions involved in buildings and construction. But even here, significant work is needed, notably on accounting for emissions post-construction and that involved at the end of the building’s life.
The reality is that whilst there has been work to estimate the embodied carbon of individual transport modes to some degree, a good multi-modal estimation of embodied carbon is currently lacking. It is estimated, for example, that 20-25% of a vehicles carbon emissions come from its production. Work by Mike Berners-Lee estimates that this carbon varies significantly by the type of vehicle also, with a Citreon C1 requiring 6 tonnes of CO2e to manufacture, a Ford Mondeo 17, and a Landrover Discovery 35.
The work of Berners-Lee doesn’t stop there, however. In the book How bad are bananas? it is estimated that the carbon footprint is further affected by what people eat prior to exercising. For example, your carbon footprint of a walk powered by fruit is lower than one powered by that Big Mac you guiltily ate last night.
I have found no such equivalent data for buses, HGVs, ships, planes, bicycles, and a lot of other things. To be fair, this is a relatively recent concept, and consequently the research has yet to be completed. This poses a challenge for us in setting our own net zero goals, in that our knowledge of this area is lacking. But that does not stop us from adopting some general principles to reduce carbon emissions wherever we feasibly can.
Trip distance
Earlier on, I mentioned this in regards to the offical UK data:
Data from the UK puts the different modes in a rank order by gCO2e/km2 (grams of CO2 equivalent per passenger kilometre).
This indicates that there is significant variation according to the total distance of a trip. To start with some basics, the UK National Travel Survey shows that between 2018 and 2020, 69% of all trips were less than 5 miles in length, with the private car being the dominant mode of transport across all lengths, with the exception of trips less than 1 mile where walking is the dominant mode of transport.
The calculation of the environmental impact of these trips is a simple one. Assume a set emission level per mile (or kilometre), and multiply by the number of miles (or kilometres). This means that one long distance trip will always emit more than a short distance trip. But the total emissions is also a factor of the frequency of such trips. This can lead to some very peverse policy ideas if implemented literally.
To take an example, lets say you live in London. Is it more environmentally friendly to fly to New York for a holiday once a year, or to fly to Inverness for 3 short breaks a year? Lets discount the access and egress trips, and just focus on the flights. Lets also use the Department for Transport’s direct and indirect emissions per mile for planes, with a straight line distance between Heathrow Airport and JFK Airport in New York (3446 miles) and Inverness Airport (444 miles). The result is:
London to New York: ((3446 x 0.209 kgCO2) + (3446 x 0.043 kgCO2)) x2 = 1740 kgCO2
London to Inverness: (((444 x 0.209 kgCO2) + (444 x 0.043 kgCO"2)) x2) x3) = 671 kgCO2
Would there really be a policy initiative that says “save the climate, and fly to the Highlands a few times a year rather than going to New York?”
Reducing the frequency of long distance trips is a sound carbon reduction strategy, as the longest trips are more carbon intensive per trip. In fact, by just removing a transatlantic flight that could reduce your own carbon emissions substantially. But by the simple fact that we undertake more trips at a local level, the majority of the carbon saving can be found there.
Utilisation
The idea here is really simple. For modes that are not walking and cycling, the more people you pile into the vehicle, the more efficient in emissions terms that vehicle becomes. In the UK, vehicle occupancy rates are calculated as so:
Buses: 9.2 passengers per bus
Light rail: from 22 passengers per vehicle to 110 per vehicle
There are no statistics for heavy rail or aviation. But from just the UK data we can immediately see that changing the utilisation of vehicles can change how you see the issue significantly. For example, lets take increasing the utilisation of cars, and see what impact that has on carbon emissions per passenger kilometre.
Simply put, putting an extra person in a car immediately makes their trips less polluting than using a bus. And a fully-laden petrol or diesel car is on a par with using a train.
There is an immediate counter to this - increasing the use of trains and buses will also make those modes more environmentally friendly, not to mention changing them over to using electricity. But this is the point. If we utilise the spare capacity in our vehicles more, then carbon emissions across all modes of transport will fall.
This also presents a challenge for public transport. To encourage more people to use public transport, frequencies of services may need to be improved in order to make the service attractive and to reduce crowding. This in turn results in more vehicle trips, which results in more under-utilised capacity to attract more people onto public transport. Which in turn reduces utilisation and…you get where I am going with this.
So what to conclude?
The clear and obvious conclusion is a simple one: decarbonise everything and always choose the lowest emitting mode in terms of carbon. I don’t think its that simple, and I need to do some more data digging to understand this all better. But an obvious few points emerge to me from just this analysis:
There is an immediate quick win to be had by reducing long distance trips, or at the least substituting for more national trips by sustainable modes. For example, why holiday in America when you could take a train to Scotland?
There needs to be a significant policy initiative to drive up vehicle occupancy across all modes of transport.
Walk and cycle more, obviously.
Embodied carbon needs urgent research.
My thoughts on all of this are still developing. But this is what the data so far says. Any thoughts?