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.

Driving

Thinking about emissions from driving is conceptually harder than any of the other actions we look at. The reason is that people buy cars, and making a car emits a truly prodigious amount of CO₂. Berners-Lee notes that people are particularly prone to underestimate this because they use lifecycle analysis rather than the (harder but better) input-output method. Here are some of the figures he gives for the UK:

For comparison, the average person in the UK emits 13 tons a year from everything they do. What this means is that if in a spirit of eco-goodwill you were to go out tomorrow and replace your perfectly functional car with an electric vehicle, you would substantially increase your lifetime emissions.

Most of those cars are around the 10 ton mark, so we can use our rules to figure out that each car made increases the Little Planet’s 2100 temperature by about 0.03 °C. (The Range Rover and other large cars cause twice that warming or more.)

Figures for the US are somewhat higher as cars are larger; Berners-Lee’s old (2011) edition cites 17 tons for a Ford Taurus. Manufacturing one of those would warm the Little Planet by 0.05 °C in 2100. A Land Rover Discovery SUV has 35 tons of embedded carbon, which is twice as much.

Things are complicated by the fact that more than one person typically owns a car, so it’s not obvious who is ‘consuming’ the car and its associated emissions. You could account all the emissions of a car to the first owner, or you could spread it out over all the owners according to how much they drive. As we’re interested in the actual emissions reduction from your choices, not in pointing fingers, there is one correct economic way of doing the accounting, but unfortunately I don’t have the data to apply it. (And it’d probably take another whole article to explain.) Berners-Lee uses the simple approach of spreading out the emissions over all the miles driven, and we’ll stick with that.

There’s one other technical point I’ll note and then ignore for simplicity. When computing Little Planet warming, I’m assuming emissions reductions of 1.5% each year due to technological improvements. But one of those technological improvements is electric cars! If I tell you that

  • you can reduce emissions by switching to electric cars and
  • emissions from driving will go down by 1.5% a year because of better technologies like electric cars,

I’m trying to count the same emissions reduction twice. If I want to split out the gain from electric cars as a separate item, I should use a slower reduction rate; looking at fuel efficiency for new cars suggests 1% a year is a more appropriate reduction rate for non-electric cars. As noted in the methods article, switching from 1.5% to 1% only changes the final figure by 10%, so I’m not going to try to correct here.

There’s a useful fact from Berners-Lee that I’ll use several times below:

As a rule of thumb, about half the carbon impact of car travel comes out of the exhaust pipe. A few percent come from the fuel before it is burned. The rest, typically 40% of the footprint, is associated with the manufacture and maintenance of the car.

I’m going to cross my fingers and guess that maintenance is a small proportion, so the emissions split 40:60 ratio for manufacturing and for driving. I’ll call this the 40:60 rule.

I should say I am a bit puzzled because it’s hard to square those numbers with this graph, also from Berners-Lee:

According to that graph we should expect manufacture and transport of petrol to emit about half what burning it does. So I’m not sure how one gets ‘about half’ of emissions from the exhaust pipe and only ‘a few percent’ from ‘the fuel before it is burned’. But I’m not going to rely on that split for fuel, so I’ll relegate this to ‘vaguely troubling’ and move on.

The UK

Berners-Lee notes that the average UK car burns 36 miles per gallon (MPG) and emits 530g per mile or 0.33kg per km driven. This includes all the emissions needed to get the petrol into the cars, and the manufacture and maintenance of the cars.

Let’s check these figures. Since certain car manufacturers are notorious for rigging emissions tests, we’ll look for independent sources. This corroborrates that 36 MPG for petrol cars; the average including diesel cars is 38.8 MPG, but I’ll stick with 36 MPG.

This webpage uses data gathered by Which to estimate 14.3 kg of emissions per gallon burnt. Combining that with 36 MPG gives us 0.25kg per km driven. The same page estimates 0.099 kg per km of emissions from car manufacture and maintenance. Adding those gives a total of 0.34 kg per km, which is remarkably close to Berners-Lee’s estimate.

I can’t square that driving-manufacturing breakdown with Berners-Lee’s comment that ‘about half of the impact of car travel comes out of the exhaust pipe itself’. I’m also a little surprised because the webpage seems to uses a fairly crude version of life-cycle analysis and I’d expect it to give lower numbers than an input-output model. But the numbers above are the best I’ve got, so I’ll stick with them.

What about distance driven? Berners-Lee says that the ‘UK average annual distance’ is 7600 miles (12,200 km), but I’m not sure whether that’s per car, per driver or per household. The Department of Transport estimates car driving accounted for a total of 278.2 billion vehicle-miles in 2019. Dividing by the UK population of 66.8 million, we find 6,700 vehicle-km per person per year. To avoid double-counting, we will attribute that to the driver. (It’s arguably better to split it equally between all passengers, but I don’t want to complicate the main article by making people do arithmetic.) It works out at about 11 miles driven a day.

6,700 km times 0.33kg per km gives average annual emissions of 2.2 tons per person per year, which warms the Little Planet by 0.23 °C in 2100.

Using the 40:60 rule, we get

  • 0.9 tons per person per year embedded in the new cars people buy, warming the Little Planet by 0.09 °C in 2100.
  • 1.3 tons per person per year from driving those cars, warming the Little Planet by 0.14 °C in 2100.

The US

The latest edition of Berners-Lee only covers the UK, but the 2011 edition has old data for the US. It notes that the average US car drove 22.4 miles per gallon and emitted 850 grams per mile. As with the UK figure, that includes manufacture and maintenance and so is higher than figures you’ll see elsewhere.

We have to be careful about using that directly, though. The 2011 edition of Berners-Lee estimated US emissions at 28 tons per person; the 2020 edition at 21 tons per person. That’s a 25% reduction, which we can’t gloss over. (In the same timespan, UK emissions per person drop from 15 tons to 13 tons, a 13% reduction.) Some of that reduction will be due to driving more fuel-efficient cars and greener manufacturing processes.

That graph is from the EPA, and shows fuel efficiency for new cars over time. You can see that it looked to be around 22.5 MPG around 2010; my guess is that Berners-Lee used that instead of an average over all cars on the road due to a lack of data about car ages. We’ll do the same and use 25.1 MPG.

Berners-Lee links 22.4 miles per gallon to emissions of 850 grams per mile. As noted above, he says that about 40% of that is from manufacture and maintenance of the car, and the remainder from the fuel. Assuming that ratio stays the same, we can scale down the emissions figures to adjust for the MPG; at 25.1 MPG we get emissions of 760 grams per mile, of which about 300 grams per mile from manufacture and maintenance and the remaining 460 grams per mile from the fuel. Note the same EPA page suggests only 353 grams per mile for fuel, probably because they aren’t accounting for the emissions from processing and transporting fuel.

We’ll stick with 760 grams per mile, which is 0.42 kg per km.

The Bureau of Transportation Statistics estimates that in 2018 2.23 trillion miles were driven in ‘light duty vehicles with short wheel base’, which includes ‘passenger cars, light trucks, vans and SUCs’. (A small proportion of that may be driving for business, which should be counted via embedded emissions in goods and services, but I can’t find the data to make that correction.)

The US Census gives a population of 328 million in mid-2019. That’s works out at 6,800 vehicle-miles per person per year, or 10,900 vehicle-km per person per year. Unsurprisingly this is much larger than the UK figure of 6,700 vehicle-km per person per year as the US is huge and things are spread out. As before we’ll attribute that to the driver; it works out at about 19 miles driven per day.

10,900 km times 0.42 kg per km gives average annual emissions of 4.6 tons per person per year, which warms the Little Planet by 0.48 °C in 2100.

In the main article, I’m going to round the figures to 20 miles per driver per day and 0.5 °C of warming, to make it easier for people to follow the arithmetic.

Using the 40:60 rule, we get:

  • 1.8 tons per person per year embedded in the new cars people buy, warming the Little Planet by 0.19 °C in 2100.
  • 2.7 tons per person per year from driving those cars, warming the Little Planet by 0.29 °C in 2100.

Reducing Emissions

One way to emit less is simply to drive less; if you drive 20% less, you’ll reduce your emissions by 20% or so. (It’s not quite exact, because you might eventually have to replace your car because it’s old, but I think that’s a minor effect.) So driving 20% less would cool the Little Planet in 2100 by 0.05°C (average UK driver) or 0.10°C (average US driver).

Another way is to switch to a more efficient car. Smaller cars can be made with fewer emissions and emit less while driving. The most efficient car is, of course, an electric car. For the UK, Berners-Lee gives the following emissions figures for driving a mile, inclusive of vehicle manufacturing emissions:

  • 180g in a mid-sized five-door electric car.
  • 530g in an average UK car at 36 miles per gallon.

I find it hard to square these numbers with the 40:60 rule, which would tell us that 40% of that 530g, or 212g, is from the manufacture & maintenance of that average car. Berners-Lee notes that making an electric car emits more than an equally sized petrol car because the battery is carbon-intensive, so I can’t see why the overall electric car figure is lower than 212g.

Let’s run a sanity check by looking at the emissions from the electricity used. This suggests an electric car will travel around 3.5 miles on one kWh. Berners-Lee notes that that kWh involves 340g from the UK grid. So we’re talking roughly 100g/mile (UK) just from the electricity. Berners-Lee also notes that manufacture of a Renault Zoe electric car works out at 105–110g/mile if it’s driven 100,000 miles. So a total of 180g/mile does make sense if you assume the car is driven a fair bit more than 100,000 miles or so in its lifetime.

This is confusing. Let’s check car sizes. A Renault Zoe weighs about 1.5 tons; this (not UK specific) site notes that an average car weighs 1.4 tons. So that’s not it.

It might be this (from the 2011 edition):

Big, expensive new cars have more of their embodied emissions attributable to each mile of driving. An older car that is still fairly efficient could beat a new Fiat 500 by virtue of having had its embodied footprint written off.

I don’t understand what’s going on here. And unfortunately, I can’t cross-check this stuff because most of the research out there uses lifecycle analysis rather than the input-output method. Lifecycle analysis sucks. It often misses half of emissions or more.

It is clear that switching to an electric car reduces emissions to around a third to two-thirds of a non-electric car, including the embedded emissions. This suggests a figure of a half, and even if it’s based on lifecycle analysis I’m going to go with that.

Driving more slowly and avoiding stops and starts can also reduce emissions. Berners-Lee notes that driving on a congested road can triple your emissions, and that driving at 60mph saves 15% compared to driving at 70mph. And finally, emissions per mile driven are barely affected by the number of people in the vehicle, so you can save a lot by driving with more passengers. For four people, Berners-Lee notes that taking a train emits very slightly more than taking a small efficient petrol car.

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