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.

The emissions figures for food seem to be less consistent than for the other sections I have analysed. That may be because of the time horizon or GWP/GTP issues mentioned in the methods post. Given that, mixing and matching numbers from different sources is particularly likely to lead to nonsense. To get round that I’ll use different sources to estimate percentage reductions in an individual’s emissions from going vegan and vegetarian, and then convert those to warming numbers at the end.

Total Emissions from the Average Diet

When I was researching this, I found this 2018 review article which pointed me at two studies for the US and one for the UK. Let’s look at those in turn.

This 2011 input-output study for the US is methodologically solid and cross-checks its data against two databases:

TOTAL is a subtotal. Ignore it.

The fact that the two databases, CEDA and EIOLCA, agree so well, is reassuring. You can see that an average household emits 7.4 tons from food a year, which is 15% of the total 48 tons they estimate. They use an average household size of 2.5, so that’s 3.0 tons of emissions per person per year from food out of 19.2 tons.

This 2008 paper uses life-cycle analysis, so is less reliable, but still useful for cross-checking. It comes up with 8.1 tons of emissions per US household per year. That’s satisfyingly close to 7.4.

What about the UK? The review I mentioned cites this corrected graph from this 2011 paper, from which they estimate an average UK household emits 3.6 tons from food out of a total 28.6 tons, or 12.6%. On the other hand, the authors of that paper use input-output analysis to derive explicit numbers in this 2010 paper, which I’m more inclined to trust. They estimate an average household emits 6.1 tons from food a year, out of a total of 26.1 tons, or 24%. ONS gives the average UK household size as 2.4, so that is 2.5 tons of emissions per person per year from food out of 10.9 tons.

After I had dug out those figures, the second edition of Berners-Lee’s book came out, with an estimate that food accounts for 25% of the average UK emissions of 12.7 tons per year, or 3.2 tons per person per year, which is very close to the US figure. That’s a fair bit more than 2.5 tons, but as mentioned before, none of these numbers are too exact. I’ll stick with Berners-Lee for the usual reasons.

Percentage reductions from going veg(etari)an

The table I posted before conveniently has everything we need in.

It’s a nuisance that eggs are filed under meat, but it can’t be helped. Cutting out meat reduces annual household food emissions from 7.4 tons to 4.9 tons, a 34% reduction. Also cutting out dairy gets us to 4.1 tons, which is a 45 % reduction. In practice the reductions could be smaller than this, because vegetarians and vegans could eat more cereals, fruit, etc. than the average person, but I can’t find good data on that.

You might observe at this point that taking the average US person as a starting point is a little misleading; it might be better to start with the average US meat-eater. But in fact, the proportion of vegetarians in the US is only about 3%, so in practice it doesn’t change the numbers.

The website Shrink that Footprint gives this infographic:

From this we can read that going vegetarian reduces emissions by 32% and going vegan by 40%.

This study based on a survey of about 50,000 people estimates the emissions reductions from thousands of people in the UK changing diet. Here are the key findings, with some editing:

The age-and-sex-adjusted mean GHG emissions in kilograms of carbon dioxide equivalents per day were 7.19 for high meat-eaters (>= 100 g/d), 5.63 for medium meat-eaters (50–99 g/d), 4.67 for low meat-eaters (< 50 g/d), 3.91 for fish-eaters, 3.81 for vegetarians and 2.89 for vegans.

One headache is that they normalise all diets to have 2,000 kcal. I compensate for that using this data.

This sample is not representative of the population. The Vegetarian Society estimates that only 2–3% of people are vegetarian in the UK. As with the US data, this will mean that the emissions of the average person are very close to those of the average meat-eater.

The paper conveniently mentions that the average meat-eater emits 5.93kg per 2,000 kcal. Combining with the 1,972 kcal average daily intake, we find yearly emissions are 2.1 tons. The same calculation gives 1.3 tons for vegetarians, a 31% decrease, and 0.9 tons for vegans, a 57% decrease. The vegetarian figure is close to the US 34% decrease, but the vegan one is significantly more than the US 45% decrease. That could be measurement error, or it could be that people consume a higher proportion of dairy in the UK.

Finally, what about Berners-Lee? He suggests emissions reductions of 25% for going vegetarian and 40% for going vegan.

I’ve thrown too many numbers at you to remember, so let’s have a recap of the potential savings from dietary changes…

As I mentioned at the start, there’s more variation here than in other sections, particularly in the figures for going vegan. However the two based purely on input-output studies (US: 2011 and UK: Berners-Lee) are more trustworthy, and their figures are more consistent. It may be that the other approaches are overestimating the effect of going vegan because they aren’t considering the full embedded emissions from transporting vegan foods.

I’m also worried that the GWP/GTP issue mentioned in the methods article means that people are overestimating the effect of methane and so overestimating the effect of going veg(etari)an. To that end I’m happier sticking with numbers near the lower end of savings. Conveniently Berners-Lee’s numbers (25% and 40%) are the smallest, so we’ll stick with those.

Cooling the Little Planet

We saw above that the total US emissions (3.0 tons) and Berners-Lee-estimated UK emissions (3.2 tons) were very close. I’m pretty sure the US is actually higher than the UK; what’s may happening is that Berners-Lee is giving a higher figure than other sources because he is applying a 1.9x multiplier for the effect of high-altitude emissions from air transport. Given that, and for simplicity, I’m going to use the Berners-Lee figure of 3.2 tons per person per year for food for both countries.

Applying the 25% and 40% numbers above, we find going vegetarian would save 0.8 tons a year, reducing 2100 temperatures by 0.08 °C on the Little Planet. Going vegan would save 1.3 tons a year, reducing 2100 temperatures by 0.14 °C on the Little Planet.

Credit.

Finally let’s check a number I used at the start of the main post…

Cows

Cows burp, emitting methane, which causes a lot more warming than CO₂. How much? The UN estimates that 14.5% of (anthropogenic, i.e. human-caused) emissions come from livestock, and about 65% of those are from cattle. That suggests 9.4% of emissions are from cows.

But as discussed in the methods article, accounting for methane is much fiddler than accounting for CO₂. Let’s dig into that figure…

  • This estimates that livestock emitted around 119 megatons of methane in 2011.
  • 65% of that is about 80 megatons of methane from cattle.
  • Now we need to pick one of the GWP/GTP multipliers mentioned in the methods post, to convert from tons of methane into tons of CO₂-equivalent. Because methane breaks down over time, which multiplier we pick depends on what time horizon we are looking at.
  • Berners-Lee is looking over a 100-year window and so uses a multiplier of 28x. That us around 2.2 gigatons of CO₂-equivalent emissions from cattle per year. We (mankind) cause about 50 gigatons of emissions a year, so that works out at around 5% from cattle.
  • If instead you look over a 20 year window, the IPCC suggest a multiplier of 84x. That’s triple 28x, so you end up finding 15% of emissions are from cattle.

As mentioned in the methods article, there’s a case for using a GTP multiplier, which would give lower figures. But there’s also a case for including emissions due to land use change, which would give higher figures. For simplicity, I’ll stick with the 10% figure given by the UN; it’s not wrong, so much as very nuanced.

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