A few days ago, a group at Oxford University published a report in Nature on the environmental impact of different diets in the UK. https://www.nature.com/articles/s43016-023-00795-w

Broadly speaking, this new report reaches very similar conclusions to a previous study, also undertaken by an Oxford team, published in 2018. In brief, if we are worried about the global environment, then we should eat less meat. Which is hardly surprising in general, but the specific results are striking. The new report focuses on real dietary data, whereas the previous one used assumptions about diet, and is more authoritative in this respect.

The classifications are a little confusing and hence open to misinterpretation. The researchers divided people in their survey into the following categories: vegan, vegetarians, fish-eaters, low meat-eaters (<50 grams/day, averaging 28), medium meat-eaters, and high meat-eaters (>100 grams/day, averaging 140). A typical portion of meat (e.g. chicken breast, portion of mince) is 100-150 grams in UK supermarkets. So high meat-eaters are in effect defined to be people who eat a normal portion of meat once a day on average and low meat-eaters are people who do so once every 4-5 days on average.

“Fish-eaters”, confusingly, eat about 40 grams/day of fish on average. In other words, they are people who have a normal portion of fish about once every 3 days. In the study, medium and high meat-eaters actually eat more fish than fish-eaters! Given that the study finds that fish-eaters have an environmental impact which is only a little larger than vegetarians, it would have been helpful to point this out more strongly than a passing comment in the body of the text. Eating fish every day, in place of meat every day, is not the answer to our climate problems.

On average, high meat-eaters have a carbon footprint roughly double that of low meat-eaters and 4 times that of vegans. Low meat-eaters, fish-eaters and vegetarians are bunched fairly close together. Vegetarians in the study eat more cheese than any other group, which also helps to explain why their carbon footprint is not too different from fish-eaters and low meat-eaters.

Despite the potentially misleading categorization, the study is a useful counter-weight to the large body of material that emphasizes reducing food waste. Of course it’s good to reduce waste, but – in the scheme of things – the research team stresses that what we choose to eat is a much bigger issue than how much of it we leave on the plate. The authors also dismiss the argument often put forward by the farming lobby that sequestration (e.g. through grasses) somehow makes up for the damage done by livestock farming. Along with the argument that British beef is somehow OK because it’s British: there may be some differences between sources, but these are overstated and in any event they’re taken into account in the results.

Setting things in context

There is a widespread tendency to think that climate is an energy problem; that agriculture only makes up about a quarter (or a third, according to the authors of this study) of total greenhouse gas emissions, so we should focus on fossil fuels. A problem with this line of thought is the use of averages: high meat-eaters, predominantly members of the world’s richest decile, have a disproportionately large impact. The authors calculate these individuals, averaging 140 grams/day of meat, each account for around 3.7 tonnes of CO2e per annum from their diet. That is more than 50% of the average per capita total emissions (all activities, including transport, heating, power, material goods, etc. as well as eating) for the global population. In North America, where studies have estimated meat consumption to be typically 250 grams/day, the diet-related per capita emissions exceed this global average for all activities.

(There is an argument that the dietary impact is even higher than this, because the land opportunity costs – around half of the world’s total habitable land area is used for livestock agriculture, one way or another – are not fully quantified.)

This statistical point – that the Developed World diet matters more than is generally reported – is similar in kind to, albeit less egregious than, an analogous argument made with respect to flying. Airline companies are very keen to tell us that aviation only accounts for a few percent of GHG emissions. Leaving the (difficult and generally ignored) issue of radiative forcing (RF) to one side, the key point is that most of the world’s population doesn’t fly. A frequent flyer might clock up >10 tCO2e per annum from flights alone, perhaps twice as much when RF is incorporated into the analysis.

I often have to remind myself about proportionate impacts. Recently, we installed an air-source heat pump in our 5-bedroom house. I estimate that – after the first year during which the ‘cost’ of the embedded emissions in the ASHP manufacture is recovered – it will save around 1-2 tCO2e per annum. That’s about the same saving that results when a single high meat-eater converts to a low meat-eater diet …