"Finding protein-rich foods that are good for the climate can be complex. Isabelle Gerretsen digs into the data to understand which food choices can help us curb emissions.
So, what would a protein-rich, low-carbon diet really look like? Just how bad for the climate are meat and dairy? How much more sustainable is it to only eat plant-based proteins, such as tofu, chickpeas and peas? Is it better to cut out cheese or chicken? Which animal-free alternatives have the lowest emissions' output?
Meat
"There's so much emphasis on beef that people often forget about other types of meat and their impacts," says Anne Bordier, director of sustainable diets at the World Resources Institute.
Beef from a dairy herd has a lower greenhouse gas footprint than meat from a beef herd because you get more food in return for all the resources you invest in the cow (feed, land, water and fertiliser), says Bordier. "[In addition to beef], these cows produce milk, which also tends to be used as feed [for other animals]... So it's more efficient overall," she says.
Meat from small, non-ruminant animals, such as chicken, turkey, rabbit and duck, has a much lower GHG footprint than beef and lamb. Chicken, for example, has a GHG footprint almost nine times lower than beef's – generating 5.7kg of CO2e per 100g of protein.
That's "quite low", says Sarah Bridle, professor of food, climate and society at the University of York in the UK. "It is really similar to farmed fish and eggs."
Pork's GHG footprint (7.6kg) is about 6.5 times lower than beef's and 1.4 times higher than poultry's (5.7kg).
Dairy
It is cheese, not chicken or pork, that generates the third-highest emissions in agriculture, after lamb and beef.
"There's this consensus that 'being vegetarian is great', but then we sort of forget that cheese is actually pretty carbon intensive," says Marbach, noting that this is due to cows' high methane output and the fact that they require "a lot of inputs for not much output".
The GHG footprint of cheese (10.8kg of CO2e per 100g of protein) is almost twice as high as chicken's and also higher than pork and eggs (4.2kg of CO2e).
The dietary emissions can vary greatly depending on the type of cheese you're eating. Harder cheeses, such as parmesan, are more carbon-intensive than soft cheeses because they are made with more milk, says Bridle. Soft cheeses contain more water – there's 50% more water in cottage cheese than in cheddar, for example, she says.
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Yoghurt, meanwhile, is surprisingly low-carbon, 2.7kg of CO2e per 100g of protein, as not much milk is needed to produce it (much less than in the case of cheese) and there are a number of by-products, such as cream and butter, which means the GHG footprint is distributed across numerous food items,says Marbach.
Plants
A diet rich in peas, pulses and nuts can be incredibly low-carbon. Producing 100g of protein from peas emits just 0.4kg of CO2e. This is almost 90 times less than getting the same amount of protein from beef. Other pulses, such as lentils, have a GHG footprint of 0.8kg of CO2e. Tofu production, meanwhile, generates 2.0kg of CO2e per 100g – these emissions are mostly linked to the clearance of land for soy production, says Bridle.
By crossbreeding wild chickpeas with cultivated varieties, US company Nucicer has created high-protein chickpea powder, which it says also lowers the CO2e of the crop. The powder can be used as gluten-free flour in pasta and baked goods.
By increasing the protein content, Nucicer is able to produce more protein per acre and reduce the overall amount of energy and water needed, says Kathyrn Cook, the company's chief executive and cofounder. "That really helps with the environmental impact of our protein sources," she says. Chickpeas are also highly water-efficient and fix nitrogen from the air into the soil, which is vital for plant growth, she adds.
Fish and seafood
When it comes to fish and seafood, it is more difficult to calculate the GHG footprint. It can vary greatly depending on the species and how it is caught.
But farmed bivalves, including mussels, oysters, scallops and clams, have a much lower GHG, about six times lower than farmed prawns and roughly 3.5 times lower than farmed fish,says Jessica Gephart, assistant professor in environmental science at the American University in Washington DC.
Farmed bivalves scored the best across the board. However, bivalves caught in the wild did not perform nearly as well when it came to greenhouse gas emissions – they emit five to 10 times more emissions as their farmed counterparts, says Gephart.
Lab-grown protein
But how do they compare to traditional meat and dairy, when it comes to emissions?
"Plant-based meat substitutes have smaller GHG footprints than most farmed meats and cell-based meat, but wild tuna (1.2kg), insects (0.9kg), tofu (1.2kg) and less processed pulses (0.4kg) and peas (0.3kg) have the lowest footprints of all protein-rich foods," says Santo.