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By Roger Highfield on

Can a change in diet counter climate change?

Roger Highfield, Science Director, talks to Atul Jain, lead author of a study that shows the huge impact of the global food system on climate change.

To meet the increasing food demand of the Earth’s burgeoning human population, the United Nations projects that food production from plants and animals will need to increase 70% by 2050, compared with 2009.

However, a recent study in the journal Nature Food, led by Atul Jain and colleagues at the University of Illinois Urbana-Champaign, shows that food production is a major driver of climate change: global production of food is responsible for just over one-third of all planet-heating gases linked to human activities, releasing the equivalent of 17.3 billion metric tons of carbon dioxide every year.

Although carbon dioxide is the most significant greenhouse gas, the researchers also included emissions of two other gases, methane and nitrous oxide, which are 34 and 298 times more powerful at trapping heat in the atmosphere than carbon dioxide, respectively.

Rearing animals for meat – at around 57 percent of the carbon dioxide equivalent of the food sector – caused twice the climate heating pollution of producing plant-based foods, which is around 29 percent.

The remaining 14% of emissions from farming that are neither connected with food nor animal feed are for other purposes, such as bioenergy crops, cotton, rubber, and tobacco.

The new estimate of the climate impact of meat is much higher than previous estimates. For example, the UN’s Food and Agricultural Organization estimated that are only about 14% of all emissions come from meat and dairy production.

‘I’m a vegetarian, and have been since childhood, and that was one of the motivations for me to carry out this research,’ said Prof Jain, an atmospheric sciences professor. ‘I think people will be surprised about how much their food contributes to the greenhouse effect’.

The research could help identify the plant and animal based emissions from four food sectors that contribute most to emissions of three major greenhouse gases (carbon dioxide, methane and nitrous oxide), and allow policymakers to take action to reduce emissions from the top-emitting food commodities at different locations.

However, Prof Jain added that he believed change must come through personal choice and full information, with the latter having to take account of other challenges, not least ensuring that we all have an adequate well-balanced diet with sufficient macro and micronutrients, impact on land use, and challenges already faced by farmers in low and middle income countries.

Prof Pete Smith from the University of Aberdeen, a co-author of the study, said: ‘It is in countries that vastly over consume livestock products, like most industrialised countries and some middle income countries in South America and South and Southeast Asia, that the greatest shifts in diets could make a difference. This would not only help the climate but would be very beneficial for population health.’

Their study was the first to account for net greenhouse gas emissions from all major sub-sectors related to food production and consumption, taking account of geography too, making it much more granular than earlier research.

‘The unique aspect of our study is that we have developed a data-modelling framework, which calculates spatially explicit production – and consumption – based greenhouse gas emissions worldwide from the individual plant and animal based human foods from four major sectors,’ said Prof Jain. ‘This can allow individuals to calculate their own carbon footprint from food consumption.’

The framework includes a wide range of farming practices, agricultural products – totalling 171 crops and 16 animal products – and data from around 200 countries.

The framework captures emissions from four major sectors:

  • land-use changes, such as cutting down forests to make way for croplands and grazing lands;
  • farm management activities, such as ploughing soils;
  • raising livestock;
  • activities beyond the farm, such as mining, manufacturing and transportation of fertilizers.

‘Suppose you have a farm that grows various crops,’ he said. ‘Our framework can calculate emissions not only from crops that are grown for human food, animal feed but also for other purposes, such as biofuel.’

The new study showed that the countries with leading greenhouse gas emissions from animal-based foods are China at 8%, Brazil at 6%, the U.S. at 5% and India at 4%. Meanwhile, the countries with leading greenhouse gas emissions from plant-based foods are China at 7%, India at 4% and Indonesia at 2%.

In general, meat is linked with much higher emissions. To produce 1 kg of wheat, the equivalent of 2.5 kg of carbon dioxide emissions are released. In contrast, beef and pork have emission intensities of around 69 kg carbon dioxide equivalent and around 10.5 kg carbon dioxide equivalent.

The type of meat produced does matter, and the study ranks their contribution to the global total of emissions. Beef contributes more than double the next on the list, cow’s milk, then comes pork, chicken, lamb, buffalo meat, chicken eggs, buffalo milk, goat, and horse meat.

However, there is a subtlety in that if you compare emissions by weight of the animal, ruminant animals such as cattle, sheep, and goats (which produce the greenhouse gas methane when they digest food) have a much larger climate impact than non-ruminants like pigs and chickens.

When ranked by emissions per kilogram weight of animals, beef has the highest impact, followed by lamb.

‘At the Science Museum Group we made the conscious decision to remove beef and lamb from our menus, in conjunction with our catering partners. This is in support of our mission to reduce the carbon emissions in our supply chain,’ commented Hayley Marks, Sustainability Manager.

In the case of plant-based foods, rice is by far the greatest emitter of greenhouse gases, exceeding the global emissions of milk, then comes wheat, which is a little less than chicken, followed by sugarcane, maize, cassava, potato, soybean, bean, rape and mustard, and sunflower.

When it comes to novel foods, such as insect proteins, Prof Jain said that for more detailed analysis there must be more knowledge of efficient methods to farm insects, process them and transport the resulting protein at scale; ‘We don’t have a well-established system to cultivate insects, as we do with rice, wheat and so on.’

For the study, Prof Jain was assisted by his graduate students Prateek Sharma, Tzu-Shun Lin, and Shijie Shu, and postdoctoral researcher Xiaoming Xu. Researchers from the Laboratoire des Sciences du Climat et de l’Environnement, Yvette, France; the Food and Agriculture Organization of the United Nations; the University of Aberdeen, Scotland; and PlantPure Communities, Mebane, North Carolina, also contributed.

Elsewhere on the blog, find out how a global plant-based diet and lab-grown meat could both curb greenhouse gas emissions.

Discover more about the science of climate change in our short animated series.