The last few weeks have seen George Monbiot write passionately in support of measures to improve biodiversity. To the surprise of many, he also gave us a reasoned defence of eating meat. Are these two themes consistent? Can biodiversity be maintained if world food production includes a significant amount of farmed meat? A new paper in the Proceedings of the National Academy of Sciences suggests strongly that George cannot both maintain an omnivorous diet and defend biodiversity. (1) Now, and far more so in the future, livestock farming imposes stresses on global ecosystems that are incompatible with maintaining species diversity. Pelletier and Tyedmers’ article demonstrates that livestock farming pushes human society over important three global ecological boundaries: biomass use, the nitrogen cycle and climate change. In each case, failure to remain within the sustainable limits creates unmanageable pressure on biodiversity. Monbiot’s defence of meat eating is based on two assertions. First, he says that Simon Fairlie’s book Meat: a Benign Extravagance shows convincingly that livestock farming is only responsible for about 10% of world emissions. Second, a move away from the factory farming of ruminants (predominantly cattle) towards a system that fed pigs and chickens with our agricultural and domestic food waste would significantly reduce the climate change impact of livestock. It would also decrease the diversion of cereals such as maize to animal feedlots and away from human use. Both of these assertions may be true. But they don’t tell the whole story.

First, climate change. We know that temperature increases and changes in rainfall patterns are already seriously affecting the survival ability of many types of plant, animal and insect life. The global target of maintaining temperature increases at less than 2 degrees C above the pre-industrial level will require us to cut annual emissions to no more than about one tonne of CO2 per person by 2050. Pelletier and Tyedmers’ article says that emissions from livestock today are equivalent to about 52% of this level today, rising to about 70% by 2050 as the dietary habits of the rich world are adopted by today’s industrialising nations. If we are to meet our targets for emissions, large scale livestock production, particularly of methane-producing cows and sheep, is impossible. Switching to pork and chicken only makes limited difference to emissions levels.

The impact of livestock production on the possibility of staying within the other important boundaries is even worse. At present humankind uses about one quarter of the total net production of biomass across the world each year and this figure is rising as the planet’s population increases. The food we take from the world’s surface is perhaps one half of this figure, either directly or in the form of biomass eaten by farmed animals. Livestock farming accounts for about 58% of total food-related biomass use. Increasing numbers of farmed animals will require more food and the new paper estimates that livestock alone will use up 88% of the total sustainable amount of global biomass that can be appropriated by humankind for its own purposes. The increasing need to devote land to producing food for animals (for example, soybeans in what was the Amazon rainforest) necessarily implies a reduction in the space and plant diversity available to sustain threatened species.

Lastly, the paper looks at the impact of livestock farming on the amount of reactive nitrogen on the planet’s surface. Nitrogen atoms in the air are bonded with another atom of nitrogen to form a stable molecule. Humankind has found ways of breaking the bond and turning nitrogen atoms into important components of artificial fertilisers. ‘Half the synthetic fertiliser ever used on Earth has been applied in just the last 15-20 years’ say Pelletier and Tyedmers. Adding fertilisers to crops increases the amount of food produced but at a cost to the many species of plant and animal life that would lived alongside the crop. The improvements in food productivity, largely driven by the need to feed huge numbers of livestock and increasing population, have resulted in drastic ‘ecosystem simplification and biodiversity loss’. If George Monbiot wants space for threatened species to live, he cannot also allow it to be used for heavily fertilised monocultures to feed the world’s farm animals.

Argument rages over what the sustainable amount of unbonded nitrogen added to the soil can be. The paper’s authors suggest tentatively that today’s use of nitrogen is about three times the acceptable future level. This will affect crop yields adversely. Today, over half the world’s corn production is fed to animals. If we continue with livestock farming at its current levels, this means that reducing nitrogen fertiliser use will reduce the amount of grain left for human use. So it we are to give priority to maintaining biodiversity and feeding the extra three billion people by 2050, livestock farming has to reduce dramatically. Today’s ultra-intensive production techniques for food, whether they are feedlot production of cattle or highly fertilised grain production, are unlikely to be possible in the not-far-distant future

The unavoidable conclusion is that meat eating is going to have become an unusual luxury for all of us. Or it will have to be grown in a test tube. Animal sources of protein will have to be replaced by plants such as soybeans which have ecological impacts of up to two orders of magnitude lower than cattle farming.

One three separate grounds the new PNAS paper therefore says that livestock farming is difficult to accommodate without increasing the rate of biodiversity loss. Nevertheless, let’s be kind and allow George Monbiot a little meat and dairy. We’ll accept his view that pigs are better than cattle and get him a sow for his back garden, happily eating the kitchen waste. George consumes about 2,000 calories of food energy a day and we’ll assume that the unconsumed food in his household allows the sow a daily 1,000 calories that would otherwise have been thrown away. (In order to get a fat pig, he will therefore have to share with a neighbour).

A pig kept outdoors might be able to turn those 1,000 calories of food waste into 150 or 200 calories of meat. Don’t believe the figures you see suggesting higher conversion efficiencies – they assume that the pig is fed the highest quality maize and is not free to take much exercise that would burn off some of the energy value of the waste food. Even at the maximum 200 calories meat production a day - a couple of bacon rashers – George will only get about 10% of his energy intake from animal products, compared to the European average of about 30%. The unfortunate truth is that a little bit of meat and dairy may be compatible with keeping within global ecological constraints but nothing like today’s levels of meat and dairy consumption.

(1) Nathan Pelletier and Peter Tyedmers, Forecasting potential environmental costs of livestock production 2000-2050, PNAS Early Edition, October 2010