Clearly irritated that his argument in an interview in the Times had been boiled down to a 'go veggie to save the planet' headline, Nicholas Stern has issued a clarifying statement:

I think that once people understand the great risks that climate change poses, they will naturally want to choose products and services that cause little or no emissions of greenhouse gases, which means 'low-carbon consumption'. This will apply across the board, including electricity, heating, transport and food. A diet that relies heavily on meat production results in higher emissions than a typical vegetarian diet. Different individuals will make different choices. However, the debate about climate change should not be dumbed down to a single slogan, such as 'give up meat to save the planet'.

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Without representing his position as advocacy for veganism, Stern's point on food is correct: the average western diet makes a very substantial contribution to climate change. Rough calculations suggest that food production is responsible for between 15% and 20% of total UK greenhouse gas emissions. Food miles are important and the electricity consumption of a big supermarket might surprise you. But the really serious issue is the intensive farming of livestock, particularly cows and sheep, which generate as much as a half of the total emissions. One study from 2007 suggested that the CO2-equivalent emissions of global warming gases from beef production could be as much as 50 times the weight of the meat itself.

There are three elements to the problem: farmed livestock eat large quantities of grain, they belch methane and they use land that might otherwise be forest. To get a kilo of beef, the animal typically eats about eight kilos of grain. That corn or wheat took energy to grow, required a lot of artificial fertiliser and then needed to be processed into a cake for cattle. Some of the fertiliser applied to fields breaks down into nitrous oxide, a far more powerful global warming gas than carbon dioxide. Cows and sheep emit methane as bacteria in their digestive tracts digest the cellulose in plants. And, worldwide, the gradual increase in the consumption of meat creates pressure to cut down forests to create new pastureland and cropland for grains to help feed the livestock.

As countries get more prosperous, their populations tend to eat more meat. So unless we do something, the impacts of livestock farming are probably going to get worse. And, by the way, it isn't just meat. The same arguments apply, albeit with less force, to dairy products as well. The best diet from a climate point of view is probably a mixture of dried plant-based foods, such as beans and nuts, with large quantities of locally grown seasonal vegetables and fruits. It may also be best for our health and it would certainly save us money. In fact, the simplest and cheapest way of largely meeting your commitment to the 10:10 campaign would probably be to eat vegan foods for half the week. To many people this will seem a less demanding challenge than not flying for a summer holiday.

Nevertheless, the reaction to Lord Stern's statement has been unpleasantly vicious. People have seen his views as another illustration of how "climate change" will be used as an excuse for the elite to limit the choices of ordinary people. We are already being told to drive less, not to fly and to buy dim lightbulbs. Stern's comments suggest a future campaign to reduce our hamburger consumption.

Unfortunately, the many stresses on the world's ecosystems mean that either we eat less meat or change our farming and food manufacturing methods. The greenhouse gas emissions from agriculture, food manufacturing, transport and retailing are now about two tonnes a head, about as much as we can afford to emit from all our activities in 2050. Either we decide to eat a very different diet, as Stern suggests, or we try to change agriculture so that it becomes a helpful part of our drive to reduce emissions. Instead of depleting the soil and abusing animals in pursuit of cheap meat, we could put our weight behind schemes for using agricultural soils to sequester CO2. A new campaign, called Climate Friendly Food, may offer us a way of continuing to eat some meat and looking after the global atmosphere at the same time.

This article was originally published in the Guardian on Tuesday 27 October 2009.