'The average meat eater in the US produces about 1.5 tonnes of carbon dioxide more than a vegetarian every year. That's because animals are hungry and the grain they eat takes energy, usually fossil fuels, to produce,' he says.

The world's fertiliser industry uses natural gas as a basic ingredient and therefore contributes to global warming when it uses fossil fuel to manufacture the extra fertilisers needed to ensure cattle and other animals have sufficient food. For example, it requires 2.2 calories of fossil-fuel energy to produce a single calorie of plant protein, according to researchers at Cornell University in Ithaca, New York. In turn, it takes even more plant protein to make animal protein. It requires four calories of plant protein to make one of chicken protein, while the ratio for pork is 17:1; for lamb, 50:1; and for beef, a staggering 54:1. 'That is a lot of energy and a lot of grain diverted,' adds Patel.

In fact, it is a massive amount of energy, the journal Physics World has noted. 'The animals we eat emit 21 per cent of all the carbon dioxide that can be attributed to human activity,' it states.

Geophysicists Gidon Eshel and Pamela Martin from the University of Chicago have even calculated that changing eating habits to become a vegetarian does more to fight global warming than switching from a gas-guzzling SUV to a fuel-efficient hybrid car, such is the amount of Co2 generated in the production of beef, pork or lamb.