Monbiot spoke to Kim Hill about why the average New Zealander's diet is particularly bad for the planet.
"I've come to see land use as the most important of all environmental metrics," Monbiot says.
"Because every hectare of land we use for our extractive industries is a hectare of land that can't be used for wild ecosystems, such as forests or wetlands or savannahs or natural grasslands, on which the great majority of the world's species depend."
While a lot of emphasis has been placed on the impact of growing urban sprawl, Monbiot says we should be more concerned about the amount of land required for farming.
"The entire urban area occupied by humanity is 1 per cent of the planet's land," he says.
"Farming by contrast occupies 38 per cent roughly of the planet's land, and of that, 12 per cent is arable farming, and of that, almost half is producing crops for animals, the rest is producing crops for humans.
"So what about the rest of it? Well 26 per cent of the planet's land, by far the greatest use of land that humans have ever inflicted on the planet is used for pasture-fed meat production.
"And yet from pasture feeding alone, from animals which are grazed entirely on pasture, we get just 1 per cent of our protein, so this is a phenomenally profligate and wasteful way of producing our food and it carries a vast ecological opportunity cost."
Monbiot says that if we switched to a plant-based diet, our total land use could be reduced by about three-quarters. He points to metrics that show soy crops require only 2sq m of land for every 100g of protein - compared to 163sq m for beef and 185sq m for lamb.
"It's because of those [metrics] that the nation with the world's greatest hunger for land is New Zealand."
Moving to high-yield crops like soy could help us see a better future, Monbiot says, but in exploring the options in his book, he found examples were few and far between. And while New Zealand does have some very high-yield farming, it is outweighed by our "extraordinary degree of agricultural sprawl".
And it seems our love for free-range beef and lamb could be devastating for the planet.
"If everybody ate the average New Zealanders diet, we'd need another planet almost the size of Earth to feed us. And that's because of the very high proportion in New Zealanders' diets of free-range lamb and beef.
"Because we haven't got an extra planet, we cannot arrogate to ourselves the right to eat that diet. It's simply wrong to do so, it is unethical to eat that diet."