When you think about the growth of human population over the last century or so, it is all too easy to imagine it merely as an increase in the number of humans. But as we multiply, so do all the things associated with us, including our livestock. At present, there are about 1.5 billion cattle and domestic buffalo and about 1.7 billion sheep and goats. With pigs and poultry, they form a critical part of our enormous biological footprint upon this planet.

Just how enormous was not really apparent until the publication of a new report, called “Livestock’s Long Shadow,” by the Food and Agriculture Organization of the United Nations.

Consider these numbers. Global livestock grazing and feed production use “30 percent of the land surface of the planet.” Livestock  which consume more food than they yield  also compete directly with humans for water. And the drive to expand grazing land destroys more biologically sensitive terrain, rain forests especially, than anything else.

But what is even more striking, and alarming, is that livestock are responsible for about 18 percent of the global warming effect, more than transportation’s contribution. The culprits are methane  the natural result of bovine digestion  and the nitrogen emitted by manure. Deforestation of grazing land adds to the effect.