veganism and environmental justice

Lets all think back to Mr. Keener's AP biology class. Any given ecosystem can support significantly more herbavores than carnivores. That's because 90% of the energy is lost at each tropic level. The majority of the food that an animal eats is used to keep it warm and kicking. Only 10% goes to making flesh. Meaning if you were to eat the grain, rather than eating the cow fed by the grain, you would only need to grow 1/10th of the grain, which would require 1/10th the amount of land. Furthermore, the processing of animal products is extremely energy intensive, requiring refrigeration, rapid transportation, and a shit ton of water to clean slaughter houses.

Livestock are responsible for 18 percent of greenhouse-gas emissions as measured in carbon dioxide equivalent, reports the FAO. This includes 9 percent of all CO2 emissions, 37 percent of methane, and 65 percent of nitrous oxide. Altogether, that's more than the emissions caused by transportation.
csmonitor

That statistic is a little misleading, because many more people eat meat in the world than drive cars (so you're not off the hook for driving everywhere). In terms of cutting down on your GHGs researches concluded that dietary changes could make more difference than trading in a standard sedan for a more efficient hybrid car, which reduces annual CO2 emissions by roughly one ton a year.
(from Diet, Energy & Climate Change by Gidon Eshel and Pamela Martin)

So when the U.N. added it all up, what they found is that eating chickens, pigs, and other animals contributes to "problems of land degradation, climate change and air pollution, water shortage and water pollution, and loss of biodiversity," and that meat-eating is "one of the ... most significant contributors to the most serious environmental problems, at every scale from local to global.

And on the issue of global warming, the issue the New York Times deems critical enough to demand that we "change [our] lifestyles" and for which Al Gore and the IPCC received the Nobel peace prize, the United Nations' scientists conclude that eating animals causes 40 percent more global warming than all planes, cars, trucks, and other forms of transport combined, which is why the Live Earth Global Warming Survival Handbook says that "refusing meat" is "the single most effective thing you can do to reduce your carbon footprint" [emphasis in original].

There is a lot of important attention paid to population, and that's a critical issue too, but if we're consuming 11 times as much as people in China and 32 times as much as people in the third world, then it's not just about population; it's also about consumption.
www.alternet.org

From a human rights perspective, it is important to keep in mind that forest loss and land reposition contribute to sometimes violent removal of indigenous populations from their land. Cattle grazing and soy production (something like 80% of which is used to feed livestock) are two of the biggest factors contributing to deforestation in the Amazon. It is also worth pointing out that something like 60% of soy production in the Amazon is funded by three American agrigiants (Cargill, ADM, and I forget the third).

Here's an article about indigenous murders & suicides linked to land shortage and agriculture in Brazil. The main crop they're planting is sugarcane to make Ethanol (Brazil gets most of their fuel from ethanol) but the secondary crop is soy.
ipsnews.net

Livestock now use 30 per cent of the earth's entire land surface, mostly permanent pasture but also including 33 per cent of the global arable land used to producing feed for livestock, the report notes. As forests are cleared to create new pastures, it is a major driver of deforestation, especially in Latin America where, for example, some 70 per cent of former forests in the Amazon have been turned over to grazing.

At the same time herds cause wide-scale land degradation, with about 20 per cent of pastures considered degraded through overgrazing, compaction and erosion. This figure is even higher in the drylands where inappropriate policies and inadequate livestock management contribute to advancing desertification.

The livestock business is among the most damaging sectors to the earth's increasingly scarce water resources, contributing among other things to water pollution from animal wastes, antibiotics and hormones, chemicals from tanneries, fertilizers and the pesticides used to spray feed crops.
www.un.org