Environmental
(Foreword: the site author maintains that veganism is neither about health nor the environment (see this video for an explanation why)—however, the fact is that these arguments about health and the environment do come up, and as such it is important that vegans are armed with the knowledge to answer them.)
1. resource usage
Given that the amount of animals massacred each year in these industries (150 billion) far outweighs the number of humans currently on earth, animal agriculture is by far the least sustainable entity on the planet when it comes to resource usage. Indeed, the USA alone could comfortably feed 800 million people with the grain it sets aside to chickens, pigs, cows, turkeys, sheep, and so on.
With an ever-growing human population (set to reach nearly 10 billion by 2050), and thus a rising demand for animal products with more human mouths to feed (so more animals being bred, requiring even more crops and water), the likelihood is that we will head towards a global resource war if immediate action is not taken. So if you ever wondered what WW3 would be about if it were to happen: it'd probably be over hamburgers and ice cream.
2. Land usage
Currently, around 30% of the world's total ice-free surface is used for animal agriculture (so this includes grazing land for livestock, and land used to grow crops fed to livestock), meaning less space available as human population continues to grow.
Veganism, however, minimises land usage, as far fewer resources are needed to feed you as a vegan. Going vegan cuts out the middle moo, so to speak, as there is no one being being housed on any land, requiring any food, in order to feed you. Rather, the food goes directly from the soil, to your plate, without anyone (i.e. a farmed animal) eating anything in-between.
3. greenhouse gases
Emissions from farm animal excrement is a leading factor in global warming. One study by environmental experts Robert Goodland and Jeff Anhang showed that livestock and their byproducts actually account for at least 32.6 billion tons of carbon dioxide per year, or 51% of annual worldwide greenhouse gas emissions—that's more than the entire transportation sector. By boycotting animal agriculture, you massively lower your carbon footprint—even more than you would do by avoiding driving and flying.
4. soil erosion
Livestock production accounts for 55% of soil erosion from agricultural land in the US each year (see page 162 of 'Livestock's Long Shadow').
5. ocean dead zones
Humans are eating seafood at such a huge rate that by 2050, the oceans could be devoid of fish. 'Ocean dead zones', as they are called, are being seen in an alarming number of areas already across the globe.
6. deforestation
Perhaps one of the most alarming statistics about animal agriculture's cancerous impact on our planet is its accountability for global deforestation. Between 1 and 2 acres of rainforest is destroyed every second worldwide to clear space for animal agricultural purposes, e.g. grazing land or space for feed, with 80% of total Amazon deforestation attributed to cattle ranching.
Rainforests are vital to the earth's ecosystem, breathing oxygen into the planet and harbouring around 50% of the earth's plants and animals. The Amazon alone produces around 20% of the earth's oxygen.
So desperate is Big Ag to cover up its secrets that it is destroying our earth through deforestation, that activists have even been assassinated trying to protest it.
Many myths go around that vegans cause more rainforest destruction than non-vegans due to buying soy products (e.g. tofu, soy milk, and mock meats), but around 80% of the world's soy is set aside for animal feed—so the best way to reduce your global soy consumption is actually to go vegan!
7. species extinction
Vegans are often asked why it makes sense to go vegan if farmed animals would end up going extinct if they weren't being bred any more (as if sparing billions of innocent creatures from a life of exploitation is a bad thing), but ironically, if one cares about species extinction, the best thing one can do is to go vegan, given that animal agriculture is the leading cause of species extinction worldwide, due to ocean dead zones, habitat destruction, hunting wild animals to protect livestock from predation (e.g. foxes, lions, bears), and deforestation.
In fact, UK animal agriculture alone has already been linked to the extinction of 33 species at home and abroad. So if you take into account the problem that every country causes worldwide, the overall figure would be staggering.