I'm not sure cutting back on animal products will help. What will help is the way animals are raised - stop CAFO (concentrated animal feeding operations, i.e. feedlots). And the way crops are raised. Right now field crops (think corn, wheat, soybeans) are basically mining the soil. There are areas where 6-10" of topsoil have been lost, and that topsoil was full of carbon. Plowing puts air deeper in the soil and increases oxidation, which releases carbon and methane into the air. Intensive field crops also destroy masses of wildlife, and the trend to bigger fields has intensified that loss. Bigger fields also mean more use of insecticides, since beneficial insects (i.e insects that eat insect pests) lose habitat.
A shift to intensive grazing grass-fed beef (mob grazing, which actually increases soil carbon) and pastured pigs and poultry, less mono-culture of annuals and more poly-culture of perennials, would all go a long way to diminish the effects of contemporary agriculture. Mob grazing mimics what happened with the bison, and happens with the big herds in Africa - plant material gets trampled into the ground and becomes humus, which is sequestered carbon. Good rotational grazing also messes up parasite life cycles, which means a lot less de-worming (and a lot of livestock parasites have developed immunity to a lot of de-wormers). Grain-fed cattle also have a more acidic rumen, produce more methane, and the combination of that with antibiotics in feed leads to E. coli from them that are more dangerous for us, since the newer E. coli strains are acid resistant (our stomach acidity protects us from a lot of pathogens).
This may make food more expensive, but food prices are abnormally low (especially in the US). Basically the push for low food prices has driven farmers to exploit their resources instead of husbanding them.
So much this. I've seen any number of posts and blogs about people who were vegetarians but began working in sustainable farms, or started a homestead, or whatever, and realized that they couldn't do it and still be vegetarians/vegans. "There's a reason our image of a farm is a polyculture with many different animals." (I believe that was Sharon Astyk? A farmer and eco-advocate, among other things.) Animals do a
lot around the farm that has now been replaced with fertilizers, insecticides, and gas-powered equipment. Oil is the only reason we
can have monoculture farming.
I don't actually want to discourage anyone from becoming vegan. Do it, it's not going to hurt anything. If nothing else, I think many people could easily reduce their intake of meat. I don't think meat is unhealthy at all--but we raise unhealthy animals and, surprise! they're not super healthy for us to eat. Having a cow that's sick and half-dead when it gets to slaughter because it's been force fed grains rather than grass...it should be obvious that that's not the ticket to good health. So eat less, support better practices and farms/farmers when you do buy meat, eggs, and dairy.
A lot of people focus on meat as being the horrible part of agriculture because it's easy. But giant fields of nothing but corn and soy are terrible for the environment too. Monocultures in general are
awful. I've done a lot of reading and research into farming practices and the issues surrounding farms. It's not an issue of meat vs. grains, it's that the entire system is so ridiculously stupid from start to finish. No, GMOs won't save us all from starvation. They have massive problems of their own, like built in genes so that you can't save seeds from them. In a world of climate change, when you can't rely on global systems anymore, how do you expect to get seeds?
Start gardening. Grow what food you can, because if nothing else those skills are going to be in high demand in a future world where people are starving to death.
And maybe don't eat fish, since not only is much of it laden with mercury but our oceans are, if possible, more fucked over than the land, and they're just as crucial to human survival. Plus, something like 40% of the plastic in the oceans is related to fishing. If you care about ocean cleanup, don't eat fish.