Humans have a taste for meat, which means that as countries develop economically a larger and larger fraction of their diet is meat. Unfortunately, it takes a lot of grain to feed a cow, grain that could be used to feed people who are not lucky enough to be on the income elevator. Growing and processing that grain takes energy, and hence increases the amount of carbon dioxide in the atmosphere, thus making global warming worse. Instead of feeding people, we then feed a cow, and give the cow to people. So if you eat a cow, you are depriving people of food and polar bears of habitat. (Not to mention aggravating the cow-fart problem.) The world's population is expected to stabilize in a few decades, but by then it will have doubled. It's hard to imagine that all of those people will be able to eat as we do.
If enough people stop eating meat, the problem will go away. The demand for meat will fall, and so will the price of grain. As poorer nations develop, they will perhaps not find the eating of meat so attractive. Being vegetarian to make all these good things happen, rather than for health reasons or to avoid killing animals, is called economic vegetarianism. Now, what are the odds of a lot of people becoming vegetarians? Pretty slim.
Nonetheless, every time I bite into a burger or a steak, I cringe at the thought of all the grain that went into the cow that I'm eating. So I have decided to become a 3/7 vegetarian. That is, I will eat meat four days a week. I have a simple system: I eat meat on Wednesdays and weekends, counting Friday as a weekend day. Monday, Tuesday, and Thursday I abstain, or, more positively, I eat yummy, yummy vegetables.
What about cheese and eggs? They're produced by animals. Don't worry! They're okay! For one thing, becoming a vegan is a lot harder that becoming a vegetarian, even fractionally. But it just seems reasonable to me that it takes less grain to produce 1000 kcal of cheese than 1000 kcal of meat because you get to use the same cow again; you don't have to build one from scratch.
Perhaps this will be the start of a movement. It's not hard, people! You don't have to eat tofu and TVP; you can maintain your unhealthy lifestyle with ice cream and pastry if you want. And if you do eat tofu on Monday, you have just 48 hours until meat is on your plate again.