The ethical argument is simple, and it's actually related to the health argument: most people would agree that needlessly harming other people or animals for your own pleasure is wrong. For humans, eating meat is unneccesary. Meat-eating is obviously harmful to animals, it wastes crops that could be used to feed people, and it harms the planet. Therefore, eating meat is simply wrong.
First, let's bust the biggest myth about vegetarianism, the notion that you have to eat meat or fish to stay healthy. In fact, this is not necessary:
"It is the position of the American Dietetic Association and Dietitians of Canada that appropriately planned vegetarian diets are healthful, nutritionally adequate and provide health benefits in the prevention and treatment of certain diseases"
(American Dietetic Association and Dietitians of Canada, "Vegetarian Diets", June 2003 [emphasis added]; consult with your physician before changing your diet -- see below).
Therefore, for most people, meat-eating a luxury, not a necessity. Some luxuries, like getting a nice massage, are virtually pollution-free, and they harm nobody. But meat-eating is a luxury that definitely harms animals and the planet. Pound-for-pound, calorie-for-calorie, and nutrient-for-nutrient, a vegetarian diet results in less waste and pollution than a meat-based one. For example:
"Animal protein production requires more than eight times as much fossil-fuel energy than production of plant protein while yielding animal protein that is only 1.4 times more nutritious for humans than the comparable amount of plant protein, according to the Cornell ecologist's analysis."
(Cornell Science News, 1997, "U.S. could feed 800 million people with grain that livestock eat, Cornell ecologist advises animal scientists. Future water and energy shortages predicted to change face of American agriculture" [emphasis added].)
In fact, getting enough protein on a vegetarian diet is easy (see vegetarian nutrition and cooking links below).
GHG emissions from fossil fuels are just part of the harm. Meat production also wastes enormous quantities of water:
"According to the USDA, growing the crops necessary to feed farmed animals requires nearly half of the United States' water supply and 80% of its agricultural land" (Wikipedia).
Meat production endangers drinking water quality, as well. Remember the Walkerton Tragedy in Ontario, Canada? The official report said that
“The primary, if not the only, source of the [E. coli] contamination was manure that had been spread on a farm near Well 5. The owner of this farm followed proper practices and should not be faulted” (CBC).But would the tragedy have happened without any animal farming? Meat production can even make it lethal to eat vegetables that are grown nearby (remember the California Spinach tragedy in 2006?).
A needless luxury that harms the planet, causes the death and suffering of millions of animals every year, wastes land and crops that could feed people, and may even harm your own health simply must be unethical.
If you'd been following vegetarian literature, you might be wondering why I haven't even mentioned the livestock-methane connection. Others may wonder why I've ignored the religious angle. I think that these arguments are either conditional or unnecessary. Livestock farming produces an estimated 37% of human-released methane (LEAD [PDF p. 6 of 19, printout p. 271]), a very potent greenhouse gas (Wikipedia). But changing animal feed may reduce or eliminate this effect in the future (e.g. Reuters/USA Today via Green Car Congress). This might reduce the meat industry's GHG emissions, but even completely "methane-free" meat would not change any of the other points above, which I think are sufficient on their own. Many world religions happen to preach that vegetarianism is either mandatory or desirable (Wikipedia). But even if you're a "radical atheist" like Douglas Adams (interview), I think that you would find the case for vegetarianism to be compelling on its own terms.
In case you're wondering, we became lacto-ovo vegetarians in 1989-90, and we have gradually moved to a mostly vegan diet (Wikipedia). Our health is good, and the animal-related ethical considerations are the same as ever. Our daughter is growing up healthy, strong, bright, caring -- and vegetarian. Meanwhile, the number of environmental reasons supporting our choice keeps increasing.
Here's an idea: read up on vegetarian nutrition and cooking. Consult your physician -- in fact, have a full check-up. Discuss your plans with your physician. Make sure that you're getting enough physical activity. Now, try going without meat for a day. Then try a week. Then a month. Then three months. At any time, consult your physician if you have any health concerns or questions. Have regular check-ups. Look at your bank account. Look at yourself in the mirror.
If you still decide to go back to eating meat after a year, I'd love to know why. If the objective reason is your own health, then you would have something like a "necessity defence" (Merriam Webster/FindLaw). I suspect that this would be fairly rare. Otherwise, what would be your excuse?