Personally, I believe that reducing our overall greenhouse gas emissions is extremely important. It's safe to say that by throwing massive amounts of carbon into the atmosphere, we are suffering some serious side effects. Pollution, global warming, ozone depletion and more extreme weather patterns can all be linked to greenhouse gas emmissions.
As far as the vegetarian diet, I don't believe that it is inherently more ethical. Eating meat is natural - as seen everywhere in nature. But I do believe the treatment of animals, especially with farming becoming more industrialized, is becoming an issue. I think that transitioning back to smaller, more eco-friendly, and more animal-friendly farms is preferable.
I would be happy to help you if you needed a longer essay.