"...If everyone became vegan and so ate only fruit and vegetables, then the reduction in greenhouse emissions for the whole of food consumption would be a mere 7%. The widespread adoption of vegetarianism would have even less impact, while organic food production actually leads to a net increase in greenhouse gas emissions. Those are the conclusions of a research paper published in the journal Progress in Industrial Ecology..." (source)
Can you imagine the heartbreak this study is having on the high and lofty, judgmental environmentalists?!
The moral of the story? Man, in his infinite smallness and insignificance, has no cogent impact on the ebb and flow of the Earth's climate. But, don't throw your veggie pita wrappers on the ground, because what the Earth looks and smells like...YOU DO make a difference.