"Each time you have a plant-based lunch like a PB&J you'll reduce your carbon footprint by the equivalent of 2.5 pounds of carbon dioxide emissions over an average animal-based lunch like a hamburger, a tuna sandwich, grilled cheese, or chicken nuggets. For dinner you save 2.8 pounds and for breakfast 2.0 pounds of emissions.
Those 2.5 pounds of emissions at lunch are about forty percent of the greenhouse gas emissions you'd save driving around for the day in a hybrid instead of a standard sedan.
If you have a PB&J instead of a red-meat lunch like a ham sandwich or a hamburger, you shrink your carbon footprint by almost 3.5 pounds of greenhouse gas emissions."
As they note, you'll also conserve 133 gallons of water, and save 24 square feet of land from deforestation. In addition, you won't be contributing to the moral and environmental nightmare that is factory farming, or contributing needlessly to the world's food shortage.
And that's just lunch.
Reducing your consumption of meat doesn't have to involve becoming a complete vegetarian, any more than reducing your consumption of fuel has to mean selling your car. Every little bit helps.