The diet-related carbon footprint of the average omnivore is about 4.1 metric tons of carbon per year. The average vegan has a diet-related carbon footprint of 0.9 metric tons. If most of that vegan food is organic, the carbon footprint goes down to 0.6 metric tons. Changing to a mostly organic vegan diet saves about 13% of carbon emissions.
Source: The Nature Conservancy