Was looking into this because of that recent Paul McCartney article that was shared, and wanted to share what I found:

Here’s a good breakdown of differences between vegan & vegetarian diets in terms of climate impact: https://www.nature.com/articles/s43016-023-00795-w

Emissions:

• Vegan:............. CO2: 2.16, CH4: 04.39, N2O: 0.71
• Vegetarian:........ CO2: 3.33, CH4: 20.21, N2O: 0.98
• medium meat-eaters: CO2: 5.34, CH4: 40.88, N20: 1.73
• high meat-eaters:.. CO2: 7.28, CH4: 65.40, N2O: 2.62

So vegans have 30% of the emissions as high-meat-eaters, and the differences between vegans and vegetarians are significant with regards to their emissions, particularly methane emissions due to the significantly higher consumption of cheese by vegetarians.

(EDIT: it has been suggested it’s worth clarifying that vegan diet having 30% CO2 emissions means that there was a 70% reduction in CO2 emissions, and that methane emissions were reduced by 93% going from high-meat to vegan.)

Vegetarians ate significantly more cheese (30 g / day) than even meat-eaters (19 g / day), despite eating less dairy overall otherwise.

Also should be noted that there is a big gap between biodiversity impact between vegans and vegetarians, with vegetarian diets causing nearly double the number of species extinctions per day than vegan diets.

I was surprised that water use was so similar between vegans and vegetarians considering how much cheese vegetarians eat.

I still would recommend a vegetarian diet to meat eaters, as it’s still a massive improvement (and in my experience, it’s easier to become vegan once vegetarian), just thought it was interesting to actually quantify differences between veganism and vegetarianism in terms of climate impact.