How many vegans are needed to reach the Kyoto protocol target?

Question

This is a pretty mathematical question. Given that:

• vegan diet has a greatly reduced environmental impact due to the smaller amount of greenhouse gases emissions (useful links: 1 2)

• world population is increasing over time

How many people are needed (in %) to switch to a vegan diet in order to reach a decrease of GHG emissions of 10% from the 1990 level? Calculations are ok for any country, but it would be quite interesting to have them for United States or for some european country.

(Note: if I missed some relevant data, just spot it and find it on the web)

Answer 1

TLDR: The short answer is that it's not possible. Greenhouse gas emissions come from a diverse set of sources, and only the weakest targets can be met by focusing solely on one area. Even if every person on earth became a vegan, that would not be enough, although it would help significantly.

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I'm going to start with a major simplifying assumptions to ease calculation. Any greenhouse gas emissions attributed to agriculture would alleviated with a vegan diet. With this assumption, a 100% vegan world results in zero greenhouse gas emissions due to agriculture. This is obviously wrong, but will give us an upper bound for how much diet changes can affect total greenhouse gas emissions.

US

In the US in 2015, agriculture was responsible for 9% of greenhouse gas emissions.¹

Assuming veganism can eliminate 100% of agricultural emissions, if every person living in the United States transitioned to a vegan diet, that would not be enough to achieve an emissions decrease of 10% over 2015 levels, nevermind 1990.

Global

Globally, all of agriculture, forestry, and other land use accounts for 24% of greenhouse gas emissions.^{2, 3}

Warming projections which keep the probability of staying below 2°C of warming by 2100 "about as likely as not" require emissions to be reduced by at least 25% by 2050 at the low end of the range.³

Even in optimistic scenarios, the elimination of all agriculture, forestry, and other land use emissions globally are insufficient, thus we can easily say that a vegan diet alone will not be able to reach our emission targets.

Answer 2

In 2006, the UN published the study Livestock's Long Shadow and stated, that the livestock sector accounts for 18% of the greenhouse gas emissions. As stated in the source cited by the questioner, there is an Oxford University study on the Dietary greenhouse gas emissions of meat-eaters, fish-eaters, vegetarians and vegans in the UK concluding that "an average 2,000 kcal high meat diet had 2.5 times as many GHG emissions than an average 2,000 kcal vegan diet." Naively taking these numbers as a basis, the global GHG emissions would drop by 10.8% if everyone became vegan:

 100
- 18       [livestock]
+ 18 / 2.5 [equivalent vegan diet]
= 89.2

100% - 89.2% = 10.8%

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The questioner asked for a "pretty mathematical" approach. Of course the potential of climate change mitigation through dietary change exceeds the direct emission savings. Especially establishing carbon sinks on the released land (livestock accounts for 80% of total anthropogenic land use), e.g. through forestation or the introduction of swamps, can additionally extract GHG from the atmosphere.