I worry that longtermism can be used to justify, or rationalize (depending on your view), too much. Imagine turning back the clock to when many of the things we consider morally wrong and abhorrent were more commonplace and were widely accepted: sexual harassment, marital rape, human slavery, etc., and sticking one's neck out in opposition to any of them would at least cost some social capital if not more.

Does the longtermist in any of these contexts really not have any obligation to engage in any costly opposition to the wrongs because it would detract from their longtermist projects? It seems it would require an awful lot of confidence in the longtermist's ability to affect the future to argue so. And it feels terribly convenient for the longtermist to argue they are in the moral right while making no effort to counteract or at least not participate in what they recognize as moral wrongs.

My view can be boiled down to this: First, we should be wary of arguments that tell us that doing things that we believe to be wrong are fine to do. Second, we should think hard about how much certainty we have about our ability to have longterm effects.

To summarize:

Eating meat -> narrower moral circle not sufficiently valuing the welfare of artificial sentience and/or wild animals -> existential risks (mostly suffering risks)

Animals and longtermism (although not specifically about your own diet):

• Why I prioritize moral circle expansion over artificial intelligence alignment by Jacy Reese
• Should Longtermists Mostly Think About Animals? by Abraham Rowe
• Why I'm focusing on invertebrate sentience by Max Carpendale
• Why Digital Sentience is Relevant to Animal Activists by Brian Tomasik for Animal Charity Evaluators
• The Relevance of Wild Animal Suffering by Tobias Baumann
• Arguments for and against moral advocacy by Tobias Baumann
• Will Space Colonization Multiply Wild-Animal Suffering? by Brian Tomasik
• Risks of Astronomical Future Suffering by Brian Tomasik
• Long-term vs. short-term focus by Sentience Institute

Comparing diet to charity (often older charity cost-effectiveness estimates):

• Four practices where EAs ought to course-correct by kbog
• Don't sweat diet? by Gregory Lewis
• Vegetarianism for Meat-Eaters by Scott Alexander
• When should an Effective Altruist be vegetarian? by Katja Grace
• Revenge of the Meat People! by Rob Bensinger

Animal charities and interventions, with newer estimates:

• See Animal Charity Evaluator's comprehensive reviews on their top charities.
• Corporate campaigns affect 9 to 120 years of chicken life per dollar spent by Saulius Šimčikas for Rethink Priorities
• Corporate Campaigns for Animal Welfare by Marinella Capriati for Founders Pledge
• Charity Entrepreneurship's reports on corporate campaigns

See also the comments.

I think there are also psychological effects of eating meat that might cause people to not give animals the moral weight they would think they deserve upon careful reflection.

Self-plugging as I've written about animal suffering and longtermism in this essay:

http://www.michaeldello.com/terraforming-wild-animal-suffering-far-future/

To summarise some key points, a lot of why I think promoting veganism in the short term will be worthwhile in the long term is values spreading. Given the possibility of digital sentience, promoting the social norm of caring about non-human sentience today could have major long term implications.

People are already talking about introducing plants, insects and animals to Mars as a means of terraforming it. This would enormously increase the amount of wild-animal suffering. Even if we never leave our solar system, terraforming just one body, let alone several, could near double the amount of wild-animal suffering. There's also the possibility of bringing factory farms to Mars. I'm studying a PhD in space science and still get shut down when I try to say 'hey lets maybe think about not bringing insects to Mars'. This is far off from being a practical concern (maybe 100-1000 years) but it's never too early to start shifting social norms.

I'd call this mid term rather than long term, but the impacts of animal agriculture on climate change, zoonotic disease spread and antibiotic resistance are significant.

I'd like to echo Peter's point as well. We don't ask these questions for a lot of other actions that would be unethical in the short term. There seems to be a bias in EA circles of asking this kind of question about non-human animal exploitation. I'm more arguing for consistency than saying we can't argue that a short term good has a long term bad resulting in net bad.

I agree with the previous answers- that is, I think the best argument here has to do with moral circle expansion affecting the long term future.

In addition, eating meat could increase existential risk through its effects on worsening climate change and the emergence of natural pandemics.

See this chart to compare greenhouse gas emissions per kg of different food products to see how much more animal products contribute to climate change. In total, animal agriculture contributes around 14% to 18% of all anthropogenic greenhouse gas emissions.

Animal agriculture also contributes to the emergence of zoonotic pandemics (which are 60% of all pandemic outbreaks). It is hard to know by how much, but the authors of this ebook estimate 70% of zoonotic diseases come from industrial animal agriculture (with uncertainly 50% - 90%). Also, see this article on the subject by Liz Specht from GFI.

I am very uncertain about this, but potentially, eating meat could also increase the chances of a global power conflict over resources like land, water, and energy because animal products require much more of these resources per calorie of food.

Also, I wonder if antibiotic resistance could increase the chances of an existential pandemic caused by airborne bacteria (something like an extremely contagious and extremely deadly version of multidrug-resistant tuberculosis). If that is the case, then eating meat would probably make this worse since the use of antibiotics in farm animals is a significant contributor to the spread of antibiotic resistance. Again, I'm very uncertain here - it's just a thought.

Michael St. Jules posted a link to this post on Facebook and I wrote a reply (formulating it rather quickly I might add), and Peter Hurford suggested that I should copy and paste this reply as a response on the EA Forum, which I am now doing. These thoughts were hastily formulated and are highly fallible and critical feedback is entirely welcome.

" Thinking about the future a million years from now, moral circle expansion is clearly an extremely important concern (failure of moral circle expansion could be catastrophic, and could have catastrophic negative consequences both in terms of causing harm and failing to prevent harm, with each one of those by itself outweighing all gains to positive human well-being, under any plausible non-speciesist moral theory).

Achieving moral circle expansion earlier on plausibly has positive flow-on effects which exponentially grow over time, since if the attainment of complete non-speciesism by the human community occurs one day sooner, then the harm thereby prevented may be such that under other scenarios harm not prevented would have exponentially grown. So, one million years from now, positive flow-on effects from achieving moral circle expansion one day sooner could be significant. So a very strong imperative to work on moral circle expansion as soon as possible right now, including psychologically undermining one's own natural tendency towards speciesism and signalling to others that one is doing so, as long as there are no substantial costs to doing so.

Costs of being vegan are in fact trivial, despite all the complaining that meat-eaters do about it. For almost everyone there is a net health benefit and the food is probably more enjoyable than the amount of enjoyment one would have derived from sticking with one's non-vegan diet, or at the very least certainly not less so. No expenditure of will-power is required once one is accustomed to the new diet. It is simply a matter of changing one's mind-set. The flow-on effects of signalling a strong commitment to non-speciesism to those in one's immediate circle are highly positive. Some complain that one must pay a social cost. Sure, I found that too at least at first, but twenty years later my friends all highly respect me for sticking to my guns. In any case, the fact that there is a social cost to be paid is precisely the point: this is the thing that must be fought against. The tables need to be turned so that it is meat-eaters who feel on the defensive.

From long-termist considerations, the case for going completely vegan starting today, for almost everyone, unless you have some significant reason to believe you would be at risk of major health problems (which is statistically rare indeed), is very strong. "

Full disclosure, not in original FB post: Over 25 years of being vegan, I have occasionally, like Brian Tomasik, deviated from full vegan purity and been just lacto-vegetarian for a while. I now think that this is on the whole not justified.