No, it's to choose the best n options, up to the point at which you reach your cost limit. Depending on the limit, and the options you face, n could be 0, or 20, or 100; but there's no particular reason to think it should be 1.

I guess I wasn't sufficiently clear there. My point is that you need to do a cost-benefit analysis, pick your best choice and then do a new cost-benefit analysis rather than just follow through with your 2nd and 3rd best choice from your original analysis. You can't assume that your 2nd best choice becomes your new best choice after taking your best choice.

If you're hungry and you decide your first choice is to buy a mars bar and your second choice a snickers and you buy and eat the mars bar you can't assume that your next action should be to buy and eat the snickers - the situation has changed and you need to re-evaluate.

Of course, assuming a fixed upper bound on willingness to sacrifice runs counter to the idea that the sacrifices you're willing to bear should depend on the benefits obtained.

Not a fixed upper bound, just a limit. Anyone who cares about reducing their carbon footprint will reach a point where they are not currently willing to make any further sacrifices for a further carbon emissions reduction because to do so would conflict with their other goals. What I'm saying is that each choice you make changes the calculation a little when considering future choices.

The thing is, the opportunity cost of becoming vegetarian isn't like the opportunity cost of $5.

Not exactly no. The thing is that one might be willing to pay more than one currently does to continue eating meat. If the cost of meat doubled for example I would not reduce my consumption by 50%, I'd cut back elsewhere. I choose to spend a certain amount of money on meat because it represents better value than my next best opportunity. The reason I currently spend money on meat is that I value the meat more than the money (or other alternate uses of the money). You have to take that into account when considering the opportunity cost of becoming vegetarian.

Assuming the costs can simply be summed

They can be, if you denominate them in human welfare.

No, the benefits can be denominated in (general) human welfare. The costs are denominated in your own personal welfare. Money can serve as a convenient proxy for that to aid in calculation but I'm not sure you can give any direct measure, the best you can do may be a preference ordering.

If there is a more effective yet equally costly option O, but the total benefit of O+Vegetarian is still greater than the total cost of O+Vegetarian, then Vegetarian is still worth it (as is O).

You've got to consider opportunity cost and marginal utility. If you valued reducing carbon emissions above all else then your best course of action would probably be suicide. Assuming some upper limit on the cost you're willing to pay to reduce carbon emissions, your best strategy is to choose the option that provides the greatest reduction for the least personal cost. If given your preferences and available options, buying some carbon offsets is your most cost effective option and becoming vegetarian is your second most cost effective option, it does not follow that you should do both.

While the benefits can be seen as fixed for the purposes of your decision (since whatever you do will have such little impact that you can sum them without worrying about diminishing marginal utility) the costs cannot, if they are significant relative to your total resources. This is more easily seen when you're talking about choices that can directly be represented with money but is still true when the costs are not purely financial. Assuming the costs can simply be summed you would have to conclude that you should spend all your money on carbon offsets if you thought it was wise to spend any of your money on them (since if the benefit of O is greater than the cost of O then the benefit of 100xO should be greater than the cost of 100xO).

Surely all that's required is that it have a net positive effect?

Net positive taking into account all of the personal costs, yes. It's not enough that it merely reduces emissions, it needs to reduce emissions more effectively than other equally costly options. I get the sense that we're largely in agreement there though.

My original question was an attempt to ascertain whether the reduction in greenhouse gas emissions was truly your primary reason for choosing vegetarianism (with the choice made by weighing up the costs and benefits of various ways of reducing emissions) or whether it was a convenient 'added benefit' given a choice that was made partly or wholly for other reasons. The (seemingly) more common animal welfare justification for vegetarianism seems more directly linked to the particular decision to not eat meat than does a carbon emissions argument.

FWIW, I'm a little unsure about the value of buying local for a couple of reasons.

I'm not particularly advocating buying local as a better option, it was just an example of the kinds of factors that one might need to consider.

I tend to think that if there is a significant negative externality to carbon emissions that is not currently reflected in prices, the optimal solution would be to impose some kind of carbon tax to reflect that hidden cost. This would avoid the need for individuals to try and make complex cost benefit calculations for themselves on optimal carbon reducing choices.

I don't think it's very likely that it is politically feasible to implement such a tax though so if I considered the issue important I might attempt to make lifestyle choices that reduced my own personal impact. Under those circumstances I'd want to make choices efficiently. It's not clear to me that vegetarianism would be the best choice but since I don't consider reducing my own carbon footprint a priority I haven't done a lot of research on the issue.

Is that impression based on anything in particular?

Not really, I just suspect that if one's primary concern is reducing one's carbon footprint, it seems like it would be a bit too convenient if a comprehensive cost benefit analysis came out with the answer 'become a vegetarian'. That seems like an overly simple answer to a very complex question. All else being equal, eating less meat is probably going to reduce carbon emissions but were you to take into account the full picture (perhaps preferring locally sourced produce over imported, preferring food that you can walk to the store to buy over food that you have to drive to a specialty store to purchase, taking overall nutritional content into account, etc.) and consider other lifestyle changes in addition to dietary then I just find it unlikely that 'stop eating meat' is the uncomplicated best course of action.

I am prepared to believe that the answer to the question 'Will eating less meat tend to lower my carbon footprint?' is yes. I am very skeptical that the answer to the question 'All things considered, what is the best way for me to lower my carbon footprint?' is a simple 'Become a vegetarian'.

I think the fact that it's possible to adapt pretty easily to a meat-free diet strengthens the case for others doing (or at least trying) it.

It's a data point for others to consider, sure.

P.S. What makes you assume I'm male?

Given the male/female ratio here (discussed at length elsewhere) it's my default assumption unless a username seems obviously male or female. In the absence of a good gender neutral pronoun I tend to use he, though in this case I did assume you were male.

If your main reason for eating a vegetarian diet is to reduce your carbon footprint, how effective is your dietary choice? My impression is "not particularly".

Implicit in my question was an assumption that the person making the choice places some inherent value on meat consumption (they like the taste, or they believe it has health benefits for example). If that is not the case then the question of environmental justifications is irrelevant if it is in fact true that eating vegetarian is cheaper.

Vegetarians who do not feel they are giving anything up by not eating meat and are indeed saving money have already adequately explained their choice. Bringing additional justifications related to environmental benefits is only relevant if they wish to persuade others who do feel they would be giving something up by giving up meat to become vegetarian.

The original poster seemed to be saying that giving up meat was originally motivated by environmental concerns and that it was initially a sacrifice ("I used to enjoy meat a lot") but that he doesn't really miss it any more so he didn't appear to be attempting to persuade anybody. My question was whether he considered alternative ways to alleviate the environmental concerns without paying the perceived cost of giving up a food that he enjoyed.

Inspired by mattnewport's comment, here are additional questions for any vegetarians: if vat-grown meat were developed, would you eat it? Would there be any ethical issues with eating it?

Environmental and efficiency concerns are my main motivation, particularly GHG emissions.

Would it be accurate to say that your primary concern is that there are negative externalities involved in meat production that are not reflected in the price of meat products? If the largest negative externality that concerns you is GHG emissions do you feel that your reasons for not eating meat would be eliminated if the negative externalities were priced in through some kind of energy tax or cap and trade system? Did you ever consider eating meat and purchasing carbon offsets to make up for the unpaid negative externality? It sounds like you don't miss meat much any more but you say that you used to enjoy it so presumably there would have been some additional price you would have been willing to pay in the form of a carbon offset or perhaps a charitable donation of some kind?

1. I avoid all meat, as well as milk. I'm working to reduce other dairy products, but cheese is proving stubborn.
2. Environmental and efficiency concerns are my main motivation, particularly GHG emissions. I have no particular concern for animal welfare.
3. I have three general exceptions. The first is that I'll try types of food that I've never eaten before if offered the opportunity. (I would totally try fried locusts.) The second is that I'll eat things that I or a close friend or family member has caught/killed (non-farmed). The third is that I'll eat meat to avoid serious social awkwardness. I've also thought about making an exception for food that would otherwise go to waste, but decided that it could create bad incentives.
4. Any kids would presumably be raised as practical vegetarians, because that's what I (and my partner) cook.
5. I've encouraged others, with some success, to reduce and/or change the mix of meats they eat. (I've not really tried to convince anyone to become totally vegetarian.) I've found that non-vegetarians tend to be more open to my reasons for being vegetarian (which are fundamentally anthropocentric) than to concerns about animal welfare.
6. I do the standard stuff to keep up proteins: legumes, soy products etc.
7. My attitude to others doesn't depend much on their dietary choices. (Some of my best friends are omnivores.) My attitude to others' dietary choices depends on their reasons for doing whatever it is they do. Difficult to give a general answer.
8. I've been vegetarian for a little over two years. I've had three "lapses", one for each of the exceptions listed in 3.
9. I used to enjoy meat a lot, but except for seafood, don't really miss it at all. In fact, the longer I go without eating it, the less appealing it seems. I really enjoy good vegetarian food.