Are you a global warming denier? If you know that cuts of 80-90% to carbon emissions are ultimately needed to prevent dangerous warming but haven't actually made deep cuts to your own personal contribution, then in a way you are.
It is also worth noting that our carbon emissions are composed of many small things. It is therefore impossible to meet targets such as the Paris accord and avert disaster if we ignore anything that individually would have a small impact.
If you are not yet vegetarian or vegan, following the advice in this article could cut your carbon (equivalent or CO2e) food
print by easily more than 60% and your entire carbon footprint by around 20%. If you are already vegan realistic cuts are probably about half of that or less. (Stats are for UK/Europe, % could be a bit lower in US/Australia and a bit higher in developing countries.)
Beef and lamb are the worst offenders, then cheese (hard cheese is worse than soft cheese), milk, chicken, and other meats and perhaps some fish. Eggs are better than most of the above, but worse than most vegetables. Almost all animal products are worse than most vegan foods and the difference can be very large. Cutting out animal products has a larger impact on carbon than other things I discuss, perhaps reducing your food-related emissions by 50% or more, but that is not the focus of the article since many of you have already done that.
Flying Food vs Seasonal Food
Air freighted food and food grown in indoor heated greenhouses typically have carbon emissions several or many higher than the impact from locally grown plants outdoors. In some cases air freighted asparagus for instance is even worse for the environment than beef (per kilo of per $ of food). Removing unseasonal products could be the most obvious and perhaps largest way for a vegan to reduce their carbon emissions.
The first thing to do is read packets and look if they are produced locally or internationally. If there's no info (or no packet), the best thing is to buy what's in season. See links at bottom of article for identifying seasonal foods.
I assume that the odds of food being local are slightly higher at stalls by the side of the road, farmer's markets, and smaller shops rather than supermarkets. I haven't done any research to check but it seems logical.
As a very good general rule, foods rank in this order: 1 local, 2 national, 3 shipped, 4 local but grown in a heated environment, and 5 the worst is air. Shipping is far more energy efficient than flying and adds very little to the footprint whereas heating is carbon intensive. This means that something sent from the other side of the world by ship is better (carbon wise) than a tomato grown out of season in a heated building in your garden. The difference in carbon between the 3rd and 4th ranked above is very large, and is the key. Eat as much as you want of 1, 2 and 3 (excluding animal produce) and almost none of 4 and 5.
If it's not seasonal, and it's the kind of food that doesn't last (i.e. can't be shipped), such as asparagus, lettuce, tomato, and other vegetables, strawberries or soft tropical fruits, and peppers it may be wise to avoid it out of season, or eat sparingly. Some foods, such as apples, oranges, bananas and wine, travel well and last longer before spoiling so are likely sent by boat and a low carbon choice all year round. Mostly eating local foods could cut your carbon foodprint by 5%, religiously making sure you don't eat a single air freighted or greenhouse product could mean more like a 10% cut is achievable.
That is not a huge cut but local food is also is likely fresher, tastier and cheaper, so it's a win-win.
Which Plant Foods Are Worse?
According to the book I read, rice is one of the worst vegan foods for carbon footprint due to excessive fertilizer use, long distance travelled from tropics to first world, and methane bubbling out of rice fields. Although other statistics I found suggest it's not that bad. Bread seems to be better as a staple though. As long as you are not eating rice constantly as your major staple food, it's probably not worth worrying about too much.
In fact, do not focus at all on whether broccoli is better than carrots, or pasta is better than bread, or bananas are better than raspberries. It doesn't matter. Almost all plant foods not air freighted and not grown in heated environments are low carbon, and the differences between them are small, and are not worth worrying about.
Cutting down on wastage, e.g. by food rotation, is the next biggest saving for many people as many people waste 10%-30% of all their food. If you're already on top of your food wastage and ready to go the next level, note that the book I read suggests buying misshapen fruit and veg to stop retailers throwing it away.
Packaging and Recyling
Packaging accounts for about 6% of carbon emissions from food so avoiding products with excessive packaging, could save around 3% of carbon emissions from food. Recycling could save a similar amount as landfill decomposition releases some gases into the atmosphere.
Note that the gas or microwave energy used to heat food is a small % of the total carbon footprint. Eating a largely raw diet has a notable carbon saving but things like efficient cooking methods are not critical, although not completely negligible either.
Drinking tap water rather than bottled water is a no brainer, easy saving when we consider the packaging, transport and so on. If you are in the habit of filling the shopping trolley with mineral water, that could be 1% of your total carbon emissions (total, not just food).
The book How Bad are Bananas (Berners-Lee), which covers all carbon emissions not just food, is my most important source for this article, a few others are below.
(although the article is for meat eaters the chart is worth a quick look)
When foods are in season