Eating local makes only small improvement to your environmental impact

here from the social change lab suggests that the British public think that eating locally produced meat is better for the environment (see The British public is totally wrong about how to improve the environment)

Many assume that food grown in foreign countries is imported via airplane, but actually only 0.16% of global food transportation happens by air. Most of it, 60%, happens by sea, which releases about 50 times less greenhouse gas emissions than air transport. It's likely that you can get all the benefits of locovorism though avoiding a few key foods that always need to be flown for example.

Transport is a small contributor to emissions. For most food products, it accounts for less than 10% (Poore and Nemecek 2018), and it’s much smaller for the largest GHG emitters. In beef from beef herds, it’s 0.5%. that means local beef is at best 0.5% better for the environment than beef from afar.

In other words, going ‘red meat and dairy-free’ (not totally meat-free) one day per week would achieve the same as having a diet with zero food miles.

Many of the foods people assume to come by air are actually transported by boat – avocados and almonds are prime examples. Shipping one kilogram of avocados from Mexico to the United Kingdom would generate 0.21kg CO2eq in transport emissions.11 This is only around 8% of avocados’ total footprint.12 Even when shipped at great distances, its emissions are much less than locally-produced animal products.

Which foods are air-freighted? How do we know which products to avoid?

They tend to be foods which are highly perishable. This means they need to be eaten soon after they’ve been harvested. In this case, transport by boat is too slow, leaving air travel as the only feasible option.

Some fruit and vegetables tend to fall into this category. Asparagus, green beans and berries are common air-freighted goods.

==10% and surely plane and tanker makes up a small fraction of that ==

It's what you eat

A bigger factor is what you eat. The most sustainably produced beef still emitted more greenhouse gases than the least sustainably produced tofu.

Weber and Matthews (2008) investigated the relative climate impact of food miles and food choices in households in the US.5 Their analysis showed that substituting less than one day per week’s worth of calories from beef and dairy products to chicken, fish, eggs, or a plant-based alternative reduces GHG emissions more than buying all your food from local sources.

The researchers estimated that the average American household’s food emissions were around 8 tonnes of CO2eq per year. Food transport accounted for only 5% of this (0.4 tCO2eq). This means that if we were to take the case where we assume a household sources all of their food locally, the maximum reduction in their footprint would be 5%.

it doesn't actually mean this but close enough: this statement assumes all food is using the most emissions it can. In reality this average is pushed down by the fact that many people attempt to eat local.

Downsides to eating local

Going local has some very real costs too: limits international trade which prevents poorer countries growing their economies to become more efficient

There are also a number of cases where eating locally might in fact increase emissions. In most countries, many foods can only be grown and harvested at certain times of the year. But consumers want them year-round. This gives us three options: import goods from countries where they are in-season; use energy-intensive production methods (such as greenhouses) to produce them year-round; or use refrigeration and other preservation methods to store them for several months. There are many examples of studies which show that importing often has a lower footprint.

Hospido et al. (2009) estimate that importing Spanish lettuce to the UK during winter months results in three to eight times lower emissions than producing it locally. The same applies for other foods: tomatoes produced in greenhouses in Sweden used 10 times as much energy as importing tomatoes from Southern Europe where they were in-season.

Source

https://scienceline.org/2020/05/why-eating-local-isnt-always-best-for-the-environment/