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THE popular narrative with food is simple: 鈥渆ating local鈥� is one of the best ways to shrink your carbon footprint. This is because food miles are a handy proxy for overall sustainability, as transporting harvests across the planet is a key driver of carbon emissions. It is a story that is as pervasive in the foodie media as it is intuitively plausible, but how reliable is it?

Well, it is indeed true that the food sector uses a lot of energy, contributing of all greenhouse gases. But how much of this is directly down to moving food around the globe from farm to fork? Perhaps less than you might think.

In one of the most comprehensive studies, researchers at Carnegie Mellon University in Pennsylvania report this to be just (thus making up as little as 0.8 per cent of the average carbon footprint of someone in the US). Echoing other research from around the world, which suggests transport generates just 4 to system鈥檚 carbon footprint, the study found that most emissions were actually generated from other aspects of food production, such as agriculture, processing and cooking.

To put this in context, the US Department of Agriculture household kitchen appliances generated nearly seven times as much carbon emissions as food transport. This figure is particularly pertinent considering that typical estimates of the length of US food supply chains aren鈥檛 exactly insignificant, coming in at .

In fact, considering all aspects of our food system 鈥� whether farming, processing, packaging, transport, retail, food services and households 鈥� it was the end consumer that used by far the most energy. Transport? The least.

OK, so food miles may not be the largest driver of our food system鈥檚 carbon emissions, but surely every little bit helps when it comes to reducing our carbon footprint? The tricky thing is that cutting transport distance by swapping to a more local producer can often involve an environmental trade-off as the carbon emissions of agricultural systems can vary wildly.

One UK government study found that tomatoes trucked hundreds of kilometres from sunny Spain had a that was less than a third that of those grown in heated glasshouses in the chilly UK. Conversely, it found that the opposite was the case for crops like potatoes, where the carbon emissions of production were similar, meaning transport from further afield had a greater overall impact. In contrast to the simple 鈥渓ocal = more eco-friendly鈥� narrative, the authors therefore concluded that: 鈥淎 single indicator based on total food kilometres travelled would not be a valid indicator of sustainability.鈥�

鈥淭omatoes trucked from Spain to the UK had lower emissions than those grown in heated glasshouses in the chilly UK鈥�

When it comes to our carbon footprint, transport methods vary enormously too. For example, as more emissions than sea freight, shipping food to the UK all the way from South-East Asia would involve far less carbon than the same product popped on a short flight from Italy. In this context, a single air food mile is the equivalent of almost 8 road food miles and over 75 shipping food miles, making the concept of distance as a direct indicator of emissions highly problematic.

Finally, even within the same transport method, the concept of food miles can belie the true complexity of calculating carbon emissions, such as economies of scale. In a study at the University of Bath, UK, researchers aimed to compare the emissions when consumers bought fruit and vegetables directly from local farms or through an organic box delivery scheme. The box scheme involved a carbon-hungry system of cold storage, packing and transport to and from a regional hub, on top of delivery to the consumer鈥檚 door. Yet despite this, the efficiencies of scale meant than if all these consumers made individual car trips direct to the farm of 6.7 kilometres or more.

So is eating local really a better option? It depends. There are many reasons why you might be choosing to do so, aside from environmental concerns. Indeed, there are many other ways to measure environmental impact other than carbon emissions. But when we look at the evidence, only one thing is clear: food miles alone really aren鈥檛 a good proxy for sustainability. In fact, sometimes they are incredibly misleading. While we can鈥檛 discount them entirely, we should view them in the context of a suite of other factors, recognising that they often make up a tiny fraction of food鈥檚 carbon impact.