We’re always told “buying local” is best for the environment.

However, a new book suggests food miles are a poor indicator of a product’s total carbon footprint, and could even be misleading.

Bananas imported from the Dominican Republic, apples from New Zealand and oranges from Brazil are among the most carbon-friendly foods UK consumers can buy, according to Professor David Reay, a climate scientist from the University of Edinburgh.

“I went in thinking really distant food stuff like bananas coming from the Dominican Republic or tea from Assam in India would have really high carbon footprints because of the food miles,” said Professor Reay.

“Actually when you look at the life cycle, food miles are not a major part. It was how they were produced and how people used them when they got to the UK that matters. So that was a real eye-opener.”

Most oranges consumed in the UK come from Brazil and are shipped across the Atlantic, but still have low carbon footprints. This is because 60 per cent of an orange’s life-cycle footprint has been embedded in their flesh before it has left the farm gate, according to Professor Reay’s new book, Carbon-Smart Food.

Most of this comes from fertilisers, pesticides and the fuel used by machinery during harvesting.

They are then processed and sorted. If the orange is used to make juice, 22 per cent of the total footprint is down to transport and distribution.

Professor Reay said: “If you just assume everything closer to you is better, so if you’re thinking these blueberries or cucumber from Holland in January will be low carbon, they won’t be. It’s going to be intensive production with a high carbon footprint.

“This will massively outweigh the food miles of bringing them from a country where the climate is right at that time of year.”

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According to government data, buying a tomato grown in the UK has three times the footprint of a tomato grown in Spain.

Lettuces grown in the UK during winter are cultivated in poly tunnels which require lots of energy to keep them warm. In terms of carbon emissions, it is more environmentally friendly to buy them from Spain during the winter and in the UK during the summer.

But food miles do matter if the product has been transported by air. For this reason, Professor Reay says consumers should avoid eating out-of-season soft fruit such as raspberries and blueberries.

A 100g box of blueberries grown locally or imported via ship will produce around 100g of carbon dioxide. If it’s flown in, that increases by ten times, pushing its carbon footprint up to more like 1kg.

“If you want to go into the high carbon footprint foods then once it’s been air freighted you’re in real trouble. That’s when the food miles absolutely soar in terms of emissions. We should have a blanket ban on air freight,” said Professor Reay.

Despite the fiendishly difficult business of counting carbon he says the message “buy local” needs to go with “in season”.

“The food mile message is a little more nuanced than it’s sometimes presented,” he added.

Reducing waste would also make a significant difference to the carbon footprint.

According to 2017 research, food waste accounts for 8 per cent of total greenhouse gas emissions. If food waste were a country it would come behind China and the US as the largest emitters on the planet.

Professor Derek Stewart, an agri-food sector expert from the James Hutton Institute, said the book was interesting but some of the data could be challenged, notably on the grounds that it only looked at carbon dioxide emissions.

Ships use dirty fuels that emit nitrogen and sulphur as well as carbon dioxide which has a significantly larger greenhouse gas impact than carbon dioxide alone, he said.

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Professor Stewart added: “The carbon footprint is too nebulous for most people and in fact forms only part of a food supply chain.”

Dr Eugene Mohareb, a lecturer in sustainable urban systems at the University of Reading, commended the overall analysis of food miles and the impacts of production.

“In an ideal world, embodied carbon from production and transport would be communicated to the consumer on food products so that they could make informed decisions. But in the absence of that, eating produce in season can be a good guide,” he said.

Food systems contribute to between 20 and 30 per cent of emissions globally and Dr Mohareb says the UK is unlikely to reach net zero by 2050 unless these emissions are reduced.

“There are many things that we as consumers can do to reduce this impact. For example, we found that a simple measure, such as using grocery delivery instead of collecting by car just half of the time, reduced someone’s total impact on the food system by around half a percentage point,” he said.