As a climate scientist I understand that our greenhouse gas emissions are impacting the climate. Like many people I try to minimise my environmental impact – I ride my bike whenever I can, I eat very little meat, and I used to care about food miles.
That’s right, I used to.
Food miles are easy to focus on, they’re easy to measure, and easy to explain. The only downside is that they’re a terrible way to estimate the total greenhouse gas emissions from food.
Apples are a great example. The UK imports a lot of apples from New Zealand – about $108 million (US dollars) worth in 2012. From the perspective of food miles, these apples are about as bad as you can get; it’s almost 18,000 km from New Zealand to the UK. There aren’t many countries that are farther apart than these two. Surely then, eating British apples in the UK must be much better for the environment. Right? Wrong.
This book by three New Zealand academics provides a summary of the energy use and greenhouse gas emissions from growing apples in New Zealand and the UK. First, let’s look at the emissions from transporting the apples to the UK.
This looks pretty bad – there are definitely emissions caused by transporting the NZ apples to the UK.
But, how big are those emissions in comparison to the emissions from actually growing the apples? The next graph shows the emissions from growing and transporting the apples.
Now the two types of apples look pretty similar – it seems that food miles are definitely relevant. Maybe not the most important factor, but it looks like they shouldn’t be ignored.
But, earlier I said I no longer care about food miles – so what’s going on? The answer is that apples are a seasonal crop. They are harvested once a year, and then stored in large warehouses. To keep apples in good condition these warehouses have controlled atmospheres with low oxygen levels and low temperatures. It takes a lot of energy to maintain those conditions.
To make a fair comparison between apples from the UK and NZ, we need to know how long they’ve spent in storage before we buy them. Apples in New Zealand are picked between February and April, while apples in the UK are harvested between August and October. By knowing when apples are harvested and using information from Saunders, Barber and Taylor (2006) about the emissions released by storing apples it’s possible to work out the total emissions for apples eaten in each month.
The greenhouse gas emissions from storing apples means that it’s better for the planet if I eat New Zealand apples between February and July. I was pretty surprised by this.
For six months of the year thinking only about food miles would give you the wrong answer.
Update: Geoff asked a great question about the total impact of eating UK apples all year compared with eating the lower emissions choice. Some quick internet searching reveals that there are about 6 apples per kilogram, so eating an apple a day is about 1 kg of apples a week, or 4.3 kg of apples per month.
The total emissions from eating only UK apples is about 12.4 kg CO2e per year.
The total emissions from eating the lowest emissions apples is about 10.4 kg CO2e per year.
The difference is fairly modest in absolute terms – 2 kg of CO2e is a bit less than is released by burning one litre of petrol – but large in relative terms – about 20%.
Saunders, C., Barber, A., & Taylor, G. (2006). Food Miles – Comparative Energy / Emissions Performance of New Zealand ’ s Agriculture Industry.
If you’re a details person and are interested in looking at the workings behind these graphs, they can be found here.