Ben Caldecott of Climate Change Capital argues in the Guardian that ‘sustainable’ aviation requires the use of biofuels. He suggests a target of about 60% bio-based ingredients in the fuel that powers planes at UK airports . He doesn’t begin to address the implications for food supply, or show how biofuels will reduce global emissions. My calculations suggest replacing 60% of the UK’s aviation kerosene with fuels of biological origin would use all of the UK’s home produced cereal and oil seeds crops and substantially increase food imports. Furthermore, to replace the food used to make aviation fuel on farmland elsewhere in the world would result in a net increase in greenhouse gas emissions. The inconvenient truth is that biofuels are never an answer to climate change problems. Put crudely, photosynthesis in growing plants captures energy provided by the sun. This energy can either be used to fuel human beings, providing them with the two or three kilowatt hours a day they need to function, or it can be used to create power for other purposes. For example, the energy in corn (maize) can be turned into alcohol that replaces petrol in a car. Or it can provide food for human beings or cattle.

A kilogramme of wheat contains about 3,000 (kilo) calories, equivalent to about three and a half kilowatt hours. Biofuel processing plants use the energy in foods to create liquids that can power engines and jet turbines. Ben Caldecott wants us to switch to 60% biofuels in aviation fuel. How much food would that require?

In 2011, the UK used about 11.4 million tonnes of aviation fuel. The total energy value in this kerosene was about 133 terawatt hours. (Contrast this with the UK’s total electricity use of about 350 TWh, about three times as much).

Britain produced about 24 million tonnes of grain and oil seeds. This was mostly wheat but also included barley, oats and oil seed rape. The energy value of this was about 84 terawatt hours. So if every single food grain produced in Britain this year was turned into liquid fuel at 100% energy efficiency, we’d only cover about 60% of our needs for aviation fuel. But even in the most efficient conversion process, only about half of the energy value in grains can be turned into fuel. Even if Britain turned every single grain produced this year into kerosene, the country would barely meet a third of its need for aviation fuel.

No problem, Ben Caldecott and other biofuel fans might say: we simply need to import more food. The question that arises is whether growing more food elsewhere would increase greenhouse gas emissions to a greater or lesser extent than the savings from reduced oil use in airplanes. Unfortunately, even simple calculations show that conventional agriculture produces more emissions than aviation per unit of energy. Growing food using conventional agriculture uses large amounts of energy to produce nitrogen and phosphorus fertilisers. More importantly, nitrogen applications to fields increases the emissions from soils and watercourses of nitrous oxide, a far worse global warming gas than CO2. The net impact on global emissions of producing an extra tonne of food is probably at least 550 kilogrammes of carbon dioxide equivalent. (Much, much more if it is new land converted from forest or grassland to arable). And unfortunately the saving of CO2 from replacing kerosene with oil seeds is far less than 550 kg per tonne of food.

As study after study has shown around the world, biofuels don’t save emissions. As importantly, every tonne of food that is converted to liquid fuel increases the price of basic foodstuffs for poor people. Ben Caldecott's article welcomes increased air travel. He and his colleagues at Climate Change Capital should ask themselves whether feeding the aviation industry is more important than avoiding hunger and starvation. The numbers simply don’t support the view that aviation can become more ‘sustainable’ by switching from fossil fuels to biologically sourced equivalents.