The basic assumption with biofuels is that plants absorb as much carbon dioxide while growing as they release when burnt in an engine. If you use them as a fuel, their net impact on the climate is close to zero, except for emissions from farming machinery and fertilizers.

However, this does not take into account a relatively new concept that scientists dryly call "indirect land-use change.” Put simply, if you take a field planted with grain and switch that crop to something that can be used to make a biofuel, then somebody will go hungry unless the missing grain is grown elsewhere or farming yields are massively improved.

The rush to biofuels means the quantities of land needed are huge, if all renewable fuels would come from plantations. Satisfying the demand of the European Union alone will require an additional 4.5 million hectares of land by 2020, based on an average of 15 of the studies for the Commission. That is an area roughly equal to Denmark.

This leads to a number of open questions:

• What gives Europe the right to lecture developing countries on how they should use their land?

• Why impose tighter standards for the vegetable oils that are burnt in cars than those that are used in the kitchen?

• How do we account for waste animal fats that are as likely to end up in cosmetics and beauty products as they are in the fuel tank of a car?

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