Bringing biodiesel to the oil-rich United Arab Emirates may sound like carrying coals to Newcastle. Yet the sea change in renewable-energy economics has stimulated the green entrepreneurs of the Neutral Group to do just that.

Most transport still relies on liquid-fuel combustion engines. To deal with the impending decline in petroleum-based fuel, we must either change our vehicles or find sustainable liquid-fuel alternatives.

I wrote recently (21 March) about the potential of microbial algae, which capture energy from sunlight into oily substrates for biodiesel. The Neutral Group has taken another route. It collects used cooking oil produced by McDonald's outlets in the UAE and converts it into biodiesel, which fuels the burger giant's distribution trucks. Recycled cooking oil can't meet the world's liquid-fuel needs but the Neutral Group has shown that micro-green economics can work in a closed-loop system.

Biofuels still pose challenges, such as greenhouse-gas emissions. Debates about the rival demands of biofuels and food crops continue. However, in June, an article in Nature envisioned benefits, particularly in Africa, where land is abundant but good agricultural management rare. Linking biofuels and food-crop growth.