Guardian: Ruth Kelly Our cars and other forms of transport are the third-largest source of carbon dioxide emissions in the UK and the only one likely to have increased by 2020. Any serious attempt to tackle climate change requires us to dramatically step up our efforts to reduce these emissions. So a clean, renewable energy that can be mixed with fossil fuels to power our cars has great attraction. This is exactly what supporters of biofuels believe they offer. They say they are one of the …