Green Central: click here to read Times Online's environment blog

At first sight, biofuels are hugely exciting. They seem to offer a way to wean transport systems off oil. The city of Curitiba in Brazil, for example, has been famously running on sugar cane for years. The dream of a cleaner world, running on plant-power, is deeply attractive.

But the current generation of biofuels cannot save the planet. In individual cities and towns, they can make a difference. But at any scale, the dream dwindles.

The picture is complicated because there are many different types of biofuel, and their effects differ depending on where and how they are grown. Brazil has an agricultural surplus and uses relatively little energy in fertilising, extracting and purifying the ethanol from sugar cane. This is not true in other countries where these processes are more industrialised.

The first issue is efficiency. In most countries, using ethanol reduces carbon dioxide emissions by only about 13 per cent compared to petrol. This is partly because of the pollution caused by the production process, and partly because ethanol gets only about 75 per cent of the mileage of petrol.

The second issue is land. If cultivated land is turned over to biofuels, it reduces the space to grow food. So demand for biofuels will tend to lift prices for cereals and oilseeds. Some people argue that the European Union’s timing is disastrous because it has set a target for biofuels just as food price inflation is taking off. Others, like Britain’s National Farmers Union say that developing country farmers would benefit if the EU and US stopped swamping their markets with surplus grain, which has kept prices artificially low.

The NFU proposes to switch the 3.5 million tonnes of surplus feed wheat that we currently export to bio-ethanol, and to use 750,000 hectares of set-aside land to grow oilseed rape for bio-diesel. But those are relatively small quantities. One study has suggested that a quarter of the EU’s arable land would be needed to meet the EU’s 5.75 per cent target.

The alternative is to use uncultivated land. But this could destroy habitats. Particularly if tropical forests or peatbogs are cleared to plant crops, that will release more greenhouses gases than it saves. There are already very real concerns about the loss of rainforests in Southeast Asia, home to orangutans, which are being cleared to make way for palm oil plantations. The rainforests of Southeast Asia and Brazil are important carbon sinks at the forefront of the war on global warming. The apparent eagerness of these countries to clear their forests is why biofuels have been dubbed “deforestation diesel”.

Another option is to import biofuels. But that is the worst of all worlds, as it risks both raising food prices and razing forests, with significant quantities of energy used to deliver them.

So what next? The Royal Society, in a recent report, argues that more research is needed into biofuels, and that policy is running too far ahead of R&D. Biotechnology offers hope of converting plants to fuel more cheaply and cleanly. Biofuels could be part of the answer to climate change, but grown in the wrong way they could exacerbate it. It looks as though the world needs to pause for thought.

Times recommends

• Energy-saving ideas from switched-on professor
• Rooks are as clever with tools as chimpanzees
• Burping of the lambs blows roast off menu