By Richard Black
Environment correspondent, BBC News website
When biofuels sprang to political prominence a few years ago - endorsed by Tony Blair among others - the world was rapidly running out of options for a liquid fuel that could keep the wheels of transport turning after the oil ran out.
Biofuel crops such as jatropha could be good for Africa, the EU believes
Hydrogen had been the bright-eyed new kid on the block. But as the practical difficulties and economic realities raised their ugly heads, the new kid was quickly revealed as an unpleasant, drug-addled hoodie.
Fuel cells with a low tolerance for pollution or vibration, a finite supply of already expensive platinum for catalysts, a range smaller than a Tour de France cyclist could achieve in a day, high leakage from pipes… the closer one looked, the spottier the hoodie's face appeared.
Research into hydrogen continues, and rightly so - the least promising kids can turn into prime ministers.
But for a fleeting moment, biofuels appeared to be the real deal, promising a future that was achievable within a few years at a reasonable price, and carried a homespun whiff of living off the land to boot.
The need is still there. As the Gallagher Review of biofuels, commissioned by the UK government and just published, puts it, there is "little sign of the developed countries losing their appetite for travel, and millions of new motorists expected in rapidly developing countries such as India, China, Russia and elsewhere…"
Put simply, as long as we continue to see personal transport as meaning riding around in metal boxes on wheels capable of taking us from one end of the country to the other on a single tank of gas, biofuels are technically and economically the only thing that can substitute for petrol and diesel.
Which is why the report from Ed Gallagher, chairman of the UK's Renewable Fuels Agency, continues: "We cannot afford to abandon biofuels as part of a low carbon transport future".
However, he suggests, we should put the brakes on.
"Current evidence suggests that the proposed EU biofuels target for 2020 of 10% by energy is unlikely to be met sustainably," he concludes.
"The introduction of biofuels should therefore be slowed... we therefore propose targets for renewable transport fuels of between 5% and 8% for the EU for 2020."
Trial by science
The UK's domestic target of fuelling 5% of vehicle traffic from plants by 2010 should be put back by three to four years, Professor Gallagher concluded - a recommendation which UK Transport Secretary Ruth Kelly endorsed.
This all stops short of the abrupt end to the biofuel bonanza that some environment and development groups have been advocating. But it does mark a cooling in the policymaker's love affair with the technology.
So what has changed?
Demand for crops such as soya has led to felling of tropical forests
For Gallagher, one study stood out from the plethora of reports detailing the climatically crazy felling of tropical forest to grow biofuel crops and the equally large dossier of reports blaming biofuel demand for raising food prices.
It was published in the journal Science in February. And the US academics writing it showed that conventional sums failed to show the whole picture.
Making ethanol from US-grown corn was supposed to bring a 20% saving in greenhouse gas emissions compared to using petrol.
But Timothy Searchinger's group showed the real impact was a doubling of greenhouse emissions, as developing world farmers cleared forests and grasslands for new agricultural fields to grow corn to fill the gap in the food market.
The smiling adolescent on whose shoulders policymakers piled all their desperate hopes had turned into a nasty little climate assassin.
UK Transport Secretary Ruth Kelly has vowed to take Ed Gallagher's message to Brussels. But signs are that the European Commission, which is really the engine and steering rack of EU policy, will not want to listen.
Commission energy spokesman Ferran Tarradellas told BBC News that the 10% target can be met sustainably.
And rather than the biofuel rush automatically driving food prices up and making life more miserable in poorer countries, he said the demand could bring economic gains in tropical countries with a conducive climate.
Hydrogen fuelling stations are not a regular feature along the world's roads
"African countries have a comparative advantage for biofuel production, in particular sugar cane and other high-energy crops," he said.
"So if these countries get the necessary assistance, this could be used as a tool to increase the productivity of their agricultural sectors, and that is badly needed in these countries."
The Gallagher Review is equivocal on this. "There is some potential for the poor to benefit from biofuel production," it says - but only where land is plentiful and where the crops are grown according to a set of social and environmental criteria.
It contains much in this vein - biofuels can do x and y, but only if z. The basic message is that if you get the science right - doing all the sums, not just the convenient ones - then biofuels can be positive for the climate.
That may be especially true of so-called "second generation" fuels, a term that encompasses a variety of technologies in gestation such as fermenting agents that digest waste plant material, genetically-modified trees that melt to ethanol at the touch of a catalytic wand, and algal vats that pump out fuels on demand.
As concerns have mounted over existing fuels, these new technologies have donned some of the shining armour. But Gallagher has a cautionary note - they can take more land than the crops currently used - so beware the tarnish.
The key issue is really sustainability; can regulations be drawn up and agreed internationally, bearing in mind the rules of the World Trade Organization, that guarantee crops will only be grown and harvested and distributed and owned in such a way that the climate and local societies benefit?
Looking at history, it is hard to be too optimistic. We did not manage whales, humanity's first global industrial-scale source of oils, very sustainably; and it is hard to argue that in countries such as Nigeria, the oil industry has developed with the needs and rights of the entire population in mind, or that on a global basis we are using it in such a way as to maximise its availability.
But there it is; we need some liquid to put in our cars other than water and brake fluid. Otherwise we might have to walk.
So far, biofuels are all there is. And the wheels have to be kept turning.