Biofuel carbon debt may take centuries to repay

Most biofuels may increase greenhouse gas emissions because clearing grassland or forest to plant them releases carbon dioxide, two groups of US researchers have independently concluded. 

The new analyses show that large amounts of trapped carbon are released into the atmosphere when vegetation burns or decays as land is cleared. This up-front ’carbon debt’ can take centuries to claw back via emissions gradually avoided by using biofuels instead of fossil fuels. 

Only biofuels made from waste products or grown on abandoned lands do less harm than good, the researchers say. 

’Does the carbon you lose by converting forests, grasslands and peatlands outweigh the carbon you ’save’ by using biofuels instead of fossil fuels? The answer is no,’ said Joe Fargione, of US environmental charity the Nature Conservancy, who co-authored one study together with researchers from the University of Minnesota [1]. 

In the best possible case that Fargione’s team examined, sugarcane ethanol grown on converted Brazilian savannah would need to replace petrol emissions for 17 years just to repay the carbon released when the savannah was converted. Other examples, such as soybean biodiesel from cleared Amazonian rainforest, took centuries to break even.

Using existing cropland to grow biofuels might seem to avoid the problem. But as the second land-use study points out [2], this would trigger higher crop prices, and farmers would respond by clearing more forest and grassland to replace crops. Indirectly, therefore, sequestered carbon would be released into the atmosphere anyway.

 ’Simply put, most of the biofuels people think will save greenhouse gases, won’t,’ said Tim Searchinger, of Princeton University, who co-authored the second study. 

Worrying figures 

The problem is hardly surprising, Searchinger told Chemistry World: both direct and indirect carbon debts have been routinely noted in previous papers analysing biofuel emissions savings. But these caveats had no numbers attached and so were eclipsed by headline figures. Now that the US teams have quantified the problem, the debts look more worrying. 

The studies raised immediate questions for biofuels policies, Searchinger said. The EU, for example, has proposed strict certification and sustainability standards so as not to use biofuels produced from rainforests. But that means that farmers would simply turn cropland to biofuels and clear forests to grow more crops instead. ’The current policy will have virtually no effect and needs to be dramatically revised,’ he said. 

Amid the gloom, there were some positive options available, both reports concluded. Biofuel produced from municipal solid waste, manure, housing waste, or corn husks which would otherwise decompose in soil were sustainable - though the latter option would have to be carefully managed not to exacerbate soil erosion. And biofuels grown on grasslands in poor soils - or perhaps from algae grown on deserted lands - would also avoid a carbon debt, though yields would have to improve. 

But investigating these options would probably ’take more time than . we’ve provided for,’ said Searchinger. ’Industry is under enormous pressure to adjust the numbers to justify biofuels that probably don’t deserve it,’ he added.

Richard Van Noorden