Current methods of producing biofuels release so much carbon that it could take hundreds of years for them to make any greenhouse gas savings, a recent study has found.
Research led by David Tilman from the University of Minnesota has shown that whether biofuels offer carbon savings depends on how they are produced. Converting heavily vegetated land such as rainforest, savannah or grassland in Brazil, the US and southeast Asia can release between 17 and 420 times more CO2 than the annual reduction in emissions provided by using biofuels in place of fossil fuels.
Growing demand for alternatives to petroleum is increasing the production of biofuels from food crops such as corn, sugarcane and soybean, with both agricultural land and native ecosystems being converted to biofuel production. The soils and plant matter on this land are large stores of carbon, and together they contain almost three times as much carbon as is found in the atmosphere. Converting the land to grow crops for biofuel manufacture releases this as CO2 via burning or decomposition.
After these initial emissions, there is a prolonged release of greenhouse gases as roots and branches decay. The amount of CO2 released over the first 50 years of this process is the ‘carbon debt’ of land conversion. Biofuels made from crops grown on converted land can repay this carbon debt if the net greenhouse gas emissions generated by their production and combustion are less than those of the fossil fuels they replace, but in many cases this may take centuries.
Tilman and his colleagues estimated the time taken to repay the carbon debt of biofuels produced from various crops and habitats. They found that only two would pay off their greenhouse gas emissions in under 50 years – sugarcane ethanol and soybean biodiesel, both grown on converted Brazilian Cerrado savannah, have repayment periods of 17 and 37 years respectively.
Most repayment periods are far longer, rising to an estimated 420 years for palm biodiesel grown on converted tropical peatland rainforest, as the required drainage causes additional emissions due to peat decomposition. There is also a 48-year repayment even when converting farmland that has been under the US Conservation Reserve Program for 15 years, as such systems gradually recover their carbon store.
In fact these figures may be an underestimate, since the researchers assumed cleared land to be in a fixed state, whereas it could still be accumulating carbon. In this case, the debt would be increased by loss of future carbon storage.
The production of biofuels must release as little stored carbon as possible if they are to successfully mitigate climate change. Biofuels made from plant waste, or native perennial plants grown on abandoned agricultural land, create little carbon emissions and offer immediate reductions in greenhouse gas generation, as well as reducing the displacement of food crops that causes the price of food to rise.
However landowners may prefer land clearance that results in greenhouse gas emission if they can make a profit producing biofuels but do not receive payments for carbon management.
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