Calculating carbon payback times for biofuels

Biofuels can only contribute to reducing greenhouse gas (GHG) emissions if crop production does not lead to further deforestation in tropical regions. New research introduces the concept of ecosystem carbon payback time (ECPT), which can be used to help identify the best means of expanding biofuel production worldwide, without causing unacceptable levels of carbon emissions from crop production.

Industrialised regions, such as the EU and the US, are unlikely to have the land base needed to meet growing demand for biofuels. Rising demand is spurring production of biofuel crops, such as sugarcane and palm oil, in tropical areas including Brazil, Malaysia and Indonesia. Tropical forests are up to 400 times more valuable in reducing global GHG by acting as net carbon sinks than as areas for biofuel production.

Tropical ecosystems store 340 billion tonnes of carbon, more than 40 times than the amount of emissions arising from global fossil fuel combustion. The carbon stored in these ecosystems is released when forests and grasslands are cleared, burned and converted to agricultural land.

The study defines ECPT as the number of years needed to produce carbon savings from burning biofuels instead of fossil fuels, to compensate for carbon emissions incurred by clearing land to grow biofuel crops. The ECPT of different regions and crops was calculated, using new detailed databases of crop locations and yields and IPCC estimates of global vegetation and soil carbon stocks. Estimates of the carbon payback time are provided for a range of locations and feedstocks across the tropics. Future scenarios of improvements in agricultural productivity and biofuel efficiency, as well as increasing difficulty in extracting fossil fuels, are also evaluated.

The findings suggest that the ECPT is longest when low-yielding crops, such as cassava, maize and soybeans, replace carbon-rich land sources such as tropical forests. In these situations, it can take 300-1500 years to compensate for the loss of carbon stocks. The greatest carbon savings are provided by growing the highest yielding tropical biofuel crops, sugarcane and oil palm, on previously cultivated land. In some cases, immediate carbon savings can be made. For example, the expansion of biofuel crops into former cocoa plantations in West Africa.

However, the authors warn that in other cases, switching from food to biofuel crops can have a knock-on effect and result in increased overall emissions, as in southeastern Brazil where smallholders are being pushed further into highland forests to grow food crops. The study concludes that no foreseeable changes in agricultural or energy technology will be able to achieve meaningful carbon benefits if crop-based biofuels are produced at the expense of tropical forests. Careful consideration of ECPT is needed for any expansion in crop-based biofuels in tropical regions.

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