Most analyses of the benefits of biofuels focus chiefly on the crop/fuel cycle, ignoring the value of longterm carbon ‘sinks’ such as forests and grasslands. Yet, to be comprehensive, our approach to climate change must also look at the alternative of maintaining or restoring these areas, compared to destroying them and turning them over to arable production of fuel crops.
When arable land is restored to forest instead of using it for biofuel production, carbon stores build up in the soil and vegetation and outweigh the emissions avoided by the production of biofuel (see illustration). Converting cropland to tropical forest can sequester 20-30 tonnes CO2/hectare per year – two- to threefold higher than the emissions avoided by sugarcane-derived bioethanol. In temperate regions, forest regrowth is slower but the rates of carbon sequestration are still two- to threefold higher than the avoided emissions from biofuels produced from temperate crops. The sequestration rates fall as forests mature, but only after 50 to 100 years might the cumulative avoided emissions exceed the carbon sequestered by forest restoration.
Where, directly or indirectly, natural forests or grasslands are converted to arable land to permit the production of a fuel crop, the loss of carbon stored in the biosphere must be factored in. In the tropics, the amount of carbon released into the atmosphere in conversion of secondary forest to burnt cropland is approximately 600 tonnes of carbon per hectare; the conversion of primary and logged forest releases approximately 700 tonnes carbon per hectare. Most of this loss occurs through burning and biodegradation in the months following the initial clearance, and its impact on globalCO2
and warming is immediate.
Balancing this amount of carbon in the atmosphere with the emissions avoided through the use of biofuels would take 30 to 50 years. Furthermore, removal of forest cover to make way for biofuel crops may reduce downwind rainfall, causing a cascade of further forest loss, further reducing the biosphere’s capacity to sequester carbon and accelerating warming.
Replacing diverse natural habitats with monocultures of arable crops drastically reduces the range of plants and animals that an area supports. This is particularly true in the tropics, where the forests are the most biologically diverse regions on the planet and where forest loss has already eliminated or endangered many species. Intensive agriculture also reduces amenity value: there is less countryside for people.
The next few decades are a window of opportunity for us to develop real low carbon societies. In this window, to minimise the net flux of carbon dioxide to the atmosphere, it is clearly better to maintain existing forests and regenerate forest on available arable land, rather than to produce biofuels from arable crops. Compared with large-scale use of bioethanol and biodiesel, this will result in lower atmospheric carbon dioxide levels and it will provide a larger more effective carbon sink for the future.
Dr Renton Righelato is chair of the World Land Trust and a Visiting Research Fellow at the Environmental Systems Science Centre at the University of Reading
This article first appeared in the Ecologist March 2007