I suppose we should have known it was all too good to be true. What could be wrong with using plants for fuel? They take carbon out of the air, so burning them up in the tank just puts it back up there — no net change, right?
We’ve already heard about the massive quantities of synthetic fertilizer and water required to keep a biofuel crop like corn going. And there is the distinctly distasteful problem that the corn would do a lot more good feeding someone. Indeed, the headlong rush into converting cropland to biofuel cultivation is already raising food prices.
As if that weren’t bad enough, it turns out that even where the major biofuels would seem to have a hands-down advantage over petroleum, i.e., on the balance sheet for net carbon emissions, the story is quite a bit less rosy than it appears.
A new study by Fargione et al in Science has crunched the numbers to show why. The key advance here is that these authors calculated the amount of carbon present in both the standing biomass (i.e., trees) and in the underlying soils on the land that is cleared for biofuel crops. They then estimated how much of that carbon is released into the atmosphere, and over how long a time span, by burning or microbial decomposition in exposed soil. It turns out that a large quantity of carbon is lost gradually from the soil over the course of decades after land has been cleared. Fargione and colleagues call this the “carbon debt” from land conversion:
“Over time, biofuels from converted land can repay this carbon debt if their production and combustion have net [greenhouse gas, GHG] emissions that are less than the life-cycle emissions of the fossil fuels they displace. Until the carbon debt is repaid, biofuels from converted lands have greater GHG impacts than those of the fossil fuels they displace.”
Greater GHG impacts than petroleum. The figure below shows the numbers for several major types of biofuel operations that involve land conversion.
The figure shows for each of nine scenarios (A) the carbon debt, i.e., the CO2 emissions from soils and biomass lost or degraded during habitat conversion, (B) the percentage of that debt due to biofuel production as opposed to other uses, (C) the annual carbon repayment rate, meaning the greenhouse gas reduction from fossil fuel use displaced by the biofuel production, as well as carbon storage in soils, and — here’s the kicker — (D) the number of years required to repay biofuel carbon debt. The results are, to put it mildly, sobering. For example, conversion of native grassland (if you could find some) to cornfields for ethanol production requires 93 years to break even. In Indonesia and Malaysia, where vast swaths of virgin rainforest are being burned down every year to plant oil palms, it would take four centuries to repay the debt. As the authors note:
“Our analyses suggest that biofuels, if produced on converted land, could, for long periods of time, be much greater net emitters of greenhouse gases than the fossil fuels that they typically displace. All but two—sugarcane ethanol and soybean biodiesel on Cerrado—would generate greater GHG emissions for at least half a century, with several forms of biofuel production from land conversion doing so for centuries. At least for current or developing biofuel technologies, any strategy to reduce GHG emissions that causes land conversion from native ecosystems to cropland is likely to be counter-productive.”
Now, lest I be accused (again) of being pessimistic, there is some good news of sorts here if you hunt for it. The main point is that not all biofuels are created equal. As I’ve discussed before, you can avoid the carbon debt by brewing fuel from plants growing on land that is too degraded to produce much of anything else, or from harvesting of natural prairie vegetation that does not require the land clearing that sends all that wood and humus and soil carbon up in smoke. Doing that on a commercial scale is of course not as straightforward as growing corn or soybeans, but these data emphasize that it is well worth exploring.
Maybe that way we can minimize the hot air.