Archive for February, 2008
A pair of articles that just became available in Science (1-2) and a slightly earlier article (3) have found that biofuels like ethanol produced from corn are net greenhouse gas (GHG) losers over decade to century timescales (also reported by the Times here).
The debate about whether plant-derived ethanol saves GHG’s over its life cycle compared to gasoline has been going on for years. The basic story is that bio-ethanol and gasoline emit similar amounts of CO2 at the tailpipe, but on the bio-ethanol side, when plants regrow they capture some of that carbon back from the atmosphere. However there are other energy inputs to make fertilizer, run farm equipment, process the plants, and transport the fuel. On balance, looking across many studies, it appears that ethanol from corn provides a modest GHG benefit over gasoline, and technology in development to make ethanol from a whole plant (“cellulosic ethanol”) would provide a big benefit (4).
The new contribution of the first three references is to look at the carbon lost from plants and soil when you convert land from some other use to grow biofuel crops like corn, soy, palm, or sugarcane. The stunning result is that you lose so much carbon converting a piece of land to cropland, that it takes decades or centuries of making biofuels from it to even break even on GHG emissions. What is really disturbing is that as prices for corn and ethanol have gone up, people are already chopping down tropical rainforest to grow more biofuel crops. Even if you make sure to buy ethanol produced from land that was already cropland, that is just pushing land for food crops to be converted somewhere else (since the total demand for food is relatively inelastic).
Even in the U.S., taking out-of-use cropland, of which we have millions of acres, and converting it to grow corn for ethanol, results in a carbon debt that takes about 50 years to pay back (1). That is essentially because when ag land is out of use for while, it turns back into grassland or forest. With future, more efficient means of producing biofuels, the payback times will be shorter, but still substantial.
Does this mean biofuels are a Bad Idea? My opinion is, on a grand scale, yes. There are some exceptions. Fuels from agricultural wastes are still a good idea. And cellulosic fuels from a careful mix of native grasses grown on marginal land is still a good idea with significant potential. But can we rely on biofuels to make deep cuts in transportation sector emissions? Probably not. And this is an important salvo in the climate policy debate.
And there is a larger story underpinning the findings of these papers, and that is that land-use change is a key element of climate policy, one which hasn’t got enough attention so far. There is a huge amount of carbon currently stored up in natural lands and we are rapidly setting it free, whether by converting it to cropland to feed a growing and increasingly meat-hungry population, converting it (perversely, it seems) to grow biofuels, or clearing it for urban development. Continuing to do so will undercut our efforts to reduce greenhouse gas emissions.
There is also an interesting connection here to my research on capturing CO2 from air. In principle, one could extract CO2 from the atmosphere using industrial methods (as opposed to using plants), add a lot of energy, and get liquid fuels. The industrial method uses at least 10,000 times less land than the corn-ethanol method, and so basically avoids the land-use change issues we are talking about. On the other hand, the biomass method runs largely on solar energy but the industrial method would require a huge amount of energy from somewhere to synthesize the fuel. Back in the ’70′s, Steinberg proposed doing it with nuclear (5). Now maybe we can find something better.
- Fargione, J.; Hill, J.; Tilman, D.; Polasky, S. and Hawthorne, P. “Land Clearing and the Biofuel Carbon Debt.” Science, 2008. DOI: 10.1126/science.1152747
- Searchinger, T.; Heimlich, R.; Houghton, R. A.; Dong, F.; Elobeid, A.; Fabiosa, J.; Tokgoz, S.; Hayes, D. & Yu, T. “Use of U.S. Croplands for Biofuels Increases Greenhouse Gases Through Emissions from Land Use Change.” Science, 2008. DOI: 10.1126/science.1151861
- Righelato, R. & Spracklen, D. V. “Carbon Mitigation by Biofuels or by Saving and Restoring Forests?” Science, 2007, 317, 902-.
- Hammerschlag, Roel. “Ethanol’s Energy Return on Investment: A Survey of the Literature 1990-Present.” Environmental Science and Technology. 2006, 40, 1744-1750.
- Steinberg, M.; Dang, V. D. “Production of synthetic methanol from air and water using controlled thermonuclear reactor power: Technology and energy requirement”. Energy Conversion. 1977, 17, 97-112.