Do biofuels actually cause more greenhouse gases?


I recently received a pointer to this blog article which references a NY Times piece about articles in Science that state that biofuels actually increase global warming by pulling land into the agricultural pool that was previously a carbon sink. The first of these Science papers is focused on the ethanol industry in the U.S.

During the past 14 years, 15 separate studies have shown that ethanol has a net positive energy balance. Only one study has contradicted it, but the researchers of that study (Pimental and Patzek) wrote the same paper 4 times so you may hear that the ratio is 15:4. It’s the one that always gets quoted (usually unknowingly) when someone tells you it takes more energy to produce a gallon of ethanol than you can get out of it. Now it appears ethanol opponents will have another study to quote, this time about biofuels creating additional greenhouse gases.

In looking in the supporting materials in Science Express, I found this curious assertion:

If corn-based ethanol could not receive a credit for removing carbon from the atmosphere – deleting the feedstock uptake credit from the GREET model– it would increase greenhouse gas emissions by 48%. It follows that if the use of land to grow corn for ethanol has the net effect of reducing land-based carbon sequestration, the overall effect will be a bigger release of greenhouse gasses.

In other words, they are stating that when comparing greenhouse gases from corn to gasoline, corn should not get a credit for having removed carbon from the atmosphere. Instead they think it should be compared to growing a forest or prairie in the place of farmland which would allow the carbon to be sequestered year after year. Forests and prairies give back carbon to the atmosphere every year when their leaves and grasses die. In the case of forests, every few decades the trees die, or burn, or are used for some other purpose and thus also give back their carbon in a brief instant of geological time. Unless you’re burying the carbon deep under the earth’s surface or oceans, any carbon taken in by plants is given off in a few months or decades. Soils also have a limited capacity to hold carbon and eventually reach a homeostasis after only a few decades. So I consider the logic used in this study to be flawed.

But I will expect that every biofuel opponent will quote it with abandon, never realizing that the authors of the paper are not comparing biofuels with fossil fuels, but rather biofuels with some imaginary state of affairs where forests that capture but do not release carbon to the atmosphere have been replaced by farmland.

All land capable of sustaining plants, whether it be used for farming, prairie, or forest eventually reaches a homeostasis when it comes to CO2 sequestration. Farming allows us to take advantage of the CO2 to carbohydrate conversion that occurs on land whereas prairies and rainforest that go unharvested do not. But in the end, they all return CO2 back to the atmosphere in a relatively short span of geological time. The only counter-examples are swamps that can, over the course of millions of years, turn vegetation into coal by trapping a tiny percentage of carbon each year.