Counting All the Carbon: A Waxman-Markey Wart
by Bill Chameides | July 2nd, 2009
posted by Erica Rowell (Editor)
When lands are changed from one use to another to grow biofuel crops, the change tends to lead to greater emissions of carbon dioxide into the atmosphere. So assessing the climate impact of biofuels should take that fact into account. (NREL)
To love something means accepting it, “warts and all.” Well, here’s one Waxman-Markey wart I don’t even like: its failure to count all the carbon.
In theory bioenergy is climate neutral (see figure). The carbon dioxide (CO2) that is emitted when biomass is burned comes from carbon that the plant took in from the atmosphere during photosynthesis. That carbon is part of a circle going nowhere: plants take CO2 out of the atmosphere in photosynthesis, and CO2 goes into the atmosphere when the biomass is burned. No net CO2 emissions and no net climate effect.
Unfortunately, it is not that simple. You have to count all the carbon.
Embedded CO2 Emissions: Bioenergy Is Rarely Climate Neutral
Producing biofuels such as ethanol often requires energy from fossil fuels and thus CO2 emissions from those fossil fuels. These “embedded emissions” must be counted when assessing the climate impact of bioenergy, and as a result, bioenergy is rarely climate neutral.
Nevertheless, the embedded emissions of biofuels on a gallon to gallon basis are usually smaller than the embedded emissions associated with fossil fuel production, and thus bioenergy is often a good climate bet from this point of view.
A far less straightforward issue relates to changes in land use. To explain this, let’s return to the concept of bioenergy and climate neutrality but reverse the order of things — instead of CO2 out and CO2 in, let’s analyze it as CO2 in and CO2 out.
CO2 into the atmosphere: We start by harvesting biomass, for example, in the form of corn or timber. We burn the biomass to make energy and in the process we emit CO2. These emissions of a greenhouse gas warm the atmosphere even though they came from biomass. No climate neutrality there.
CO2 out of the atmosphere: But normally, whatever crop we harvest and burn would be planted again for more fuel. As that replacement crop grows, it removes the CO2 that was emitted in the first step.
So there’s our circle going nowhere again, this time CO2 out and CO2 in. No climate impact, right? Not necessarily.
This scenario envisions a steady state in land use. For example, once a cornfield, always a cornfield, season after season. But what happens if an old-growth forest with an enormous store of sequestered carbon in its trees and soil is cut down and planted with, say, corn, which is then harvested and burned and then replanted in, say, soybeans this time?
The carbon released from the conversion of the old-growth forest is never removed, and this represents a net flow of CO2 into the atmosphere. In this scenario the carbon emissions are from a direct land-use change.
Indirect Land-Use Change — The Wrench in the Bioenergy Works
Far more complicated and contentious are emissions from indirect land-use change.
Suppose a U.S. farmer who’d been selling corn for food decides to sell that corn for bioenergy instead. That would produce an imbalance between supply and demand for corn for food that will induce another farmer somewhere to fill that food-corn gap. If that other farmer, perhaps living in South America or Africa, does so by converting an old-growth forest to cornfields, that change will lead to a net emission of CO2.
Now, these emissions come from an indirect land-use change since the change does not occur on the same farm. But beware of the word indirect: the CO2 that ends up in the atmosphere is just as real as CO2 from burning fossil fuels or direct land-use changes.
So is indirect land-use change an issue? It looks like it. A couple of papers in Science last year (see here and here) concluded that indirect land-use changes, occurring primarily in Brazil to compensate for the use of American corn for ethanol, make corn ethanol a larger net emitter of CO2 than gasoline — forget climate neutral.
In a more recent Science paper, Marshall Wise of the University of Maryland and colleagues projected that any climate-mitigation strategy that ignored land-use changes when assessing emissions from bioenergy would:
- lead to the conversion of almost all of the world’s unmanaged forests and pastures to farms for bioenergy crops,
- cause a huge jump in corn prices, and
- because of the excess CO2 emissions, require a “negative” emission rate of fossil fuel CO2 to stabilize atmospheric concentrations at 450 ppm.
Ag-Food Fight With EPA
EPA took these findings seriously: it included indirect land-use changes in its assessment of corn ethanol emissions and concluded that corn ethanol did not qualify as a “renewable fuel.” That decision pissed off Representative Collin Peterson (D-MN), who represents a corn-growing district. In the run-up to the vote on the climate bill, Peterson threatened to take his marbles and his House ag-friends home and not vote for Waxman-Markey.
Alas, compromise is the name of the game in Washington, and Peterson got what he wanted before the bill passed in the House. Indirect land-use changes will not be included in the bioenergy accounting ledger for at least five years while the National Academy of Sciences (NAS) studies the issue. Then the agriculture department, energy department, and EPA will all have to approve the NAS study, and, assuming they do, Congress will have to decide what to do when folks like Peterson and his friends can once again threaten a hissy fit.
Maybe the Senate will get it right.filed under: carbon dioxide emissions, climate change, faculty, global warming
and: bioenergy, biofuels, carbon, carbon cycle, greenhouse gas emissions, land-use changes