In a post—“When do biofuels really balance carbon?”—on his blog Cars and Climate, Dr. John DeCicco at the University of Michigan Energy Institute emphasizes again the importance of a rigorous examination of the common “biofuels recycle carbon” argument. (Earlier post.)
Summarizing his earlier conclusions from an open access paper published in the journal Climactic Change, DeCicco first observes that:
CO2 is always flowing in and out of the atmosphere, whether or not products of the biosphere are being used to supply fuel.
The total amount of carbon in the world is fixed. Whether as food for biological processes, CO2 in the atmosphere, fuel for motor vehicles or in living biomass such as forests, wetlands and other carbon-rich ecosystems, carbon utilization occurs in a closed system.
|DeCicco’s simple model. Terrestrial carbon cycle is shown in the left of the diagram. An increase in Net Ecosystem Production (NEP) is necessary for biofuels to have a potential climate benefit. Click to enlarge.|
Using a simple model to show the carbon flows associated with fuel use in addition to the basic carbon cycle flows, DeCicco shows that regardless of whether the source of carbon in a fuel is biomass or fossil, the amount of CO2 emitted when burning the fuel is essentially the same per unit of useful energy.
In other words, using a biofuel (such as ethanol or biodiesel) instead of a fossil fuel (such as gasoline or diesel from petroleum) does not appreciably change the rate at which CO2 flows into the atmosphere, e.g., from vehicle tailpipes or jet engines.
Therefore, to reduce CO2 buildup in the atmosphere, the emissions from fuel combustion must be balanced by increasing NEP [Net Ecosystem Production], that is, speeding up how quickly the biosphere removes CO2 from the atmosphere.—John DeCicco
For biofuels to show a CO2 benefit, the net CO2 flow rate from the atmosphere to biosphere must be positive, DeCicco argues. If this condition is not met, biofuels cannot provide a climate mitigation benefit—i.e., biofuel use is not carbon neutral, and that’s before considering the extra emissions involved in growing the feedstock and processing it into fuel as well as considering the land-use change impacts.
It is the need for an increase in the net rate of CO2 removal from the atmosphere that is missed when assuming that biofuels are inherently carbon neutral. Sadly, the simplistic assumption of carbon neutrality has been broadcast widely for many years; it is also hard-coded in the lifecycle analysis computer models used to claim that biofuels reduce CO2 emissions.—John DeCicco