When do biofuels really balance carbon?

The belief that biofuels are inherently carbon neutral rests on an assumption that the CO₂ absorbed when their feedstocks are grown balances out the CO₂ emitted when the fuel is burned. That's why it's said that biofuels recycle carbon, in contrast to fossil fuels' one-way flow of carbon into the atmosphere.

However, the extent to which CO₂ emissions actually get balanced needs to be verified, not just assumed, even as obvious as it may seem. A careful analysis shows that biofuel carbon flows balance out only under certain conditions. Moreover, those conditions are at best only partly met for the biofuels now being used. To see why, it is necessary to understand some fundamental concepts about CO₂ flows between the biosphere and atmosphere.

Figure 1. The main flows of the terrestrial carbon cycle:
P = net primary production; R = heterotrophic respiration.

The first point is that CO₂ is always flowing in and out of the atmosphere, whether or not products of the biosphere are being used to supply fuel. This always-circulating carbon cycle is illustrated in Figure 1, where P is the downward flow of CO₂ from the air into the biosphere and R is the return flow of CO₂ back up into the atmosphere. 

P stands for net primary production (NPP), which is the amount of carbon embodied by plants as they grow after subtracting plants' own metabolic release of CO₂. R stands for heterotrophic respiration (often designated R_h), which is the CO₂ released by organisms that consume plants.

Heterotrophic respiration includes all the CO₂ exhaled by humans and other animals as they metabolize food, but the vast majority of R is from the decomposition of biomass in soils, forest litter and crop residues. In nature, no carbon is wasted; ecologically speaking, it is all put to good use whether or not it is utilized commercially. On average, P exceeds R, which enables carbon to accumulate in the biosphere.

The second point is that the total amount of carbon in the world is fixed. Otherwise put, 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. This point might seem trivial (it's just conservation of mass), but unfortunately it is neglected in the lifecycle analysis (LCA) models commonly used to analyze biofuels. This error is due to the fact that those models fail to properly represent the terrestrial carbon cycle.

With these concepts in mind, we can perform a simple but scientifically rigorous analysis of what happens when a biofuel substitutes for a fossil fuel. The model is depicted in Figure 2, which shows the carbon flows associated with fuel use in addition to the basic carbon cycle flows illustrated in Figure 1. Also shown is the P-minus-R difference, which is termed net ecosystem production (NEP); it is given as a downward arrow and reflects the net flow of carbon from the atmosphere to the biosphere. 

Figure 2. An increase in Net Ecosystem Production (NEP) is necessary
for biofuels to have a potential climate benefit.  

At the center of the figure is fuel combustion. Whether the source of carbon in the fuel is biomass (B) or fossil (F), the amount of CO₂ emitted (E) 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, that is, speeding up how quickly the biosphere removes CO₂ from the atmosphere. Mathematically, this condition is written as

            d(NEP)/dt > 0

and it means that the change in the net CO₂ flow rate from the atmosphere to biosphere must be positive. If this condition is not met, biofuels cannot provide a climate mitigation benefit. In other words, biofuel use is not carbon neutral, and that's even before considering the extra emissions involved in growing the feedstock and processing it into fuel. Moreover, it's also before considering the land-use change impacts that have become so prominent in the biofuels debate. 

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.

Because the feedstocks (such as corn and soybeans) used to make biofuels are harvested largely from land already in production, there is little or no overall increase in NEP. For example, a corn field doesn't remove CO₂ from the air any faster when the harvest gets turned into ethanol than when it is used for feed and food.

NEP can be increased by using crop residues to make fuel, as now being pursued at a small scale through cellulosic ethanol production. NEP then increases because R decreases, e.g., by collecting corn stover that would otherwise decompose and thereby reducing the CO₂ emissions from cornfields after grain is harvested. In any case, it is necessary to do a careful, location-specific assessment of how NEP actually changes when biofuel feedstocks are produced; one cannot just assume (as lifecycle models now do) that the carbon in a harvest fully offsets CO₂ emissions during fuel combustion. Ecologically speaking, the extent to which one can safely "starve the decomposers" by harvesting residues is likely to be limited. 

The implication is that, while it may be possible for biofuels to contribute to climate mitigation, the conditions under which they actually do so are much more restricted than is commonly assumed. Moreover, because any climate benefit hinges not on biofuel use per se, but rather on raising the net rate of CO₂ removal from the atmosphere, there are likely to be other ways to accomplish that task which are less costly and more ecologically sound. 

References

The analysis summarized here is formally presented in:

DeCicco, J.M. 2013. Biofuel's carbon balance: doubts, certainties and implications.
Climatic Change 121(4): 801-814.  http://dx.doi.org/10.1007/s10584-013-0927-9

For further explanations of NEP, see:

Kirschbaum, M.U.F., et al. 2001. Definitions of some ecological terms commonly used in carbon accounting, pp. 2-5 in Kirschbaum, M.U.F., and Mueller, R. (eds.) 2001. Net Ecosystem Exchange. Canberra, Australia: Cooperative Research Centre for Greenhouse Accounting. [PDF link]

Lovett, G.M. J.J. Cole and M.L. Pace. 2006. Is net ecosystem production equal to ecosystem carbon accumulation? Ecosystems 9: 1-4. http://dx.doi.org/10.1007/s10021-005-0036-3; also at [PDF link]