Demystifying the carbon neutrality of biomass

The recent reports by the IPCC [.pdf] and the U.S. National Climate Assessment paint a bleak picture for Planet Earth: a two to five degree rise in temperature by the end of century, which would bring havoc to cities, industries, infrastructure, agriculture systems, and economies while causing enormous destruction to biodiversity and the global ecosystem in general, unless significant actions are taken immediately to curb GHG emissions.

These reports also point toward cost-effective ways to combat climate change. The use of biomass for energy, including forest biomass, is presented as one of these solutions. The basis for bioenergy’s perceived carbon mitigation potential lies in the concept of carbon neutrality.

The basic concept of carbon neutrality is that biogenic CO₂ emitted during biomass harvest and use is eventually sequestered during plant growth, resulting in zero net emissions. This idea has made its way into various carbon accounting methodologies, including life cycle analysis (LCA). In the LCA of bioenergy, it is a common practice to either omit entirely biogenic CO₂ emissions from biomass fuel combustion (assuming that they will be eventually sequestered) or, if biogenic emissions are listed, offset them by including the CO₂ sequestered in biomass as a credit.

On a mass basis, there is nothing fundamentally wrong with the carbon neutrality assumption, provided that the forest in question is indeed allowed to regrow. From a climate change mitigation perspective, however, we are not concerned with the mass of biogenic CO₂ per se but its potential to cause global warming while it remains in the atmosphere. This is where the concept of carbon neutrality can start to break down.

In recent years, a growing body of literature on the time-dependent impact of biogenic CO₂ emissions has clearly identified that there is a warming effect during the period while biogenic CO₂ remains in the atmosphere before being sequestered during plant regrowth. The longer biogenic CO₂ stays in the atmosphere, the higher the warming impact. This time-dependent global warming impact of biogenic CO₂ prevents biomass energy from being “global warming neutral,” even though it may become carbon neutral on a mass-exchange basis. For biomass with shorter harvest cycles, carbon neutrality may still be an acceptable approximation, but for biomass with longer harvesting cycles this new development has proven that a long-held view of the carbon neutrality of biomass is wrong.

Alissa Kendall and her colleagues at the University of California—Davis were among the first to suggest the time-dependent impact of CO₂ emissions. Kendall et al. found that CO₂ emissions occurring at different times in the life cycle of bioenergy would not have the same global warming impact as one-time upfront emissions. However, in lifecycle analysis we implicitly convert an upfront emission into a smaller repeated emission by amortizing emissions (amortization is the process of spreading one-time emissions across several years for analytical purposes). Kendall et al. pointed out that amortizing upfront emissions would underestimate their global warming potentials and suggested that we use time correction factors to capture the true warming potential of amortized GHG emissions.

Since then, a number of researchers have quantified GWP_bio factors and used them to account for the global warming impact of biogenic CO₂ emissions from biomass/biofuel combustion, building on this concept of time dependence. A GWP_bio factor represents the relative global warming potential of 1 kg of biogenic CO₂ emissions when compared to 1 kg fossil CO₂. GWP_bio factors correct the global warming potential of biogenic CO₂ emissions depending on how long biogenic CO₂ stays in the atmosphere, which in turn corresponds to harvesting cycles (rotation periods). .

For example, using a simplified model of biomass harvest and growth, Francesco Cherubini, an LCA expert at Norwegian University of Science and Technology, has quantified GWP_bio factors for rotation periods ranging from 1 to 100 years.

The implication here is that biogenic CO₂ emissions for a biomass system with a rotation period of one year (annual crops) would have a near-zero global warming impact relative to fossil CO₂. On the other hand, biogenic CO₂ emissions from a biomass system with a rotation period of 100 years—such as a forest—would have a warming impact 43% that of fossil CO₂. What this implies is that for every 1 kg of biogenic CO₂ released and eventually sequestered by a 100-year rotation forestry system, there would still be global warming impact equivalent to 0.43 kg of CO₂.

To further develop the science on the time-dependent impact of biogenic CO₂, researchers like Geoffrey Guest and Bjart Holstmark have been trying to develop more robust GWP_bio factors specific to various forest management regimes.

GWP_bio factors can easily be applied in LCA of biofuels and bioenergy, in the same way we use global warming potentials for non-CO₂ gases. All we have to do is multiply the amounts of biogenic CO₂ emissions by the GWP_bio factors specific to the relevant rotation periods or harvesting cycles.

A recent ICCT study does this [.pdf]. The study illustrates that the incorporation of GWP_bio factors in carbon accounting has negligible impact on carbon payback periods and carbon intensities of biofuels derived from annual crops and perennial energy crops such as switchgrass, due to short harvesting cycles and quick carbon sequestration. However, for a forestry system with a longer rotation period, the incorporation of GWP_bio factors has a significant impact. For example, for a short rotation forestry (25 years) system analyzed in the study, including a GWP_bio factor increased the carbon payback period for biochemical ethanol from 116 years to 147 years, as shown in the chart below.

Impact of GWP_bio on carbon payback period for biochemical ethanol derived from short-rotation forestry (25 years)

Even though the science on the time-dependent impact of biogenic CO₂ emissions continues to evolve and solidify, we have yet to see it spill over into the policy arena. There are many policies—such as the Renewable Energy Directive, the Low Carbon Fuel Standard, and the US Renewable Fuel Standard (RFS2)—which assume the carbon neutrality of tailpipe or stack biogenic CO₂ emissions. In view of mounting evidence that biogenic CO₂ emissions are not necessarily carbon neutral, earnest discussions on GWP_bio factors and how we can incorporate them in existing policies with minimum administrative costs are the need of hour.

It’s important to note that the GWP_bio factors we’ve presented here are all derived on the assumption that an existing forest is harvested now and combusted, and then regrown. At the landscape level, we could find a rather different picture if the total forest estate expands in response to bioenergy production, especially if that expansion preempts the first harvests. In that case we could have a situation where you could think of CO₂ as being temporarily removed from the atmosphere and then returned, and the GWP_bio factor could work in reverse, showing an enhanced benefit from the scheme. This shows how important it is to analyze the likely impacts of new policies at the system level rather than only at the level of a single plot, especially where forestry is concerned.

All in all though, at a time when there is growing interest on utilizing forest resources for bioenergy, including forest residues, a due consideration of GWP_bio factors in climate change mitigation policies would go a long way toward avoiding perverse incentives, reducing the risks of policy failures, and achieving the intended climate change mitigation goal.