Comprehensive carbon accounting for identification of sustainable biomass feedstocks
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What the Senate missed in counting forest bioenergy emissions
It’s usually a bad idea when lawmakers overturn scientific consensus; think North Carolina banning the use of estimates for sea level rise in making long-term coastal land-use policies. Now, Congress has proposed to prescribe how EPA does carbon accounting for bioenergy. On April 20, the Senate passed an energy bill containing an amendment that requires EPA to “reflect the carbon-neutrality of forest bioenergy and recognize biomass as a renewable energy source, provided the use of forest biomass for energy production does not cause conversion of forests to non-forest use.” This bill must now be reconciled with the version of the energy bill that has passed the House of Representatives, which does not include the amendment. At the time of writing, it remains to be seen whether the amendment defining forest biomass as “carbon neutral” will be passed into law.
Think about it: burning a standing tree and calling it carbon neutral makes no sense. That tree has been storing carbon for years and would have kept on storing it for years to come if it hadn’t been burned – not terribly unlike the carbon savings you’d get from leaving oil in the ground. The only good reason to expect bioenergy from whole trees to save any carbon is because if other trees grows in their place they will over time sequester back the same amount of CO2 that was emitted by burning the first trees. And to be fair, the Senate bill did caveat that forest biomass could only be considered carbon neutral if it “does not cause conversion of forests to non-forest use” – meaning that trees must be allowed to grow back in the same place others were cut down. But it takes a long time to grow trees – depending where you are, anywhere from twenty to a hundred years or more for trees to return to the size of those that were felled. During that time, CO2 in the atmosphere has increased. In fact, over short timescales bioenergy actually emits more CO2 than coal, due to the lower energy density of biomass and additional emissions that are caused from biomass harvest and transport. CO2 is emitted when the forest soil is disturbed, from equipment used to harvest and transport the biomass, and from production of fertilizer that is needed to support growth of the next cycle of trees under normal forestry management. One also has to consider the additional carbon sequestration that would have taken place from continued growth if the trees had not be harvested – this lost opportunity has the same effect as increased CO2 emissions at the time of harvest.
Over time, the CO2 sequestration from growing the next round of trees will offset the short-term CO2 increase, and eventually bioenergy will have delivered carbon savings compared to coal. But how long do you have to wait? The well-known Manomet study published in 2010 for the State of Massachusetts found that in that state it would take 100 years to pay back the carbon debt of harvesting forest biomass for energy. Note that this simply means that after 100 years, the net CO2 emissions will have been the same whether biomass or coal was used for energy – it does not mean that carbon neutrality has been reached! In fact, no credible science claims that carbon neutrality is ever reached from using whole trees for energy, unless in exceptional circumstances such as establishing a forestry plantation on degraded, biomass-free land and later harvesting it. A recent review paper found that under normal circumstances, it takes 17 to 400 years to pay back the carbon debt of using bioenergy from whole trees instead of coal, natural gas, or heating oil. These payback periods are generally substantially longer than the time period needed to take action and mitigate the worst effects for climate change—that’s why many climate policies usually count emissions over shorter time periods.
If we use a reasonable timeframe of 30 years and include all emission sources associated with harvesting and processing biomass for energy, we see that using whole trees for electricity does not deliver climate benefits compared to coal. This figure is based on results in a study done by my colleagues Anil Baral and Chris Malins in 2014, and is generally in agreement with the literature, although there is variation among studies based on the ecological system that is modeled. Baral and Malins found that the carbon balance is even worse for liquid biofuel pathways from whole trees. With these results in mind, it is striking that Congress is proposing to consider all types of forest biomass, including whole trees, to be “carbon neutral.” If this definition applied to accounting under the Clean Power Plan, bioenergy-heavy states would be able to meet their Clean Power Plan obligations without providing any climate benefits at all. This would be a real perversion of our climate incentives.
Using whole trees for biomass is not going to be an effective climate mitigation strategy. That doesn’t mean that all bioenergy is bad for climate – in fact, there are many biomass sources that could provide substantial greenhouse gas reductions when used for either electricity or liquid biofuels. According to Baral and Malins, sustainably produced energy crops and agricultural and forestry residues (excluding stumps) can deliver high greenhouse gas savings, ranging from 82% to 98% for biomass electricity compared to coal, and from 55% to 102% for liquid biofuel pathways compared to petroleum. Forest thinnings, produced when plantation mangers selectively cut down young trees to make room for others to grow larger, is often thought of as a sustainable feedstock, but like larger whole trees, was found to increase greenhouse gas emissions compared to coal on a 30-year timescale. It’s clear that proper carbon accounting is needed for all biomass pathways, and EPA should be allowed to follow the best available science in determining the greenhouse gas impact of any bioenergy feedstock.
