Bioenergy has its limits for cutting emissions

Many climate policies and models consider biomass carbon-neutral. The argument is that carbon emitted during burning the biomass is balanced out by the carbon that plants and trees sequester. But that understanding is flawed.

Biomass is indeed renewable, and burning biomass or biomass-derived fuels can offset fossil fuel use. However, cultivating and harvesting biomass, transporting it, and processing it for energy or to make liquid fuels all emits greenhouse gases. Exactly how the biomass is used—whether directly or turned into fuel—also makes a difference.

Now researchers at MIT have calculated the extent to which bioenergy can cut greenhouse gas emissions from fossil fuel use when bioenergy’s emissions are taken into account. In a new study published in the journal Nature Energy, they also quantify the best use of global bioenergy resources to replace fossil fuels.

Offsetting electricity and heat with bioenergy is 1.6–3.9 times more effective at cutting emissions than offsetting liquid fuels, the team found. Their calculations show that, depending on fossil fuel demand and the amount of bioenergy available in 2050, the use of bioenergy could cut emissions by 4.9–38.7 billion tons.

Steven Barrett and his colleagues first calculated how much global bioenergy will be available in the future by factoring land availability and energy crop yields. They came up with three bioenergy availability scenarios: low, medium, and high.

Then they computed the best way to distribute the bio-resources for various uses—liquid fuels, heat, and electricity—in order to slash the most emissions compared with fossil fuels. This let them estimate the total amount of bioenergy that could be deployed, as well as the related emissions reductions, for the three availability scenarios.

Finally, they considered three different levels of projected global demand for fossil fuel-derived energy. For each level, they estimated emissions reductions given the three bioenergy scenarios.

This showed that bioenergy could meet 10–97% of projected fossil fuel energy demand in 2050, corresponding to greenhouse gas emissions reductions of 9–68%. The highest reductions occurred in a future scenario where fossil fuel demand was medium and bioenergy was highly available.

These findings indicate that “[greenhouse gas] emissions mitigation via the use of bioenergy is constrained not only by the availability of biomass, as considered in previous assessments of bioenergy potential, but also by the [lifecycle] emissions of final bioenergy,” the researchers said.

The researchers caution that these are maximum numbers. Bioenergy adoption faces additional constraints such as existing investments in fossil fuels, as well as economic and policy factors.

Source: Staples MD, R Malina, and SRH Barrett. The limits of bioenergy for mitigating global life-cycle greenhouse gas emissions from fossil fuels. Nature Energy. 2016.
Photo: Andrew Walsh, Flickr Creative Commons

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