Humans have used bioenergy since the Stone Age, burning wood and other organic materials. However, the last 200 years have been dominated by fossil fuels, making economic growth and higher standards of living possible for parts of the Earth’s population. Burning fossil fuels has caused rising levels of carbon dioxide, leading to the greenhouse effect.
Can we replace these fossil fuels by going back to bioenergy? No, not if we want to use as much energy as we do now. Field and co-workers (2008) estimated that globally we could only get a few percent of the current energy use from bioenergy, because we also need to grow food and maintain ecosystems and biodiversity. We could save agricultural land by shifting to a more vegetarian diet, but it would still not be sufficient, in particular in light of the still growing world population.
|Effect of harvest for bioenergy used to replace coal on forest carbon stock changes and total greenhouse gas (GHG) emissions (stand level, from Ter-Mikaelian et al. 2014b). A. Accumulation of carbon in an unharvested forest stand. B. Carbon in the stand regenerating after harvest. C. Harvested biomass is used to produce wood pellets; life cycle GHG emissions from obtaining and producing wood pellets are lower than life cycle and combustion emissions of coal, resulting in a GHG benefit of using wood pellets to replace coal. D. Carbon sequestration parity is achieved when the sum of carbon in the regenerating stand and the GHG benefits of using wood pellets to replace coal reaches the amount of carbon in the stand if it had remained unharvested; carbon debt repayment is achieved when the sum of carbon in the regenerating stand and GHG benefits of using wood pellets to replace coal reaches the pre-harvest amount of carbon in the stand. Figure © authors of the article.
Are biofuels carbon-neutral? The point of using biofuels is that no new carbon is added to the carbon cycle like fossil fuels do. But biofuels are most often not carbon-neutral, in particular not in the short term . Annual crops such as rapeseed or wheat grows fast but do need energy, usually fossil during growing and harvesting. Changes in land-use also contribute also to such crops not being carbon-neutral – if forest was felled to make space for growing wheat, which still happens in many places in the world, the ecosystem contains less and less carbon. Managed forests also have a lower carbon storage the natural forests, so they should not be considered equal in climate negotiations.
Demand for bioenergy is high in the EU, due to a policy that favors bioenergy over other energy sources. So far the wood mostly comes from Europe, but imports from the U.S.A. and Russia are increasing. Further demand could lead to plantations in developing countries for import to the EU.
It is often stated that emissions from forest biomass is compensated by plants taking up carbon dioxide during photosynthesis. Forest residue left in the forest would, some say, anyhow decompose to carbon dioxide. But if the residue is left in the forest it can take anything from a few to a hundred years depending on the environment and type of residue. When combusted these residues are converted to carbon dioxide at once however.
Forests take a long time to grow. If a forest is cut down for bioenergy use, it is converted to carbon dioxide at once, contributing to the greenhouse effect just like fossil fuels do. The atmosphere does not distinguish carbon from one source of the other. It takes many years for the forest to regrow and compensate the carbon emissions. The short term (about 50 years) is important, because we could reach a climate tipping point if we don’t reduce greenhouse gases soon. A number of scientific articles by Holtsmark, Ter-Mikaelian, (see Figure) and Johnston and Van Kooten compare the climate effects of forest bioenergy and fossil fuels, including how long it takes to repay the carbon debt from burning wood.
An analysis done by a number of environmental organizations based on data from the department of Energy and Climate Change (UK) shows that electricity generated from combustion of pine trees (from plantations) can increase greenhouse gas emissions by 80% compared to coal over a 20 year period. Only after a 100 years are emissions somewhat (14%) lower than coal. Professor John M. DeCicco (University of Michigan) states that anything possible should be done to prevent carbon dioxide entering the atmosphere. The climate benefits of biofuels therefore need to be re-evaluated.