As nations try to stem emissions to keep the world from warming more than 2 degrees Celsius in line with their commitments towards the Paris Accord, replacing fossil fuels with renewable alternatives is widely seen as a big step in the right direction. A major source of energy oft-extolled as renewable is biomass from trees, which are usually harvested from managed forests either established on land that has already been deforested or planted where forests didn’t naturally grow. But a new study finds land-use like managing forests for biomass production may come at a much higher carbon cost than previously thought.
The study, published recently in Nature, was conducted by an international team of researchers who analyzed data how much biomass – organic matter – is contained in areas of terrestrial vegetation around the world. They used this, in turn, to calculate how much carbon this vegetation stores.
But they didn’t just assess current biomass and carbon stocks – they also figured out how much carbon the world’s vegetation could be storing if it wasn’t affected by human land-use.
Their results indicate terrestrial vegetation currently stores around 450 billion metric tons of carbon. But absent land-use, it could be storing more than twice that – about 916 billion metric tons.
Of the 466 billion tons of “lost” carbon, the researchers found 53 to 58 percent can be attributed to the direct clearing of forests and other land-cover changes. Deforestation has long been recognized as one of the biggest emitters of greenhouse gases and reducing it is prominently addressed in the Paris Accord, so the researchers weren’t surprised that deforestation is having a big limiting impact on carbon sequestration.
But when it came to the remainder of the missing carbon, the study uncovered some surprises. Its results showed land management that didn’t change cover type – e.g., forests stayed forests, grasslands stayed grasslands – contributed 42 to 47 percent towards the 466 billion-ton carbon deficit.
Managed forests and pastures don’t technically change an area’s vegetation type and haven’t been major discussion points for climate change policy and commitments since it’s been assumed that this type of land-use was much less intensive than outright deforestation. But the new study upends this assumption.
“Until now, these effects have been severely underestimated, and have therefore received scant consideration in global studies and models,” Karl-Heinz Erb, lead author of the study, said in a statement. Erb is a researcher with U.S.-based Institute of Social Ecology and a professor at Alpen-Adria-Universität Klagenfurt in Germany.
“Our results show that the consequences of forest management and grazing are far greater than previously assumed. Managed forests store about one third less carbon than pristine, untouched forests would,” Erb said. “This effect is not just local, but can be observed almost world-wide.”
The study notes that human land-use impacts were already pronounced before industrialization, with around 350 billion of the 466 billion metric tons of carbon lost before 1800. This adds more fodder to a growing body of research that finds using a pre-industrial baseline to measure human-caused carbon emissions may not be accurate. Such a baseline is used by the UN’s Intergovernmental Panel on Climate Change to set emissions reduction targets in the fight against global warming.
The researchers write that the loss of so much carbon to land-use in a pre-industrial world before fossil fuels became a mainstay highlights the huge impact vegetation degradation can have on the global carbon budget. As they try to reduce carbon emissions and meet international climate agreements, many countries are turning away from fossil fuels in favor of alternative energy sources like the burning of biomass harvested from managed forests. But Erb and his team warn that such efforts may not make as much of a dent in global emissions as hoped and may ultimately derail global warming mitigation strategies.
“Our results clearly show that it is not legitimate to assume that the use of biomass for energy is climate-neutral if it does not contribute to deforestation,” Erb said. “As long as models of the future carbon cycle do not explicitly and fully reflect the full effects of land management on biomass stocks, they will not be able to accurately assess the carbon cycle effects of large scale implementation of bioenergy policies. This jeopardizes the formulation of robust and reliable climate protection strategies.”
Erb and his colleagues urge that in addition to stopping deforestation, the protection of forest functions, such as carbon stocks, should be moved more into focus when it comes to land-use and climate change planning.
“Our study illustrates that land use strategies geared towards combating or mitigating climate change require a prudent and cautious approach,” Erb said. “Strategies that are too simple in their design can backfire, or may end up causing more harm than good due to the major uncertainties involved.”
Erb, K.-H., Kastner, T., Plutzar, C., Bais, A.L.S., Carvalhais, N., Fetzel, T., Gingrich, S., Haberl, H., Lauk, C., Niedertscheider, M., Pongratz, J., Thurner, M., Luyssaert, S., 2017. Unexpectedly large impact of forest management and grazing on global vegetation biomass. Nature, in press. doi:10.1038/nature25138