Just because climate protection is on the label doesn’t necessarily mean that climate protection is part of the package, so to say. Some measures that reduce carbon dioxide emissions are only climate-friendly and sustainable at first glance. On closer examination, they emerge as having no climate-protecting effect and may even have the opposite one.
As researchers at the Max Planck Institute for Biogeochemistry in Jena and the CSIRO in Canberra, Australia, have shown, this is particularly applicable to the use of bioenergy. To this effect, the scientists analysed climate-protection measures that require large areas of land, such as the cultivation of bioenergy plants. In the process, they uncovered a key conflict: land is not available in infinite qualities and far too much additional land will be needed in future, not only for climate protection but also for food and energy generation. Thus, in the researchers’ view, the only solution is a sustainable approach to land management that weighs up the different demands on land and avoids the creation of wastelands.
For combating climate change, scientists and politicians discuss wide-ranging measures. For example, they call for a reduction in the burning of fossil fuels and the generation of energy from renewable sources like biomass. In addition, the protection of tropical rainforests, which store huge amounts of carbon, is considered particularly important for the stabilisation of the climate. Emissions of nitrous oxide and methane from agriculture also need to be reduced and large areas should be afforested if possible.
However, opinions differ as to which solutions are sustainable and climate-friendly, and which ones are not. Ernst-Detlef Schulze, Emeritus Director at the Max Planck Institute for Biogeochemistry, and Josep Canadell, a scientist from the Commonwealth Scientific and Industrial Research Organisation (CSIRO), analysed different options which use plant biomass to mitigate climate change. In their study they considered the total effect of all gases with climate impact, that is, nitrous oxide and methane in addition to carbon dioxide. Although humans release far smaller volumes of the former two gases than the latter, their harmful effect on the climate is considerably greater. The greenhouse effect of nitrous oxide is 256-times stronger than that of carbon dioxide, and that of methane is 28-times stronger.
Bioenergy and climate protection: the exception not the rule
When considered from this perspective, the authors of the study are critical of the climate-protecting effect of bioenergy. With bioenergy, energy is generated from plant biomass: the generation of heat using wood and the production of liquid biofuels belong to this category. This form of energy generation is considered sustainable and carbon dioxide neutral, as the amount of carbon dioxide released when biomass is burned is equal to that previously absorbed by the vegetation. However, this calculation is incomplete.
For example, the nitrogen fertilisers used to excess in the cultivation of sugar cane, rapeseed and other biofuel crops release large volumes of nitrous oxide. Moreover, forests and other ecosystems, which already store a lot of carbon dioxide, are often cleared to make way for the arable land needed to cultivate these crops. Furthermore, intensive farming can reduce the carbon content of the soil when plant residues, for example straw, are not left lying in the fields but are burned instead. It takes decades for the reductions achieved in relation to the use of fossil fuels to compensate for this damage.
Ernst-Detlef Schulze and Josep Canadell also view bioenergy as problematic due to its low efficiency: “Photosynthesis only uses 0.6 percent of the Sun’s energy to build biomass,” explains Schulze. “In comparison, a solar cell attains an energy yield of around 30 percent. The same volume of energy could be generated using photovoltaics on just a fraction of the land.”
Land management set to become more important
Bioenergy has one thing in common with almost all other biological measures for reducing the carbon dioxide content of the atmosphere: they all require more land and plants, which are often suitable for use as food, fuels, construction material or to conserve biodiversity.
Josep Canadell explains: “The land area that would be needed to fulfil all of the demands like food, wood production, energy requirement and the mitigation of climate change is three to seven times higher than that actually available on Earth. To meet this land and biomass requirement only in part, people would have to move to remote, almost inaccessible and less fertile regions.”
To reconcile all of these requirements in future, the authors believe that land management will become increasingly more important. “Sustainable, integrated land management without nitrous oxide and methane emissions is the only way that both objectives can be fulfilled: that is, the mitigation of climate change and the fulfilment of human needs,” says Ernst-Detlef Schulze.
He and Josep Canadell believe that successful land management is needed above all in the tropics. Here, as in many other regions on Earth, the possibilities for increasing harvest yields are not being exploited and forest area is being cleared instead. However, the controlled use of fertilisers to avoid excess nitrogen deposition and the associated nitrous oxide and other nitrogen oxide emissions is important for the sustainable intensification of biomass production. Nitrous oxide and methane emissions from agriculture and livestock breeding are currently obliterating the climate-protecting effects of global vegetation, which removes large volumes of carbon dioxide annually from the atmosphere.