Impact of global forestry and livestock grazing on carbon budget much larger and older than previously thought

Human biomass utilization led to a global reduction of the carbon stored in vegetation by 50%, and thus to massive emissions of CO2. The use of forests and natural grasslands for forestry and livestock grazing has similar impacts as deforestation for agricultural purposes. This is the result of a study recently published in "Nature", led by Karl-Heinz Erb from the Institute of Social Ecology of the Alpen-Adria-Universität Klagenfurt and co-authored by Julia Pongratz from the Max Planck Institute for Meteorology.

Carbon stocks in vegetation play a key role in the climate system. Previous studies estimated the global amount of vegetation carbon at about 450-650 billion tonnes of carbon, which is about two-thirds of what the atmosphere contains. Nevertheless, significant characteristics of the vegetation have hardly been investigated so far. In particular the impact of different forms of land use on the amount of stored carbon is poorly understood. While research has made significant progress in quantifying deforestation in recent years, other effects and forms of land use have largely remained ignored. The new study tries to close this knowledge gap.

Compiling various recent datasets, the authors find that the vegetation of global land ecosystems currently stores around 450 billion tonnes of carbon. In a hypothetical world without land use, vegetation would store 916 billion tonnes of carbon - twice as much.

Slightly more than half of the 466 billion tonnes difference can be attributed to deforestation and other land-cover changes, mostly for agricultural purposes. The other half of the carbon loss stems from the use of ecosystems that barely change the land cover, i.e. forest management and grazing of natural grasslands. Two-thirds of their contribution is attributable to forest use and one third to livestock grazing.

These "management effects", i.e. the reduction of carbon stocks within an ecosystem type, have been drastically underestimated to date and were therefore hardly considered in global studies and models. The results of the new study show, however, that the effects of forestry and livestock grazing are significantly greater than expected. This means that stopping deforestation is essential but by itself would not be enough to mitigate climate change.

Fig. 1: The human impact on biomass. This map shows by how much vegetation carbon stocks that would be expected in the absence of human interference have been reduced by land use. Erb et al., 2017: Unexpectedly large impact of forest management and grazing on global vegetation biomass. Nature.

The comparison with existing data on the current global carbon cycle also suggests that this massive effect is much older than previously thought. A significant fraction of the stock reduction already took place prior to 1800, i.e. before industrialization started the large-scale burning of fossil fuels. This is relevant because it illustrates that biomass utilization is not necessarily climate-neutral, as is often assumed. It can be accompanied by considerable CO2 emissions.

The study also points to serious knowledge gaps and data uncertainties that are directly relevant for the development of land-use strategies to mitigate climate change. The current data basis allows for robust estimates of biomass increases in forests, as are for instance caused by afforestation, to be verified with sufficient certainty only in temperate climate zones. However, the potentially achievable effects are low there, whereas the greatest potentials are to be expected in the tropical forests, where large uncertainties hinder the verification of such efforts.

The study shows that land use strategies to tackle or mitigate climate change require a prudent and cautious approach. Overly simple strategies may lead to negative climate effects due to the great uncertainties or the hitherto neglected systemic effects between biomass utilization and biomass stock reductions.

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, Advance Online Publication, doi: 10.1038/nature25138.

Dr Julia Pongratz
Max Planck Institute for Meteorology
Phone: +49 40 41173 255
Email: julia.pongratz@we dont want


Institut für Soziale Ökologie, Alpen-Adria Universität Klagenfurt-Vienna-Graz, Schottenfeldgasse 29, A-1070 Wien

Senckenberg Biodiversity and Climate Research Centre (SBiK-F), Senckenberganlage 25, 60325 Frankfurt am Main, Germany.

Division of Conservation Biology, Vegetation Ecology and Landscape Ecology, University of Vienna, Rennweg 14, 1030 Vienna, Austria.

Max Planck Institut für Biogeochemie, Hans-Knöll-Strasse 10, 07745 Jena, Germany.

Departamento de Ciências e Engenharia do Ambiente, DCEA, Faculdade de Ciências e Tecnologia (FCT), Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.

Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, Svante Arrhenius väg 8, 10691 Stockholm, Sweden.

Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden.

Department of Ecological Sciences, Vrije Universiteit Amsterdam, Amsterdam 1081 HV, The Netherlands.