Quantifying and mapping the land-use impacts on global biomass stocks

The paper can be found here (nature): http://rdcu.be/CSg9

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We, here at the Institute of Social Ecology in Vienna, are passionately devoted to advancing the understanding of the mutual interrelations between social and natural processes. We are intrigued by assessing the magnitude of environmental impacts of land use on ecosystem functioning, and their role for socioecological dynamics. For us, understanding the ways humans intervene in ecosystems is key for informing political processes such as the assessment of climate change in the IPCC or international efforts to reach the Sustainable Development Goals.

In this paper we aimed at mapping and quantifying the impact of humans on the amount of biomass stored in vegetation. Biomass plays a key role in the global climate system. It is responsible for the largest single carbon flow between the atmosphere and the biosphere, and it stores large amounts of carbon, about one half of the carbon stored in the atmosphere. When humans use land, for instance to produce food, fibre or bioenergy, they generally reduce carbon stocks. Most prominently, humans replace forests, which contain a lot of carbon, with other ecosystems that hold much less carbon per unit area in their biomass, e.g. pastures or cropland. These land-use induced carbon losses play an important role for global warming, as they have been estimated to contribute one third to the total cumulative anthropogenic carbon emissions between 1870 and 2016. Furthermore, raising biomass production for energy provision, for instance coupled with carbon capture and sequestration (BECCS), holds high hopes to establish a “carbon negative” energy system – a necessity for most future scenarios that aim at reaching the 2°C target of the Paris Agreement.

Having this pivotal interlinkage between land use and biomass in mind, we were utterly surprised by a severe data and knowledge gap. The effect of deforestation was relatively well described in the literature. But what about the impact of other land uses, such as grazing or forestry, on biomass stocks? This became a burning question for us. In previous research, we had developed an approach allowing to tackle this question. Comparing the actually prevailing state of ecosystems with the potential natural vegetation, i.e. the vegetation that would be present in the hypothetical absence of land use but with current climate, allows us toisolate the effect of individual land use types on vegetation.

Now, we planned to do the same for biomass stocks. We started collecting data to come up with consistent accounts of the actual and potential biomass stocks of the Earth. However, while doing so we were more and more astonished by the many inconsistencies and intricacies we encountered. We realized, for example, that there is a surprising tendency to confuse potential with actual biomass stocks in the literature, mainly in ecological textbooks and databases. However, forest inventories convincingly illustrate what forest scientists accept as a fundamental insight: most actual forests are far below climax carbon stocks, mainly because a relatively large proportion of old-growth trees are replaced by younger, fast-growing trees. We also found severe data gaps, e.g. related to “other wooded lands”, a land cover category covering many ecosystems and large areas, but remaining almost undocumented. And we encountered severe inconsistencies, e.g. between different datasets on the actual biomass carbon stocks. This was surprising to us, as - for example - the difference between two prominent datasets amounted to no less than 80 PgC in the global total. This is a massive amount, i.e. roughly on fifth to one quarter of the amount of carbon currently stored in forests, and about ten times the current fossil fuel emissions. These problems in existing datasets made us realize that it is high-time to systematically analyse the impact of land use on global biomass stocks, based not on one, but on several databases.

Writing the paper and doing the calculations involved a lot of teamwork, as it needs expertise as well as datasets from various, highly specialized scientific fields. Consistently collating, analyzing and interpreting datasets of heterogeneous origin required cooperation across the “great divide” of natural and social sciences and across methodological approaches (data people, modelers, and remote sensing folks), communities that rarely cooperate and scarcely even communicate. But our group was different. We were able to assemble experts from various fields and approaches, and we managed to build upon a good working practice and a tradition of trust, developed and proven in previous projects. This cooperation was the key to keep an open, critical mind, to leave the beaten tracks.

The road to getting the paper published was long, but the results were worth the effort: We found that the biomass and thus carbon stock changes induced by land use were considerably larger than suggested by most existing studies. And, our results suggests that, when contextualized with the current knowledge on the global carbon balance, a significant land-use impact on biomass stocks occurred before the era of industrialization.

Our finding has direct implications for climate mitigation strategies: First, it illustrates that mitigating climate change needs to shift from an exclusive focus on the conservation of forests areas to a stronger consideration of biomass stocks, within and outside forests. And second, it suggests that models of the future carbon cycle need to explicitly reflect the full effects of land management on biomass stocks, not only those associated with deforestation. Excluding changes in carbon stocks not associated with deforestation, e.g. increased intensity of forest management or more intensive use of grasslands and savannas, would jeopardize the robust design of effective climate protection strategies.

Karl-Heinz Erb, Christian Lauk, Helmut Haberl

Karlheinz Erb

Professor, Institute of Social Ecology, Vienna