ferent assumptions about food demand (tied to population growth and dietary trends), yield increases and maps of protected areas as well as unfavourable albedo changes therefore limit the space for BPs and hence, tCDR potentials. The option of assumed technological development could, however, increase these potentials again.
The final analysis focuses on the bioenergy potentials and trade-offs of the mitigation scenario RCP2.6 (Chapter 5) — the only mitigation scenario of IPCC AR5 staying below the 2◦C warming target. I reconstruct the land-use patterns of this commonly used scenario to investigate under what conditions and trade-offs the documented and published potentials (van Vuuren et al., 2011b) could be reached. This will give a broader view on the rather extreme cases investigated in the chapters before.
A summary and outlook finalise the study and put the results into perspective. This thesis does not intend to give advice about tCDR but rather could enlighten the public, political and scientific discourse on tCDR.
A separated chapter presents supplementary information (SI) produced in the course of this study. For example, analyses and data sets created within this study are presented that, in the end, did not directly contribute to the finalisation of this work but added to the work of colleagues. Also, being a part of a publicly funded German project, I con-tributed to the educational “Bildungswiki Klimawandel” of the German Bildungsserver (in German) for pupils and teachers which is also presented here.
1.7 The main methods used in this study
Here, I shortly describe the spatially explicit biogeochemical vegetation model LPJmL (see Fig. 1.4), which forms the basis of this study. So far, there are no large-scale field experiments on tCDR and, moreover, I focus on possible land transitions in the future.
Computer simulations and the analysis of large amounts of generated data are therefore inevitable. A thorough description of the model is included in all Chapters 2–4 and in the SI D.4.
Fig. 1.4 shows one exemplary grid cell with an extent of 0.5◦ x 0.5◦ including all major model features. Monthly fields of precipitation, temperature and cloudiness as well as annual data of CO2 concentration drive the dynamically simulated distribution of nine plant functional types (PFT) for the period of 1901–2005 (Ostberg et al., 2015).
Each PFT summarises the main attributes and processes related to a certain group of plants such as tropical evergreen trees or boreal needle leave trees. These PFTs compete for light, water and space. Water supply is offered by rainfall, reservoirs and a
Figure 1.4: Illustration of the main components of the dynamic global vegetation model LPJmL.
river routing system. The distribution and composition of twelve crop functional types (CFT) is prescribed in each grid cell and their yields calibrated with national FAO (Food and Agricultural Organization of the United Nations) data of 1995–2005 (Fader et al., 2010). Crop land can additionally be irrigated as described by Jägermeyr et al.
(2015). Two categories cover the representation of “other” food, nutrition and fibre plants (e.g. potatoes, citrus and cotton) and pastures. The distribution of herbaceous bioenergy plants (bioenergy grasses, BG) and woody bioenergy plants (bioenergy trees, BT) is prescribed as well, following specific scenarios designed in each separate study of this thesis. Simulations for the period 2005–2100 are driven by bias-corrected climate scenarios conducted for the coupled climate model intercomparison project phase three (CMIP3) (Heinke et al., 2013).
In a complex post-processing procedure with the open source software R (https://
cran.r-project.org/) tCDR potentials including conversion pathways are calculated and impacts of BPs analysed. I emphasise that the model does not simulate feedbacks of carbon and biogeophysical alterations between the land and the atmosphere. Therefore, tCDR potentials cannot change the atmospheric CO2 concentration and thus, GMT reduction potentials can only be approximated (Chapter 2). tCDR potentials rather represent the sole carbon extraction potential in years of emissions saved by 2100 (equal
1.7 The main methods used in this study 17
to a slow-down on the trajectory by the given years) or as the additional emissions that could be balanced as presented in Chapter 2 and 3.
19
2 Limited potential of terrestrial climate engineering to delay Earth’s
anthropogenic warming 1
1This chapter was submitted with modifications as: L. Boysen, W. Lucht, and D. Gerten and V. Heck (2016). “Limited potential of terrestrial climate engineering to delay Earth’s anthropogenic warming”. Earth’s Future.
Abstract
Even though parties agreed in the Paris climate accord to limit global warming to at most 2◦C above preindustrial level, it still cannot be precluded that greenhouse gas emissions might evolve along a worst-case, business-as-usual trajectory. Terres-trial Carbon Dioxide Removal (tCDR) through biomass plantations or afforestation has recently been debated as a ‘green’ climate engineering option to lower global mean temperature (GMT) in case of such failed mitigation, yet the potentials and the wider Earth-systemic side-effects of such measures remain poorly quantified. Based on spa-tially explicit simulations with an advanced biosphere model, we here systematically quantify the potentials of tCDR to balance continuing CO2 emissions (after a GMT rise by 2◦C will have been reached by mid-century) for a range of scenarios representing different assumptions about which areas are considered for conversion to tCDR plan-tations. We find that the ability of the biosphere to balance cumulative emissions on a business-as-usual emissions pathway (akin to the Representative Concentration Path-way [RCP] 8.5) is limited to 28–67 years even if major arable areas (4.3–7.4 Gha) were converted. Spatially less extensive conversions (1.1–1.5 Gha) could ‘delay’ unabated emissions by 13–16 years and, respectively, by 40–45 years on an alternative emissions pathway with partial mitigation (akin to RCP4.5). Besides this limited potential to counteract fossil fuel emissions, any such tCDR scenario would more or less severely compromise ecosystem functioning (e.g. loss of habitats) or food production (likely exceeding future yield increase projections). We conclude that large-scale tCDR is not a viable alternative to ambitious mitigation actions.