3.3Diversified, ecologically intensive agriculture worldwide secures food supplies, protects the climate, enables resilience and conserves biodiversity.
3.2
Upgrading protected-area systems and extending them to 30% of the land area prevents destruction of ecosystems.
help make land-based CO2 removal synergistic.
3.4
Diets that are low in animal products are an important lever for overcoming the trilemma.
3.5
Sustainable bioeconomy needs a limiting framework and prioritizes material usage cycles, e.g. timber-based construction.
Trilemma
Governance Multiple-benefit strategies LEGEND
46
patterns affects the use of cropland and grasslands and their cultivation, and this has considerable con-sequences for greenhouse-gas emissions, CO2 sinks and land-use changes, which in turn can affect bio-diversity. Section 3.4 discusses ways to promote sus-tainable dietary habits as a multiple-benefit strategy.
> Design the bioeconomy responsibly and promote tim-ber-based construction: Another focus concentrates on the sustainable, economic use of biomass that is not used for food. There is potential here for using bio-based products above all in material applications for climate-change mitigation, while at the same time an excessively high or growing demand for these raw materials could undermine food security and biodiversity conservation. Therefore, in Section 3.5, the WBGU proposes a suitable framework for the bioeconomy and features sustainable construc-tion with timber as an example of a multiple-benefit strategy.
Finally, Section 3.6 takes an overarching look at the interplay and implementation of the added-value strategies.
O C O
O C O O C O O
C O O
C
O CO2
Trilemma
Governance Multiple-benefit strategies LEGEND
Restoration of forest landscapes: targets, pledges and implementation (by comparison: 170m ha corresponds to four times the area of Germany).
While land degradation causes the release of CO2, ecosystem restoration can remove considerable quantities of CO2 from the atmosphere.
half of the implemented pledges consisted of plantations or monocultures low in biodiversity.
350m ha is the target for 2030 170m ha total pledges up until 2019 150m ha is the target for 2020 27m ha implemented by 2019 Land degradation Ecosystem
restoration
49 3.1
Ecosystem restoration: organize land-based CO2
removal in a synergistic way Measures for removing CO2 from the atmosphere are no substitute for a massive reduc-tion of CO2 emissions with the aim of cutting them to zero.
However, in order to reach the climate-protection goals of the Paris Agreement, such meas-ures can hardly be avoided,
although they involve considerable uncertainties depending on method, scope and implementation and can potentially increase the pressure on land. The WBGU recommends stepping up research on costs, feasibility, permanence and land-area potential, and making early use of the diverse additional benefits of low-risk ecosystem-based approaches like the restora-tion of degraded land.
The fact that progress on decarbonizing the global economy has hitherto been slow with ongoing rises in global CO2 emissions is making it less and less likely that the Paris Agreement’s climate-protection goals can be achieved solely by avoiding future greenhouse-gas emissions. A later, permanent removal of CO2 from the atmosphere will probably be necessary. In this context, the various options for removing CO2 from the atmos-phere are being discussed more and more intensively worldwide. Section 3.1.1 offers an overview of this dis-cussion, as well as of the various methods of CO2 removal, their stage of development and potential, but also the risks their use entails for land-based ecosys-tems and their wider concomitant effects. Relying on the possibility of removing large quantities of CO2 from the atmosphere in the future is a risky strategy from the point of view of climate protection. Many methods of CO2 removal still lack technical maturity at the pres-ent time. The mitigating effect of CO2 removal on climate change is also generally uncertain. Further-more, many approaches are land-based, especially those that are currently at the focus of attention such as afforestation or BECCS. They thus generate new claims on the use of land and land-based ecosystems; if applied on a correspondingly large scale, they threaten to cause major conflicts of use over land areas and eco-systems as outlined by the land-use trilemma described in Chapter 2. The consequence would be corresponding socio-economic risks and negative ecological effects, especially on forests, grasslands, wetlands or agricul-turally used areas.
Against this background, Section 3.1.2 develops principles for a sustainable strategic approach to CO2 removal options in climate policy in order to address and minimize the multiple risks. Firstly, climate-policy strategies should emphasize consistent, early avoidance of emissions in order to limit as far as possible the amount of CO2 removal that will be necessary in the future. Secondly, they should focus more on close-to-nature, ecosystem-based methods of CO2 removal, which not only bind CO2, but in particular promise diverse multiple benefits and synergies to mitigate the land-use trilemma.
In Section 3.1.3, the WBGU takes an in-depth look at the restoration of degraded terrestrial ecosystems such as forests, grasslands and wetlands as an ecosys-tem-based option for CO2 removal. This is a proven, low-risk and cost-effective option for removing CO2 from the atmosphere. Restoration promises multiple benefits in relation to the land-use trilemma and beyond, but its potential for removing CO2 from the atmosphere is limited in terms of quantity and perma-nence of storage. At the same time, restoration is cur-rently high on the international political agenda, as shown by the upcoming UN Decade on Ecosystem Res-toration. The WBGU is therefore convinced that, in view of the current political tailwind, restoration should become a promising component of international climate and sustainability policy.
3.1.1
CO2 sinks: the starting position
Scientific knowledge of the devastating consequences that unchecked climate change will have for humans and terrestrial ecosystems has been further intensified in recent years by reports from the Intergovernmental Panel on Climate Change and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPCC, 2018, 2019a, c; IPBES, 2018a, 2019a).
Even so, global CO2 emissions again reached a record high in 2019 (Peters et al., 2020; Friedlingstein et al., 2019; Jackson et al., 2019). In addition, the necessary tightening of the national climate pledges under the Paris Agreement already seemed a long way off even before the COVID-19 pandemic. Yet the international community agreed in the 2015 Paris Agreement to limit global warming to well below 2°C and pursue efforts towards limiting it to 1.5°C. If these goals are to be met, only a limited budget of a few hundred Gt CO2 will be available for CO2 emissions in the course of the 21st century. In 2018, according to the IPCC, these budgets for limiting global warming to 1.5°C amounted to 420 Gt CO2 (for a 66% probability of achieving the climate O C O corresponds to four times the area of Germany).
While land degradation causes the release of CO2, ecosystem restoration can remove considerable quantities of CO2 from the atmosphere.
half of the implemented pledges consisted of plantations or monocultures low in biodiversity.
350m ha is the target for 2030 170m ha total pledges up until 2019 150m ha is the target for 2020 27m ha implemented by 2019 Land degradation Ecosystem
restoration
50
policy target), or 580 Gt CO2 (for a 50% probability).
For a 2°C limit, figures of 1,170 Gt CO2 (66%) or 1,500 Gt CO2 (50%) are mentioned (IPCC, 2018:108). It should be noted that these budgets are likely to be even smaller today due to the emissions of recent years.
However, they are subject to considerable uncertainty, e.g. because of possible repercussions in the Earth sys-tem that cannot be precisely predicted, or the effects of emissions of greenhouse gases other than CO2. Never-theless, a mere glance at the scale of these budgets compared to today’s annual emissions of about 42 Gt CO2 (Section 2.2.1) is enough to illustrate the cli-mate-policy challenge. This is all the more true since a wide range of socio-economic path dependencies, needs for adaptation, and distributional challenges involving corresponding political resistance indisput-ably stand in the way of large reductions in emissions in the short term.
The possible removal of CO2 from the atmosphere raises some hopes of resolving this growing discrep-ancy between the ambitious and even strengthened climate-policy goals and the lack of progress on cli-mate-change mitigation. This possibility and the differ-ent approaches to CO2 removal have been discussed scientifically since the early 1990s (Minx et al., 2017).
Following their increasingly prominent role in the reports of the IPCC since its fourth Assessment Report in 2007, they have also become more and more visible to the broader public. However, they cannot and should not be seen as a reason to reduce the pressure for action in climate policy. This is clearly shown by the following overview of the stage of development of the main methods of CO2 removal and the risks associated with their use, in terms of both climate-change mitigation and conflicts as outlined in the land-use trilemma (Sec-tion 2.2). Nevertheless, for precau(Sec-tionary reasons in view of the risks of climate change, methods for CO2 removal from the atmosphere should be further devel-oped and their sustainable potentialexplored early on.
3.1.1.1
CO2 removal from the atmosphere: concept and definition
Methods to remove CO2 from the atmosphere basically involve two steps: (1) capturing and removing CO2 from the atmosphere, either via the (targeted or accelerated) growth of biomass, by natural inorganic reactions, or using technical processes; and (2) storing the carbon in either ecosystems, terrestrial and marine biomass, min-eral compounds, or in gaseous form as CO2 in geological formations (The Royal Society, 2018:20f.). There is no uniform or recognized categorization of the different procedures. For example, processes can be distin-guished according to the form of carbon storage or
whether they are terrestrial or marine (Minx et al., 2018). The latter, i.e. processes for injecting CO2 into the ocean, will not be considered in the following, espe-cially since they are not compatible with agreements under international law (WBGU, 2013:39). Following Field and Mach (2017), a rough distinction can be made between (1) ecosystem-based processes for enriching carbon in ecosystems, for example through restoration or more sustainable management, (2) biological-techni-cal procedures that use natural processes or biomass and combine them subsequently with technical solu-tions, and (3) purely technical solutions.
Ecosystem-based (and in some cases biological-tech-nical) methods for CO2 removal ultimately use or enhance processes that already occur in the natural car-bon cycle (Section 2.2.1), such as photosynthesis (Rickels et al., 2019:150). They should, however, be clearly distinguished from the natural carbon cycle.
Oceans and the terrestrial biosphere already directly absorb over half of anthropogenic CO2 emissions today, leaving only about 45% of emissions in the atmo-sphere; in the long term, the ocean absorbs an even higher proportion of emissions (Section 2.1.1; Friedling-stein et al., 2019). By contrast, the removal of CO2 from the atmosphere is a deliberate human effort to with-draw more CO2 from the atmosphere than would other-wise happen in the natural carbon cycle (Minx et al., 2018). The removal of CO2 from the atmosphere (car-bon dioxide removal or CDR) is often referred to as
‘negative emissions’ and corresponding technologies or approaches as ‘negative emission technologies’ (NET), since the human-initiated removal of CO2 from the atmosphere can be seen as the opposite of an emission.
Misunderstandings can arise here: e.g. when distin-guishing between negative emissions that offset resid-ual emissions, and system-wide ‘net’ negative emis-sions, which only arise if more CO2 is removed from the atmosphere than is emitted into it over a certain period, e.g. one year. Moreover, calling emissions ‘negative’
can be misleadingly related to a valuation rather than to their effect on the emissions balance. To avoid such ambiguities and misunderstandings, the WBGU refers in the following to methods of removing CO2 from the atmosphere or, more succinctly, to CO2 removal.
CO2-removal should furthermore be clearly distin-guished from approaches of solar radiation manage-ment (SRM). The latter neither address the CO2 concen-tration in the atmosphere as the real cause of climate change, nor are they suitable for mitigating effects of the accumulation of CO2 in the atmosphere that go beyond the greenhouse effect, such as the acidification of the oceans. The use of SRM approaches involves much greater uncertainties and risks, different time scales of (climate) impacts, and different governance
51 requirements (Minx et al., 2018; Bellamy and Geden,
2019). The WBGU therefore rejects the use of SRMs on principle (WBGU, 2016a).
3.1.1.2
Land-based approaches for CO2 removal:
technologies, potential, concomitant effects A broad portfolio of approaches for removing CO2 from the atmosphere is currently under discussion. They dif-fer in terms of technical maturity, costs and potential.
Apart from protecting the climate, they also have dif-ferent ecological and socio-economic effects, both pos-itive and negative. These are often referred to in the literature as side effects or concomitant effects and, at their core, they also relate to the conflicts summarized in Chapter 2 as the land-use trilemma insofar as they originate from new claims on land and terrestrial eco-systems. Finally, there are differences between the dif-ferent approaches to CO2 removal relating to the relia-bility and permanence of the carbon storage achieved and thus the long-term climate-change-mitigation effect. More in-depth accounts of the different approaches can be found in several recent review stud-ies (Fuss et al., 2018; National Academstud-ies of Sciences, Engineering, and Medicine, 2019). The following section presents and classifies the key land-based approaches and their main features. In addition, Table 3.1-1 summarizes the scientific evidence from the detailed overview study by Fuss et al. (2018). Some of the information on potential given there paints a more pessimistic picture than more recent estimates, such as those included in the overview of the IPCC special report on climate and land (Smith et al., 2019b) and also taken up below. By contrast to the figures on potential in Table 3.1-1, however, it is not always clear in these estimates to what extent socio-economic con-straints and other considerations of sustainable devel-opment have been taken into account.
Overview of CO2-removal methods
Afforestation of non-forested areas and reforestation of deforested land are key ecosystem-based approaches that are currently being intensively discussed. Atmos-pheric CO2 is fixed during tree growth and stored in the form of biomass in the tree population or in wood prod-ucts (Section 3.5.3). The distinction between afforesta-tion and reforestaafforesta-tion is blurred and depends partly on the time horizons assumed (Section 3.1.3.2). For this reason, Table 3.1-1 aggregates afforestation and reforestation. In principle, (re)afforestation offers sub-stantial potential for CO2 removal, although the litera-ture reveals a wide spectrum: according to the IPCC, the potential of afforestation lies in the range of 0.5–8.9 Gt CO2 per year, of reforestation between 1.5 and 10.1 Gt
CO2 per year (Smith et al., 2019b:585). Table 3.1-1 also shows similarly wide ranges. In addition to afforesta-tion, adjusting forestry practices, i.e. improved forest management, can contribute to increased carbon sequestration in forests and forest soils. Here too, the estimated ranges of potential are considerable, e.g. 0.4–
2.1 Gt CO2per year (Smith et al., 2019b:585) or 1.1–9.2 Gt CO2per year (National Academies of Sciences, Engi-neering, and Medicine, 2019:110).
Reforestation (unless implemented as monocultures) and in some cases improved forest management are forms of ecosystem restoration. Restoration aims to restore ecosystem services (Chapter 2) and includes a higher sequestration of carbon in the biosphere as one of its added values. In addition to forest-related resto-ration, the restoration of wetlands, for example, distrib-uted across all climate zones, also promises relevant global sequestration potential of about 1.7 Gt CO2eq per year (freshwater: 0.82 Gt CO2eq per year; coastal wetlands: 0.84 Gt CO2eq per year; Smith et al., 2019a:261). The restoration of ecosystems as a strategy for sustainable land management is examined in more detail in Section 3.1.3.
Soil carbon sequestration uses a range of different land-use practices such as adapted harvest cycles, water management and nutrient management or, more generally, the conversion of agricultural methods to increase the carbon content of the soil and thus remove CO2 from the atmosphere (Section 3.3.2.5). Corre-sponding practices can be applied to different kinds of land such as pasture, farmland or even forest land. The IPCC, for example, estimates their potential within a wide range of 0.4–8.64 Gt CO2 per year (Jia et al., 2019:192); even higher estimates of the potential are sometimes found in the literature (Table 3.1-1). Soil carbon sequestration not only contributes to binding atmospheric CO2 in the soil, it also improves overall soil quality and health, i.e. soil nutrient richness, and reduces its susceptibility to erosion (Smith et al., 2019a:
264).
The most commonly discussed biological-technical approaches include BECCS, biochar application to soils, and enhanced weathering. BECCS, or bioenergy with CCS (Box 3.5-3), uses technical processes to capture CO2 from the emissions released during the use of bio-mass for heat or electricity generation and store it in geological formations (carbon dioxide capture and stor-age, CCS). In extreme cases, (technical) estimates of BECCS’s potential can reach up to about 85 Gt CO2per year (Table 3.1-1), but are considerably lower when targets for sustainable development are taken into account. According to the IPCC, between 0.4 and about 12 Gt CO2 per year is a realistic range (Jia et al., 2019:193); Fuss et al. (2018) limit the sustainable
52
Table 3.1-1
Overview of examples of different land-based CO2-removal methods based on Fuss et al. (2018). Data on potential and costs correspond to the authors' estimates in the overview study; the data in square brackets reflect the full range of estimates from the literature evaluated there. More recent estimates, such as those reported by Smith et al. (2019b) and taken up in part below, expect even greater potential in some cases. However, it is not always clear to what extent socio-economic constraints and sustainability criteria are taken into account in these estimates in the same way as in Fuss et al. (2018).
CO2-removal method Annual potential [Gt CO2/year]
Costs [US$ 2011/t CO2]
Land-use implications and concomitant
effects Permanence, saturation, upscaling,
etc.
>Economic development opportunities through technology transfers and integra-tion into biomass producintegra-tion (distribuintegra-tion effects are ambiguous, however) Negative
>Climate effects: albedo changes due to biomass cultivation, (in)direct GHG emis-sions due to induced land-use changes
>Land-use conflicts and possible exacerba-tion of the land-use trilemma: threats to food security (price increases), deforesta-tion and degradadeforesta-tion of forests, loss of biodiversity
>Possible health effects
>Soil and (ground-) water pollution due to increased fertilizer use
>Potentially high permanence of geo-logical storage
>Geological deposits limited, but glob-ally sufficient even for 1.5°C scenarios
>Main limiting factors are biomass and land: bioenergy potential in the litera-ture 60–1,548 EJ per year (where 1 EJ of bioenergy corresponds to approx.
0.02-0.05 Gt CO2 of negative emis-sions)
>Upscaling: development of infrastruc-ture for C transport and storage
DACCS
>In some cases improvement of indoor air quality (depending on the process)
>Only limited direct land-use conflicts
>Only limited direct land-use conflicts