Will Land-Use Mitigation Contribute as Expected to Achievement of Paris Agreement Goals?
Land use: Its role in the carbon budget
It is well known that land use is one of the principle sources of anthropogenic carbon dioxide (CO2) and other greenhouse gases (GHGs). The Fifth Assessment Report of the Intergovernmental Panel on Climate Change estimated that annual GHG flux from land use and land-use change activities accounted for approximately 4.3–5.5 gigatons of CO2 equivalent per year (GtCO2-eqyr-1), or about 9–11 percent of total anthropogenic greenhouse gas emissions.1 The latest global carbon budget2 estimated these emissions to be 4.9 ± 3.0 GtCO2-eqyr-1 for the decade 2007–16, about 12 percent of global emissions. But land use is a twofold opportunity as well, since a large carbon sink of about one-third of global sources (11.2 ± 3.0 GtCO2-eqyr-1) was also estimated. This global number and some recent estimates of global mitigation potential3 generated large expectations for the technical potential of enhancing the terrestrial sink while promoting other cobenefits. These "negative emissions" or "natural climate solutions" are expected to fulfill a substantial share of the mitigation gap of present nationally determined contributions (NDCs) in the 1.5°C4 and 2°C pathways. Whether or not the present sink will persist in the future and how the technical potential could be materialized are the greatest uncertainties in the future carbon cycle. Evidence of this can be found in the recent vigorous debate among scientists, with some foreseeing great potential for carbon sinks and others expressing considerable doubt.5
Land use and countries' contributions to the Paris Agreement (NDCs)
Although they have a much shorter-term vision, the bottom-up approach of NDCs, grounded in country leadership, was vital to the successful outcome at COP 21. More than 100 of the 187 countries that submitted their NDCs explicitly mentioned a mitigation role for the land-use, land-use change, and forestry (LULUCF) sector,6 expecting a significant contribution from land-use activities in their voluntary mitigation targets. A wide range of LULUCF mitigation options are being put forward by the Parties to reduce emissions and increase removals from this sector. The NDCs state options such as reducing deforestation, increasing afforestation, improving sustainable forest management, and enhancing forest carbon stock. According to a recent bottom-up exercise,7 the full implementation of announced NDCs would turn the LULUCF sector globally from a net source during 1990-2010 (1.3±1.1 GtCO2e yr-1) to a net sink by 2030 (-1.1±0.5 GtCO2e yr-1).
Land use: Filling the gap in the 1.5°C and 2°C global pathways?
Notwithstanding the NDCs that have been submitted, we are still far from on track to achieve the 1.5° and 2°C targets by 2050 and 2100. Recent work on pathways suggests that either goal will require dramatic transformations8 of not only energy systems but also the land sector (mitigation).9 Efforts in the land-use sector will need to include not only emission reductions but also substantial carbon removals.10 The main differences that can be seen in the pathways literature between the two targets is that 1.5°C scenarios require much earlier and more pronounced action. Therefore, the emissions reductions achieved in the next decade are critical to limiting warming to 1.5°C and will require more carbon removal annually from the land-use sector compared to 2°C scenarios. According to scenario modelers, this implies massive utilization of a land-use and bioenergy with carbon capture and storage (BECCS) technology with which we have little real-world experience and of whose economics, feasibility, and trade-offs we have poor knowledge. Acknowledging such concerns, some pathways avoid BECCS deployment through low energy demand and greater reliance on carbon removal measures related to the land sector. It is therefore critical that countries' choices and efforts in fulfilling their NDCs be designed to achieve the expected results by 2030, with the aim of increasing land-use sector contributions dramatically by 2050. This will require a long-term vision in land-use planning, enabling environments, well-designed incentive schemes, as well as strong governance and regulations as soon as possible in situations where this massive land-use contribution is expected.
As could be expected, the overall level of CO2 removals varies across pathways depending on mitigation choices. For example, the pathways vary in the relative contributions of BECCS and removals in the land sector. And the longer mitigation action is delayed, the lower the probability of delivering on the ambition of the Paris Agreement and the greater the need for removals from the land-use sector. This urgency implies certain priorities in the land-use sector measures, again making the reduction of deforestation a dramatic priority (in particular in countries with large stocks in the forest sector and high rates of deforestation). Despite the necessity of removals from the land sector, indicated by all pathways, our understanding of the effectiveness of land sector removals in reducing temperatures after they peak is limited. Also poorly understood are the impacts and potential negative feedbacks that peak temperatures can trigger in the sink capacity and the potential for reversals in terrestrial ecosystems.
Technically speaking, the land-use sector's potential to contribute to achievement of the Paris Agreement goals is huge. For example, recent studies11 advocate for 20 conservation, restoration, and improved land management actions in forests, wetlands, grasslands, and agricultural lands that could lead to a maximum potential (23.8 GtCO2e yr-1) 30 percent higher than previous estimates (11.3 GtCO2e yr-1). Furthermore, the papers claim that half of these actions are cost-effective climate mitigation (about US$100 tCO2e-1), representing 37 percent of the mitigation needed up to 2030, with one-third of this potential costing less than $10 tCO2e-1. Caution is necessary: sometimes global exercises can optimistically overestimate potential due to inadequate data, unrealistic assumptions, or insufficient resolution. However, even materializing a fraction of this potential would contribute substantially to global mitigation. What is preventing these potential mitigations from materializing? Are the estimates realistic? If they are, why have the mitigations not been delivered? How we can act now?
What prevents large-scale realization of land-use estimated potential mitigation?
Among the carbon-sequestering technologies being considered in the context of climate change mitigation, photosynthesis is assumed to be both the cheapest and the most efficient. The reduction of deforestation also was considered cheap and fast a decade ago. But the natural CO2 solutions differ widely in their maturity, potential, cost, and side effects. It is difficult to address, or even assess, those side effects and trade-offs at large regional scales, not to mention the global one. The implementation of land measures is highly dependent on local contexts and could be enhanced by the right combination of actions at smaller scales, rather than a single initiative at a large scale. Time and unsuccessful efforts in the last decade show that the feasibility of these measures depends on scale and their implications for land, water and energy use, as well as for ecosystem services such as biodiversity.
If the world is to achieve the United Nations Sustainable Development Goals (SDGs), a major challenge today and into the future will be to maintain or enhance the beneficial contributions of nature to the quality of life of all people without harming the earth system. New concepts and approaches are emerging in different research communities to transform old paradigms, such as the concept of ecosystem services popularized by the Millennium Ecosystem Assessment, into new ways of incorporating a range of perspectives, from those of social scientists to those of local practitioners. The land-use sector is one of the domains where innovative thinking on climate change mitigation and adaptation is most needed.
Integrated assessment models can be used and will continue to be used to link the emissions and climate targets of the Paris Agreement to the necessary transitions in the energy and land sectors. The climate change research community is moving toward including more bottom-up than top-down assessments while incorporating new ways to address cobenefits and trade-offs, for example by incorporating the Nature Contributions to People (NCP) approach by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES)12 and its reporting categories as a way to assess cobenefits and trade-offs.13 This is an important step forward, but more will be needed if we are to fully understand and address the land-use mitigation challenges, including the engagement of land practitioners and their knowledge and experiences
To make the right mitigation choices in the land sector, decisions need to be informed by adequate and timely information at the spatial scales where they are made. On the scientific side, there should be greater awareness of the information contained in national and subnational data, and of the possible uses of these resources. The scientific community could help document global land fluxes, including anthropogenic and nonanthropogenic emissions. Both inventory compilers and land practitioners have considerable scope to improve anthropogenic land-use-related estimates of GHG fluxes, which are fundamental to enhancing the understanding, supporting the implementation, and enabling the assessment of the effectiveness of countries' NDCs, and thus essential to ensuring that contributions to the Paris Agreement materialize. Developing transparent estimates from both scientific studies and GHG inventories by countries, understanding the reasons for discrepancies, and moving toward reconciling estimates is critical for the global stocktaking process and for increasing the level of ambition necessary to achieve the Paris Agreement goals.
In other words, global understanding of biophysical potential is important, but where and how the mitigation options are implemented matters. For example, although forest management was expected to slow global warming in Europe by increasing forest area by 10 percent and putting over 85 percent of forests under management, these efforts failed to result in "net" CO2 removal from the atmosphere. This was due to two factors: first, the release through wood extraction of carbon otherwise stored in the biomass, litter, dead wood, and soil carbon pools, and, second, the conversion of deciduous forests into coniferous forests, which resulted in changes in albedo, canopy roughness, and evapotranspiration from the land surface. As a result, European forest management contributed to warming rather than mitigating it.14 The lesson to be learned from this experience is that any climate framework that includes land management as a pathway for climate mitigation should account for not only land-cover changes but also the specific management systems, national policies, and laws that frame local implementation while recognizing and promoting effective tools and incentives.
Most promising land-use options and keys for success
The most promising potential contributions to mitigation include forest restoration and deforestation reduction, sustainable intensification of land-use practices, enhanced agricultural productivity, and demand-side options such as diet changes and reduced food waste. Such options are constrained by different institutional, environmental, economic, and sociocultural barriers, and not only by lack of access to appropriate technologies, practices, equipment, capacity building, or empirical site-specific research. Such institutional and governance issues are often not included in top-down modeling exercises. Understanding the integrative response of a combination of options available in a given context requires knowledge of its specific environmental conditions, social constraints and vulnerability, adaptive capacity, and available institutional support.
Actions to mitigate climate change are rarely evaluated in relation to their impact on adaptation, sustainable development goals, and trade-offs with food security. Considering benefits other than mitigation (resiliency to climate change, improved biodiversity and soil quality, etc.) when the options are selected and their implementation is designed could help overcome some of the constraints indicated above. Some of the most promising adaptation options for land and ecosystems include mitigation options such as conservation agriculture, efficient irrigation, efficient livestock, agroforestry, ecosystem restoration, deforestation reduction, and coastal protection with natural solutions.
For policymakers, the land-use sector represents an enormous opportunity. However, if the land sector is to contribute to achievement of the Paris Agreement goals, and compliance with the SDGs, this will require local engagement, and the creation of an environment in each specific context that enables barriers to implementation to be overcome. Land-use transformation required to limit warming will require integrative policies to sustainably manage competing demands on land for human settlement, food, livestock feed, fiber, bioenergy, carbon storage, biodiversity, and other ecosystem services. Strengthened governance and institutions and the choice of the right policies will be needed to address all of these issues. Are we providing policymakers and stakeholders with the tools they need? We will need to develop scalable integrated assessment models and use them as the analytical basis for holistic impact assessments capable of anticipating how different interventions may affect multiple services over space and time. These tools will be critical to the development of the long-term strategies required to maximize sustainable provision of all services, based on local capacities and aggregated to landscape, regional, or global scales. It is obvious that more transdisciplinary research is required to understand the interconnections of land with water, food, and energy.
1 P. Smith and M. Bustamante, "Agriculture, Forestry and Other Land Use (AFOLU)," in Mitigation of Climate Change: Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge: Cambridge University Press, 2014).
2 C. Le Quéré et al., "Global Carbon Budget 2017," Earth System Science Data 10 (2017): 405–48.
3 B.W. Griscom et al., "Natural Climate Solutions," PNAS 114, no. 44 (2017): 11645–50.
4 S. Roe et al., "Contribution of the Land Sector to a 1.5°C World," unpublished manuscript.
5 For an example of the former, see Griscom et al., "Natural Climate Solutions." For the latter, see C. Field and K. Mach, "Rightsizing Carbon Dioxide Removal," Science 356, no. 6339 (2017).
6 N. Forsell et al., "Assessing the INDCs' Land-Use, Land-Use Change, and Forest Emission Projections," Carbon Balance and Management 11, no. 26 (2016).
7 G. Grassi et al., "The Key Role of Forests in Meeting Climate Targets Requires Science for Credible Mitigation," Nature Climate Change 7, no. 3 (2017): 220–26.
8 J. Rockstrom et al., "A Roadmap for Rapid Decarbonization," Science 355, no. 6331 (2017).
9 J. Rogelj et al., "Scenarios towards Limiting Global Mean Temperature Increase below 1.5°C," Nature Climate Change 8 (2018): 325–32.
10 Roe et al., "Contribution of the Land Sector to a 1.5°C World."
11 E.g., Griscom et al., "Natural Climate Solutions."
12 S. Díaz et al., "Assessing Nature's Contributions to People: Recognizing Culture, and Diverse Sources of Knowledge, Can Improve Assessments," Science 359, no. 6373 (2018): 270–72.
13 Griscom et al., "Natural Climate Solutions"; G. Grassi et al., "Reconciling Global Model Estimates and Country Reporting of Anthropogenic Forest CO2 Sinks," Nature Climate Change (forthcoming).
14 K. Naudts et al., "Europe's Forest Management Did Not Mitigate Climate Warming," Science 351, no. 6273 (2016): 597–99.