Editing IPCC:AR6/WGIII/Chapter-7 (section)

==== 7.6.4.3 Ecological Barriers and Opportunities ====

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'''Availability of land and water.''' Climate mitigation scenarios in the two recent special reports (SR1.5 and SRCCL) that aim to limit global temperature increase to 2°C or less involve carbon dioxide (CO 2 ) removal from the atmosphere. To support large-scale CDR, these scenarios involve significant land-use change, due to afforestation/reforestation, avoided deforestation, and deployment of biomass energy with carbon capture and storage (BECCS). While a considerable amount of land is certainly available for new forests or new bioenergy crops, that land has current uses that will affect not only the costs, but also the willingness of current users or owners, to shift uses. Regions with private property rights and a history of market-based transactions may be the most feasible for land-use change or land management change to occur. Areas with less secure tenure or a land market with fewer transactions in general will likely face important hurdles that limit the feasibility of implementing novel nature-based solutions.

Implementation of nature-based solution may have local or regionally important consequences for other ecosystem services, some of which may be negative ( ''high confidence'' ). Land-use change has important implications for the hydrological cycle, and the large land-use shifts suggested for BECCS when not carried out in a carefully planned manner, are expected to increase water demands substantially across the globe ( [[#Stenzel--2019|Stenzel et al. 2019]] ; [[#Rosa--2020|Rosa et al. 2020]] ). Afforestation can have minor to severe consequences for surface water acidification, depending on site-specific factors and exposure to air pollution and sea-salts ( [[#Futter--2019|Futter et al. 2019]] ). The potential effects of coastal afforestation on sea-salt related acidification could lead to re-acidification and damage on aquatic biota.

'''Specific soil conditions, water availability, GHG emission-reduction potential as well as natural variability and resilience.''' Recent analysis by ( [[#Cook-Patton--2020|Cook-Patton et al. 2020]] ) illustrates large variability in potential rates of carbon accumulation for afforestation and reforestation options, both within biomes/ecozones and across them. Their results suggest that while there is large potential for afforestation and reforestation, the carbon uptake potential in land-based climate change mitigation efforts is highly dependent on the assumptions related to climate drivers, land use and land management, and soil carbon responses to land-use change. Less analysis has been conducted on bioenergy crop yields, however, bioenergy crop yields are also likely to be highly variable, suggesting that bioenergy supply could exceed or fall short of expectations in a given region, depending on site conditions.

The effects of climate change on ecosystems, including changes in crop yields, shifts in terrestrial ecosystem productivity, vegetation migration, wildfires, and other disturbances also will affect the potential for AFOLU mitigation. Climate is expected to reduce crop yields, increase crop and livestock prices, and increase pressure on undisturbed forest land for food production creating new barriers and increasing costs for implementation of many nature-based mitigation techniques ( ''medium confidence'' ) (IPCC AR6 WGII Chapter 5).

The observed increase in the terrestrial sink over the past half century can be linked to changes in the global environment, such as increased atmospheric CO 2 concentrations, N deposition, or changes in climate ( [[#Ballantyne--2012|Ballantyne et al. 2012]] ), though not always proven from ground-based information (Vandersleen et al. 2015). While the terrestrial sink relies on regrowth in secondary forests ( [[#Houghton--2017|Houghton and Nassikas 2017]] ), there is emerging evidence that the sink will slow in the Northern Hemisphere as forests age ( [[#Nabuurs--2013|Nabuurs et al. 2013]] ), although saturation may take decades ( [[#Zhu--2018|Zhu et al. 2018]] ). Forest management through replanting, variety selection, fertilisation, and other management techniques, has increased the terrestrial carbon sink over the last century ( [[#Mendelsohn--2019|Mendelsohn and Sohngen 2019]] ). Saturation of the sink in situ may not occur when, for example, substitution effects of timber usage are also considered.

Increasing concentrations of CO 2 are expected to increase carbon stocks globally, with the strongest effects in the tropics ( [[#Schimel--2015|Schimel et al. 2015]] ; [[#Kim--2017a|Kim et al. 2017a]] ) (IPCC AR6 WGII Chapter 5) and economic models suggest that future sink potential may be robust to the impacts of climate change ( [[#Tian--2018|Tian et al. 2018]] ). However, it is uncertain if this large terrestrial carbon sink will continue in the future ( [[#Aragão--2018|Aragão et al. 2018]] ), as it is increasingly recognised that gains due to CO 2 fertilisation are constrained by climate and increasing disturbances ( [[#Schurgers--2018|Schurgers et al. 2018]] ; [[#Duffy--2021|Duffy et al. 2021]] ) (IPCC AR6 WGII Chapter 5). Further, negative synergies between local impacts like deforestation and forest fires may interact with global drivers like climate change and lead to tipping points ( [[#Lovejoy--2018|Lovejoy and Nobre 2018]] ). Factors that reduce permanence or slow forest growth will drive up costs of forest mitigation measures, suggesting that climate change presents a formidable challenge to implementation of nature-based solutions beyond 2030 ( ''hi'' ''gh confidence'' ).

In addition to climate change, [[#Dooley--2018|Dooley and Kartha (2018)]] also note that technological and social factors could ultimately limit the feasibility of agricultural and forestry mitigation options, especially when deployed at large scale. Concern is greatest with widespread use of bioenergy crops, which could lead to forest losses ( [[#Harper--2018|Harper et al. 2018]] ). Deployment of BECCS and forest-based mitigation can be complementary ( [[#Favero--2017|Favero et al. 2017]] ; [[#Baker--2019|Baker et al. 2019]] ), but inefficient policy approaches could lead to net carbon emissions if BECCS replaces high-carbon content ecosystems with crops.

'''Adaptation benefits and biodiversity conservation.''' Biodiversity may improve resilience to climate change impacts as more-diverse systems could be more resilient to climate change impacts, thereby maintaining ecosystem function and preserving biodiversity ( [[#Hisano--2018|Hisano et al. 2018]] ). However, losses in ecosystem functions due species shifts or reductions in diversity may impair the positive effects of biodiversity on ecosystems. Forest management strategies based on biodiversity and ecosystems functioning interactions can augment the effectiveness of forests in reducing climate change impacts on ecosystem functioning ( ''high confidence'' ). In spite of the many synergies between climate policy instruments and biodiversity conservation, however, current policies often fall short of realising this potential ( [[#Essl--2018|Essl et al. 2018]] ).

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