Editing IPCC:AR6/SRCCL/TS (section)

=== Consequences for the climate system of land-based adaptation and mitigation options, including carbon dioxide removal (negative emissions) ===

'''About one-quarter of the 2030 mitigation pledged by countries in their initial Nationally Determined Contributions (NDCs) under the Paris Agreement is expected to come from land-based mitigation options (''medium confidence'').''' Most of the NDCs submitted by countries include land-based mitigation, although many lack details. Several refer explicitly to reduced deforestation and forest sinks, while a few include soil carbon sequestration, agricultural management and bioenergy. Full implementation of NDCs (submitted by February 2016) is expected to result in net removals of 0.4–1.3 GtCO<sub>2</sub> y<sup>–1 </sup>in 2030 compared to the net flux in 2010, where the range represents low to high mitigation ambition in pledges, not uncertainty in estimates (''medium confidence''). {2.6.3}

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'''Figure TS.5 | Mitigation potential of response options in 2020–2050, measured in GtCO2-eq yr–1, adapted from Roe et al. (2017).'''<br>
'''Figure TS.5 (continued):''' Mitigation potentials reflect the full range of low to high estimates from studies published after 2010, differentiated according to technical (possible with current technologies), economic (possible given economic constraints) and sustainable potential (technical or economic potential constrained by sustainability considerations). Medians are calculated across all potentials in categories with more than four data points. We only include references that explicitly provide mitigation potential estimates in CO2-eq yr–1 (or a similar derivative) by 2050. Not all options for land management potentials are additive, as some may compete for land. Estimates reflect a range of methodologies (including definitions, global warming potentials and time horizons) that may not be directly comparable or additive. Results from IAMs are shown to compare with single option ‘bottom-up’ estimates, in available categories from the 2°C and 1.5°C scenarios in the SSP Database (version 2.0). The models reflect land management changes, yet in some instances, can also reflect demand-side effects from carbon prices, so may not be defined exclusively as ‘supply-side’.
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'''Several mitigation response options have technical potential for &gt;3 GtCO<sub>2</sub>-eq yr<sup>–1</sup> by 2050 through reduced emissions and Carbon Dioxide Removal (CDR) (''high confidence''), some of which compete for land and other resources, while others may reduce the demand for land (''high confidence'').''' Estimates of the technical potential of individual response options are not necessarily additive. The largest potential for reducing AFOLU emissions are through reduced deforestation and forest degradation (0.4–5.8 GtCO<sub>2</sub>-eq yr<sup>–1</sup>) (''high confidence''), a shift towards plant-based diets (0.7–8.0 GtCO<sub>2</sub>-eq yr<sup>–1</sup>) (''high confidence'') and reduced food and agricultural waste (0.8–4.5 CO<sub>2</sub>-eq y<sup>r–1</sup>) (''high confidence''). Agriculture measures combined could mitigate 0.3–3.4 GtCO<sub>2</sub>-eq yr<sup>–1 </sup>(''medium confidence''). The options with largest potential for CDR are afforestation/reforestation (0.5–10.1 CO<sub>2</sub>-eq yr<sup>–1</sup>) (''medium confidence''), soil carbon sequestration in croplands and grasslands (0.4–8.6 CO<sub>2</sub>-eq yr<sup>–1</sup>) (''high confidence'') and Bioenergy with Carbon Capture and Storage (BECCS) (0.4–11.3 CO<sub>2</sub>-eq yr<sup>–1</sup>) (''medium confidence''). While some estimates include sustainability and cost considerations, most do not include socio-economic barriers, the impacts of future climate change or non-GHG climate forcings. {2.6.1}

'''Response options intended to mitigate global warming will also affect the climate locally and regionally through biophysical effects (''high confidence'').''' Expansion of forest area, for example, typically removes CO<sub>2</sub> from the atmosphere and thus dampens global warming (biogeochemical effect, ''high confidence''), but the biophysical effects can dampen or enhance regional warming depending on location, season and time of day. During the growing season, afforestation generally brings cooler days from increased evapotranspiration, and warmer nights (''high confidence''). During the dormant season, forests are warmer than any other land cover, especially in snow-covered areas where forest cover reduces albedo (''high confidence''). At the global level, the temperature effects of boreal afforestation/reforestation run counter to GHG effects, while in the tropics they enhance GHG effects. In addition, trees locally dampen the amplitude of heat extremes (''medium confidence''). {2.5.2, 2.5.4, 2.7, Cross-Chapter Box 4 in Chapter 2}

'''Mitigation response options related to land use are a key element of most modelled scenarios that provide strong mitigation, alongside emissions reduction in other sectors (''high confidence''). More stringent climate targets rely more heavily on land-based mitigation options, in particular, CDR (''high confidence'').''' Across a range of scenarios in 2100, CDR is delivered by both afforestation (median values of –1.3, –1.7 and –2.4 GtCO<sub>2</sub>yr<sup>–1 </sup>for scenarios RCP4.5, RCP2.6 and RCP1.9 respectively) and BECCS (–6.5, –11 and –14.9 GtCO<sub>2</sub> yr<sup>–1 </sup>respectively). Emissions of CH<sub>4</sub> and N<sub>2</sub>O are reduced through improved agricultural and livestock management as well as dietary shifts away from emission-intensive livestock products by 133.2, 108.4 and 73.5 MtCH<sub>4</sub> yr<sup>–1</sup>; and 7.4, 6.1 and 4.5 MtN<sub>2</sub>O yr<sup>–1 </sup>for the same set of scenarios in 2100 (''high confidence''). High levels of bioenergy crop production can result in increased N<sub>2</sub>O emissions due to fertiliser use. The Integrated Assessment Models that produce these scenarios mostly neglect the biophysical effects of land-use on global and regional warming. {2.5, 2.6.2}

'''Large-scale implementation of mitigation response options that limit warming to 1.5 or 2°C would require conversion of large areas of land for afforestation/reforestation and bioenergy crops, which could lead to short-term carbon losses (''high confidence).''' The change of global forest area in mitigation pathways ranges from about –0.2 to +7.2 Mkm<sup>2 </sup>between 2010 and 2100 (median values across a range of models and scenarios: RCP4.5, RCP2.6, RCP1.9), and the land demand for bioenergy crops ranges from about 3.2 to 6.6 Mkm<sup>2 </sup>in 2100 (''high confidence''). Large-scale land-based CDR is associated with multiple feasibility and sustainability constraints. In high carbon lands such as forests and peatlands, the carbon benefits of land protection are greater in the short-term than converting land to bioenergy crops for BECCS, which can take several harvest cycles to ‘pay-back’ the carbon emitted during conversion (carbon-debt), from decades to over a century (''medium confidence''). (Figure TS.5) {2.6.2, Chapters 6, 7}

'''It is possible to achieve climate change targets with low need for land-demanding CDR such as BECCS, but such scenarios rely more on rapidly reduced emissions or CDR from forests, agriculture and other sectors.''' Terrestrial CDR has the technical potential to balance emissions that are difficult to eliminate with current technologies (including food production). Scenarios that achieve climate change targets with less need for terrestrial CDR rely on agricultural demand-side changes (diet change, waste reduction), and changes in agricultural production such as agricultural intensification. Such pathways that minimise land use for bioenergy and BECCS are characterised by rapid and early reduction of GHG emissions in all sectors, as well as earlier CDR in through afforestation. In contrast, delayed mitigation action would increase reliance on land-based CDR (''high confidence''). {2.6.2}