Editing IPCC:AR6/WGI/Chapter-5 (section)

=== Biogeochemical Implications of Carbon Dioxide Removal and Solar Radiation Modification ===

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'''Land- and ocean-based carbon dioxide removal (CDR) methods have the potential to sequester CO''' <sub>2</sub> '''from the atmosphere, but the benefits of this removal would be partially offset by CO''' <sub>2</sub> '''release from land and ocean carbon stores''' ( ''very'' ''high confidence'' ''').''' The fraction of CO <sub>2</sub> removed that remains out of the atmosphere, a measure of CDR effectiveness, decreases slightly with increasing amount of removal ( ''medium confidence'' ) and decreases strongly if CDR is applied at lower CO <sub>2</sub> concentrations ( ''medium confidence'' ). {5.6.2.1; Figures 5.32, 5.33, 5.34}

'''The century-scale climate–carbon cycle response to a CO''' <sub>2</sub> '''removal from the atmosphere is not always equal and opposite to the response to a CO''' <sub>2</sub> '''emission''' ( ''medium confidence'' ''').''' For simultaneously cumulative CO <sub>2</sub> emissions and removals of greater than or equal to 100 PgC, CO <sub>2</sub> emissions are 4 ± 3% more effective at raising atmospheric CO <sub>2</sub> than CO <sub>2</sub> removals are at lowering atmospheric CO <sub>2</sub> . The asymmetry originates from state-dependencies and non-linearities in carbon cycle processes and implies that an extra amount of CDR is required to compensate for a positive emission of a given magnitude to attain the same change in atmospheric CO <sub>2</sub> . The net effect of this asymmetry on the global surface temperature is poorly constrained due to ''low agreement'' between models ( ''low confidence'' ). {5.6.2.1; Figure 5.35}

'''Wide-ranging side effects of CDR methods have been identified that can either weaken or strengthen the carbon sequestration and cooling potential of these methods and affect the achievement of sustainable development goals''' ( ''high confidence'' ''').''' Biophysical and biogeochemical side effects of CDR methods are associated with changes in surface albedo, the water cycle, emissions of CH <sub>4</sub> and N <sub>2</sub> O, ocean acidification and marine ecosystem productivity ( ''high confidence'' ). These side effects and associated Earth system feedbacks can decrease carbon uptake and/or change local and regional climate, and in turn limit the CO <sub>2</sub> sequestration and cooling potential of specific CDR methods ( ''medium confidence'' ). Deployment of CDR, particularly on land, can also affect water quality and quantity, food production and biodiversity, with consequences for the achievement of related sustainable development goals ( ''high confidence'' ). These effects are often highly dependent on local context, management regime, prior land use, and scale of deployment ( ''high confidence'' ). A wide range of co-benefits are obtained with methods that seek to restore natural ecosystems or improve soil carbon ( ''high confidence'' ). The biogeochemical effects of terminating CDR are expected to be small for most CDR methods ( ''medium confidence'' ). {5.6.2.2; Figure 5.36; Cross-Chapter Box 5.1}

'''Solar radiation modification (SRM) would increase the global land and ocean CO''' <sub>2</sub> '''sinks''' ( ''medium confidence'' ''') but would not stop CO''' <sub>2</sub> '''from increasing in the atmosphere, thus exacerbating ocean acidification under continued anthropogenic emissions''' ( ''high confidence'' ''').''' SRM acts to cool the planet relative to unmitigated climate change, which would increase the land sink by reducing plant and soil respiration and slow the reduction of ocean carbon uptake due to warming ( ''medium confidence'' ). SRM would not counteract or stop ocean acidification ( ''high confidence'' ). The sudden and sustained termination of SRM would rapidly increase global warming, with the return of positive and negative effects on the carbon sinks ( ''very'' ''high confidence'' ). {4.6.3; 5.6.3}

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