Editing IPCC:AR6/SRCCL/Chapter-2 (section)

==== 2.3.1.4 Gross emissions and removals contributing to the non-anthropogenic land sink ====

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The net land sink averaged 11.2 ± 2.6 GtCO <sub>2</sub> yr <sup>–1</sup> (l ''ikely range'' ) over 2007–2016 (Table 2.3.2), but its gross components have not been estimated at the global level. There are many studies that suggest increasing emissions of carbon are due to indirect environmental effects and natural disturbance, for example, temperature-induced increases in respiration rates (Bond-Lamberty et al. 2018 <sup>[[#fn:r586|586]]</sup> ), increased tree mortality (Brienen et al. 2015 <sup>[[#fn:r587|587]]</sup> ; Berdanier and Clark 2016 <sup>[[#fn:r588|588]]</sup> ; McDowell et al. 2018 <sup>[[#fn:r589|589]]</sup> ) and thawing permafrost (Schuur et al. 2015 <sup>[[#fn:r590|590]]</sup> ). The global carbon budget indicates that land and ocean sinks ''have increased'' over the last six decades in proportion to total CO <sub>2</sub> emissions (Le Quéré et al. 2018 <sup>[[#fn:r591|591]]</sup> ) ( ''robust evidence, high agreement'' ). That means that any emissions must have been balanced by even larger removals (likely driven by CO <sub>2</sub> fertilisation, climate change, nitrogen deposition, erosion and redeposition of soil carbon, a reduction in areas burned, aerosol-induced cooling and changes in natural disturbances) (Box 2.3).

Climate change is expected to impact terrestrial biogeochemical cycles via an array of complex feedback mechanisms that will act to either enhance or decrease future CO <sub>2</sub> emissions from land. Because the gross emissions and removals from environmental changes are not constrained at present, the balance of future positive and negative feedbacks remains uncertain. Estimates from climate models in Coupled Model Intercomparison Project 5 (CMIP5) exhibit large differences for different carbon and nitrogen cycle feedbacks and how they change in a warming climate (Anav et al. 2013 <sup>[[#fn:r592|592]]</sup> ; Friedlingstein et al. 2006 <sup>[[#fn:r593|593]]</sup> ; Friedlingstein, et al. 2014 <sup>[[#fn:r594|594]]</sup> ). The differences are in large part due to the uncertainty regarding how primary productivity and soil respiration will respond to environmental changes, with many of the models not even agreeing on the sign of change. Furthermore, many models do not include a nitrogen cycle, which may limit the CO <sub>2</sub> fertilisation effect in the future (Box 2.3). There is an increasing amount of observational data available and methods to constrain models (e.g., Cox et al. 2013 <sup>[[#fn:r595|595]]</sup> ; Prentice, et al., 2015 <sup>[[#fn:r596|596]]</sup> ) which can reduce uncertainty.

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