Editing IPCC:AR6/SRCCL/TS (section)

=== Terrestrial greenhouse gas fluxes on unmanaged and managed lands ===

'''Agriculture, forestry and other land use (AFOLU) is a significant net source of GHG emissions (''high confidence''), contributing to about 23% of anthropogenic emissions of carbon dioxide (CO<sub>2</sub>), methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) combined as CO<sub>2</sub> equivalents in 2007–2016 (''medium confidence'').'''  AFOLU results in both emissions and removals of CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O to and from the atmosphere (''high confidence''). These fluxes are affected simultaneously by natural and human drivers, making it difficult to separate natural from anthropogenic fluxes (''very high confidence''). (Figure TS.3) {2.3}

'''The total net land-atmosphere flux of CO<sub>2</sub> on both managed and unmanaged lands ''very likely ''provided a global net removal from 2007 to 2016 according to models (-6.0 ± 3.7 GtCO<sub>2</sub> yr<sup>–</sup><sup>1</sup>, ''likely range'').''' This net removal is comprised of two major components: (i) modelled net anthropogenic emissions from AFOLU are 5.2 ± 2.6 GtCO<sub>2</sub> yr<sup>–1 </sup>(''likely range'') driven by land cover change, including deforestation and afforestation/reforestation, and wood harvesting (accounting for about 13% of total net anthropogenic emissions of CO<sub>2</sub>) (''medium confidence''), and (ii) modelled net removals due to non-anthropogenic processes are 11.2 ± 2.6 GtCO<sub>2</sub> yr<sup>–1 </sup>(''likely'' range) on managed and unmanaged lands, driven by environmental changes such as increasing CO<sub>2</sub>, nitrogen deposition and changes in climate (accounting for a removal of 29% of the CO<sub>2</sub> emitted from all anthropogenic activities (fossil fuel, industry and AFOLU) (''medium confidence''). {2.3.1}

'''Global models and national GHG inventories use different methods to estimate anthropogenic CO<sub>2</sub> emissions and removals for the land sector. Consideration of differences in methods can enhance understanding of land sector net emission such as under the Paris Agreement’s global stocktake (''medium confidence'').''' Both models and inventories produce estimates that are in close agreement for land-use change involving forest (e.g., deforestation, afforestation), and differ for managed forest. Global models consider as managed forest those lands that were subject to harvest whereas, consistent with IPCC guidelines, national GHG inventories define managed forest more broadly. On this larger area, inventories can also consider the natural response of land to human-induced environmental changes as anthropogenic, while the global model approach treats this response as part of the non-anthropogenic sink. For illustration, from 2005 to 2014, the sum of the national GHG inventories net emission estimates is 0.1 ± 1.0 GtCO<sub>2</sub> yr<sup>–1</sup>, while the mean of two global bookkeeping models is 5.1 ± 2.6 GtCO<sub>2</sub>yr<sup>–1 </sup>(''likely range''). {Table SPM.1}

'''The gross emissions from AFOLU (one-third of total global emissions) are more indicative of mitigation potential of reduced deforestation than the global net emissions (13% of total global emissions), which include compensating deforestation and afforestation fluxes (''high confidence'').''' The net flux of CO<sub>2</sub> from AFOLU is composed of two opposing gross fluxes:
(i) gross emissions (20 GtCO<sub>2</sub> yr<sup>–1</sup>) from deforestation, cultivation of soils and oxidation of wood products, and (ii) gross removals (–14 GtCO<sub>2</sub> yr<sup>–1</sup>), largely from forest growth following wood harvest and agricultural abandonment (''medium confidence''). (Figure TS.4) {2.3.1}

'''Land is a net source of CH<sub>4</sub>, accounting for 44% of anthropogenic CH<sub>4</sub> emissions for the 2006–2017 period (''medium confidence'').''' The pause in the rise of atmospheric CH<sub>4</sub> concentrations between 2000 and 2006 and the subsequent renewed increase appear to be partially associated with land use and land use change. The recent depletion trend of the 13C isotope in the atmosphere indicates that higher biogenic sources explain part of the current CH<sub>4</sub> increase and that biogenic sources make up a larger proportion of the source mix than they did before 2000 (''high confidence''). In agreement with the findings of AR5, tropical wetlands and peatlands continue to be important drivers of inter-annual variability and current CH<sub>4</sub> concentration increases (''medium evidence, high agreement''). Ruminants and the expansion of rice cultivation are also important contributors to the current trend (''medium evidence, high agreement''). There is significant and ongoing accumulation of CH<sub>4</sub> in the atmosphere (''very high confidence''). {2.3.2}

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'''Figure TS.4 | Net and gross fluxes of CO2 from land (annual averages for 2008–2017).''' Left: The total net ﬂux of CO2 between land and atmosphere (grey) is shown with its two component ﬂuxes, (i) net AFOLU emissions (blue), and (ii) the net land sink (brown), due to indirect environmental effects and natural effects on managed and unmanaged lands. Middle: The gross emissions and removals contributing to the net AFOLU ﬂux. Right: The gross emissions and removals contributing to the land sink.
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'''AFOLU is the main anthropogenic source of N<sub>2</sub>O primarily due to nitrogen application to soils (''high confidence'').''' In croplands, the main driver of N<sub>2</sub>O emissions is a lack of synchronisation between crop nitrogen demand and soil nitrogen supply, with approximately 50% of the nitrogen applied to agricultural land not taken up by the crop. Cropland soils emit over 3 MtN<sub>2</sub>O-N yr<sup>–1 </sup>(''medium confidence''). Because the response of N<sub>2</sub>O emissions to fertiliser application rates is non-linear, in regions of the world where low nitrogen application rates dominate, such as sub-Saharan Africa and parts of Eastern Europe, increases in nitrogen fertiliser use would generate relatively small increases in agricultural N<sub>2</sub>O emissions. Decreases in application rates in regions where application rates are high and exceed crop demand for parts of the growing season will have very large effects on emissions reductions (''medium evidence, high agreement''). {2.3.3}

'''While managed pastures make up only one-quarter of grazing lands, they contributed more than three-quarters of N<sub>2</sub>O emissions from grazing lands between 1961 and 2014 with rapid recent increases of nitrogen inputs resulting in disproportionate growth in emissions from these lands (''medium confidence'').''' Grazing lands (pastures and rangelands) are responsible for more than one-third of total anthropogenic N<sub>2</sub>O emissions or more than one-half of agricultural emissions (''high confidence''). Emissions are largely from North America, Europe, East Asia, and South Asia, but hotspots are shifting from Europe to southern Asia (''medium confidence''). {2.3.3}

'''Increased emissions from vegetation and soils due to climate change in the future are expected to counteract potential sinks due to CO<sub>2</sub> fertilisation (''low confidence'').''' Responses of vegetation and soil organic carbon (SOC) to rising atmospheric CO<sub>2</sub> concentration and climate change are not well constrained by observations (''medium confidence''). Nutrient (e.g., nitrogen, phosphorus) availability can limit future plant growth and carbon storage under rising CO<sub>2</sub> (''high confidence''). However, new evidence suggests that ecosystem adaptation through plant-microbe symbioses could alleviate some nitrogen limitation (''medium evidence, high agreement''). Warming of soils and increased litter inputs will accelerate carbon losses through microbial respiration (''high confidence''). Thawing of high latitude/ altitude permafrost will increase rates of SOC loss and change the balance between CO<sub>2</sub> and CH<sub>4</sub> emissions (''medium confidence''). The balance between increased respiration in warmer climates and carbon uptake from enhanced plant growth is a key uncertainty for the size of the future land carbon sink (''medium confidence''). {2.3.1, 2.7.2, Box 2.3}