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

==== 5.2.1.1 Anthropogenic CO <sub>2</sub> emissions ====

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There are two anthropogenic sources of carbon dioxide (CO <sub>2</sub> ): fossil emissions and net emissions (including removals) resulting from land-use change and land management (also shown in this chapter as LULUCF: land use, land-use change, and forestry; in previous IPCC reports it has been termed forestry and other land use, FOLU). Fossil CO <sub>2</sub> emissions include the combustion of the fossil fuels coal, oil and gas, covering all sectors of the economy (electricity, transport, industrial, and buildings), fossil carbonates such as in cement manufacturing, and other industrial processes such as the production of chemicals and fertilizers (Figure 5.5a). Fossil CO <sub>2</sub> emissions are estimated by combining economic activity data and emissions factors, with different levels of methodological complexity (tiers) or approaches (e.g., IPCC Guidelines for National Greenhouse Gas Inventories). Several organizations or groups provide estimates of fossil CO <sub>2</sub> emissions, with each dataset having slightly different system boundaries, methods, activity data, and emissions factors ( [[#Andrew--2020|Andrew, 2020]] ). Datasets cover different time periods, which can dictate the datasets and methods that are used for a particular application. The data reported here is from an annually updated data source that combines multiple sources to maximise temporal coverage ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). The uncertainty in global fossil CO <sub>2</sub> emissions is estimated to be ±5% (1 standard deviation).

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[[File:71cb1477ccfff2ba7c5fd4bd04708f42 IPCC_AR6_WGI_Figure_5_5.png]]

'''Figure 5.5 |''' '''Global anthropogenic CO''' <sub>2</sub> '''emissions''' . '''(a)''' Historical trends of anthropogenic CO <sub>2</sub> emissions (fossil fuels and net land-use change, including land management, called LULUCF flux in the main text) for the period 1870 to 2019, with ‘others’ representing flaring, emissions from carbonates during cement manufacture. Data sources: ( [[#Boden--2017|Boden et al., 2017]] ; [[#IEA--2017|IEA, 2017]] ; [[#Andrew--2018|Andrew, 2018]] ; [[#BP--2018|BP, 2018]] ; [[#Le%20Quéré--2018a|Le Quéré et al., 2018a]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). '''(b)''' The net land-use change CO <sub>2</sub> flux (PgC yr <sup>–1</sup> ) as estimated by three bookkeeping models and 16 Dynamic Global Vegetation Models (DGVMs) for the global annual carbon budget 2019 ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). The three bookkeeping models are from [[#Hansis--2015|Hansis et al., 2015]] ; [[#Houghton--2017|Houghton and Nassikas, 2017]] ; [[#Gasser--2020|Gasser et al., 2020]] and are all updated to 2019. Their average is used to determine the net land-use change flux in the annual global carbon budget (black line). The DGVM estimates are the result of differencing a simulation with and without land-use changes run under observed historical climate and CO <sub>2</sub> , following the Trendy v9 protocol ( [https://blogs.exeter.ac.uk/trendy/protocol/ https://sites.exeter.ac.uk/trendy/protocol/] ); they are used to provide an uncertainty range to the bookkeeping estimates ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). All estimates are unsmoothed annual data. Estimates differ in process comprehensiveness of the models and in definition of flux components included in the net land use change flux. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

Fossil CO <sub>2</sub> emissions have grown continuously since the beginning of the industrial era (Figure 5.5) with short intermissions due to global economic crises or social instability ( [[#Peters--2012|Peters et al., 2012]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). In the most recent decade (2010–2019), fossil CO <sub>2</sub> emissions reached an average 9.6 ± 0.5 PgC yr <sup>–1</sup> and were responsible for 86% of all anthropogenic CO <sub>2</sub> emissions. In 2019, fossil CO <sub>2</sub> emissions were estimated to be 9.9 ±0.5 PgC yr <sup>–1</sup> excluding carbonation ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ), the highest on record. These estimates exclude the cement carbonation sink of around 0.2 PgC yr <sup>–1</sup> . Fossil CO <sub>2</sub> emissions grew at 0.9% yr <sup>–1</sup> in the 1990s, increasing to 3.0% yr <sup>–1</sup> in the 2000s, and reduced to 1.2% from 2010 to 2019. The slower growth in fossil CO <sub>2</sub> emissions in the last decade is due to a slowdown in growth from coal use. CO <sub>2</sub> emissions from coal use grew at 4.8% yr <sup>–1</sup> in the 2000s, but slowed to 0.4% yr <sup>–1</sup> in the 2010s. CO <sub>2</sub> emissions from oil use grew steadily at 1.1% yr <sup>–1</sup> in both the 2000s and 2010s. CO <sub>2</sub> emissions from gas use grew at 2.5% yr <sup>–1</sup> in the 2000s and 2.4% yr <sup>–1</sup> in 2010s, but has shown signs of accelerated growth of 3% yr <sup>–1</sup> since 2015 ( [[#Peters--2020|]] [[#Peters--2020|Peters et al., 2020]] ). Direct CO <sub>2</sub> emissions from carbonates in cement production are around 4% of total fossil CO <sub>2</sub> emissions, and grew at 5.8% yr <sup>–1</sup> in the 2000s but a slower 2.4% yr <sup>–1</sup> in the 2010s. The uptake of CO <sub>2</sub> in cement infrastructure (carbonation) offsets about one half of the carbonate emissions from current cement production ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). These results are robust across the different fossil CO <sub>2</sub> emissions datasets, despite minor differences in levels and rates, as expected given the reported uncertainties ( [[#Andrew--2020|Andrew, 2020]] ). During 2020, the COVID-19 pandemic led to a rapid, temporary decline in fossil CO <sub>2</sub> emissions, estimated to be around 7% based on a synthesis of four estimates. (Cross-Chapter Box 6.1; [[#Forster--2020|Forster et al., 2020]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ; [[#Le%20Quéré--2020|Le Quéré et al., 2020]] ; [[#Liu--2020|]] [[#Liu--2020|]] [[#Liu--2020|Liu et al., 2020]] ).

The global net flux from land-use change and land management is composed of carbon fluxes from land-use conversions, land management and changes therein ( [[#Pongratz--2018|Pongratz et al., 2018]] ) and is equivalent to the LULUCF fluxes from the agriculture, forestry and other land use (AFOLU) sector ( [[#Jia--2019|Jia et al., 2019]] ). It consists of gross emissions (loss of biomass and soil carbon in clearing or logging, harvested product decay, emissions from peat drainage and burning, degradation) and gross removals (CO <sub>2</sub> uptake in natural vegetation regrowing after harvesting or agricultural abandonment, afforestation). The LULUCF flux relates to direct human interference with terrestrial vegetation, as opposed to the natural carbon fluxes occurring due to interannual variability or trends in environmental conditions (in particular, climate, CO <sub>2</sub> , and nutrient deposition) ( [[#Houghton--2013|Houghton, 2013]] ).

Progress since AR5 and SRCCL ( [[#IPCC--2019a|IPCC, 2019a]] ) allows more accurate estimates of gross and net fluxes due to the availability of more models, model advancement in terms of inclusiveness of land-use practices, and advanced land-use forcings ( [[#Ciais--2013|Ciais et al., 2013]] ; [[#Klein%20Goldewijk--2017|Klein Goldewijk et al., 2017]] ; [[#Hurtt--2020|Hurtt et al., 2020]] ). In addition, important terminological discrepancies were resolved. First, synergistic effects of land-use change and environmental changes have been identified as a key reason for the large discrepancies between model estimates of the LULUCF flux, explaining up to 50% of differences ( [[#Pongratz--2014|Pongratz et al., 2014]] ; [[#Stocker--2015|Stocker and Joos, 2015]] ; [[#Gasser--2020|Gasser et al., 2020]] ). Another reason for discrepancies relates to natural fluxes being considered as part of the LULUCF flux when occurring on managed land in the United Nations Framework Convention on Climate Change (UNFCCC) national GHG inventories; these fluxes are considered part of the natural terrestrial sink in global vegetation models and excluded in bookkeeping models ( [[#Grassi--2018|Grassi et al., 2018]] ). LULUCF fluxes following national GHG inventories or Food and Agriculture Organization of the United Nations (FAO) datasets, including recent estimates ( [[#Tubiello--2021|Tubiello et al., 2021]] ), are thus excluded from our global assessment, but their comparison against the academic approach is available elsewhere – at the global scale ( [[#Jia--2019|Jia et al., 2019]] ) and European level ( [[#Petrescu--2020|Petrescu et al., 2020]] ).

Land-use-related component fluxes can be verified by the growing databases of global satellite-based biomass observations in combination with information on remotely sensed land cover change. However, they differ from bookkeeping and modelling with Dynamic Global Vegetation Models (DGVMs) in excluding legacy emissions from pre-satellite-era land-use change and land management, and neglecting soil carbon changes, often focusing on gross deforestation, not regrowth ( [[#Jia--2019|Jia et al., 2019]] ).

For the decade 2010–2019, average emissions were estimated at 1.6 ± 0.7 PgC yr <sup>–1</sup> (mean ± standard deviation, 1 sigma; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). A ''likely'' general upward trend since 1850 is reversed during the second part of the 20th century (Figure 5.5b). Trends since the 1980s have ''low confidence'' because they differ between estimates, which is related, among other things, to [[#Houghton--2017|Houghton and Nassikas (2017)]] using a different land-use forcing than [[#Hansis--2015|Hansis et al. (2015)]] and the DGVMs. Higher emissions estimates are expected from DGVMs run under transient environmental conditions compared to bookkeeping estimates, because the DGVM estimate includes the loss of additional sink capacity. Because the transient setup requires a reference simulation without land-use change to separate anthropogenic fluxes from natural land fluxes, LULUCF estimates by DGVMs include the sink forests that would have developed in response to environmental changes on areas that in reality have been cleared ( [[#Pongratz--2014|Pongratz et al., 2014]] ). The agricultural areas that replaced these forests have a reduced residence time of carbon, lacking woody material, and thus provide a substantially smaller additional sink over time ( [[#Gitz--2003|Gitz and Ciais, 2003]] ). The loss of additional sink capacity is growing in particular with atmospheric CO <sub>2</sub> and increases DGVM-based LULUCF flux estimates relative to bookkeeping estimates over time (Figure 5.5).

Gross emissions are on average two to three times larger than the net flux from LULUCF, increasing from an average of 3.5 ± 1.2 PgC yr <sup>–1</sup> for the decade of the 1960s to an average of 4.4 ± 1.6 PgC yr <sup>–1</sup> during 2010–2019 ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). Gross removals partly balance these gross emissions to yield the net flux from LULUCF and increase from –2.0 ± 0.7 PgC yr <sup>–1</sup> for the 1960s to –2.9 ± 1.2 PgC yr <sup>–1</sup> during 2010–2019. These large gross fluxes show the relevance of land management, such as harvesting or rotational agriculture, and the large potential to reduce emissions by halting deforestation and degradation.

More evidence on the pre-industrial LULUCF flux has emerged since AR5 in the form of new estimates of cumulative carbon losses until today, and of a better understanding of natural carbon cycle processes over the Holocene ( [[#Ciais--2013|Ciais et al., 2013]] ). Cumulative carbon losses by land-use activities since the start of agriculture and forestry (pre-industrial and industrial era) have been estimated at 116 PgC based on global compilations of carbon stocks for soils ( [[#Sanderman--2017|Sanderman et al., 2017]] ) with about 70 PgC of this occurring prior to 1750, and for vegetation as 447 PgC (inner quartiles of 42 calculations: 375–525 PgC) (Erb et al., 2018). Emissions prior to 1750 can be estimated by subtracting the post-1750 LULUCF flux from Table 5.1 from the combined soil and vegetation losses until today; they would then amount to 328 (161–501) PgC assuming error ranges are independent. A share of 353 (310–395) PgC from prior to 1800 has indirectly been suggested as the difference between net biosphere flux and terrestrial sink estimates, which is compatible with ice-core records due to a low airborne fraction of anthropogenic emissions in pre-industrial times ( [[#Erb--2018|Erb et al., 2018]] ; see also [[#5.1.2.3|Section 5.1.2.3]] ). ''Low confidence'' is assigned to pre-industrial emissions estimates.

Since AR5, evidence emerged that the LULUCF flux might have been underestimated as DGVMs include anthropogenic land cover change, but often ignore land management practices not associated with a change in land cover; land management is more widely captured by bookkeeping models through use of observation-based carbon densities ( [[#Ciais--2013|Ciais et al., 2013]] ; [[#Pongratz--2018|Pongratz et al., 2018]] ). Sensitivity studies show that practices such as wood and crop harvesting increase global net LULUCF emissions ( [[#Arneth--2017|Arneth et al., 2017]] ) and explain about half of the cumulative loss in biomass ( [[#Erb--2018|Erb et al., 2018]] ).

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