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

==== 5.2.1.5 CO <sub>2</sub> Budget ====

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The global CO <sub>2</sub> budget (Figure 5.12) encompasses all natural and anthropogenic CO <sub>2</sub> sources and sinks. Table 5.1 shows the perturbation of the global carbon mass balance between reservoirs since the beginning of the industrial era, circa 1750.

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'''Table 5.1 |''' '''Global anthropogenic CO''' <sub>2</sub> '''budget accumulated since the Industrial Revolution (onset in 1750) and averaged over the 1980s, 1990s, 2000s, and 2010s''' . By convention, a negative ocean or land to atmosphere CO <sub>2</sub> flux is equivalent to a gain of carbon by these reservoirs. The table does not include natural exchanges (e.g., rivers, weathering) between reservoirs. Uncertainties represent the 68% confidence interval ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ).

{| class="wikitable"
|-
!
! 1750–2019

Cumulative

(PgC)

! 1850–2019

Cumulative

(PgC)

! 1980–1989

Mean Annual Growth Rate

(PgC yr <sup>–1</sup> )

! 1990–1999

Mean Annual Growth Rate

(PgC yr <sup>–1</sup> )

! 2000–2009

Mean Annual Growth Rate

(PgC yr <sup>–1</sup> )

! 2010–2019

Mean Annual Growth Rate

(PgC yr <sup>–1</sup> )

|-
| colspan="7"| '''Emissions'''

|-
| Fossil fuel combustion and cement production

| 445 ± 20

| 445 ± 20

| 5.4 ± 0.3

| 6.3 ± 0.3

| 7.7 ± 0.4

| 9.4 ± 0.5

|-
| Net land-use change

| 240 ± 70

| 210 ± 60

| 1.3 ± 0.7

| 1.4 ± 0.7

| 1.4 ± 0.7

| 1.6 ± 0.7

|-
| Total emissions

| 685 ± 75

| 655 ± 65

| 6.7 ± 0.8

| 7.7 ± 0.8

| 9.1 ± 0.8

| 10.9 ± 0.9

|-
| colspan="7"| '''Partition'''

|-
| Atmospheric increase

| 285 ± 5

| 265 ± 5

| 3.4 ± 0.02

| 3.2 ± 0.02

| 4.1 ± 0.02

| 5.1 ± 0.02

|-
| Ocean sink

| 170 ± 20

| 160 ± 20

| 1.7 ± 0.4

| 2.0 ± 0.5

| 2.1 ± 0.5

| 2.5 ± 0.6

|-
| Terrestrial sink

| 230 ± 60

| 210 ± 55

| 2.0 ± 0.7

| 2.6 ± 0.7

| 2.9 ± 0.8

| 3.4 ± 0.9

|-
| '''B''' '''udget imbalance'''

| 0

| 20

| –0.4

| –0.1

| 0

| –0.1

|}

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[[File:021e8d9bec3c516832577661fc51eb23 IPCC_AR6_WGI_Figure_5_12.png]]

'''Figure 5.12 |''' '''Global carbon (CO''' <sub>2</sub> ''') budget (2010–2019)''' . Yellow arrows represent annual carbon fluxes (in PgC yr <sup>–1</sup> ) associated with the natural carbon cycle, estimated for the time prior to the industrial era, around 1750. Pink arrows represent anthropogenic fluxes averaged over the period 2010–2019. The rate of carbon accumulation in the atmosphere is equal to net land-use change emissions, including land management (called LULUCF in the main text) plus fossil fuel emissions, minus land and ocean net sinks (plus a small budget imbalance, Table 5.1). Circles with yellow numbers represent pre-industrial carbon stocks in PgC. Circles with pink numbers represent anthropogenic changes to these stocks (cumulative anthropogenic fluxes) since 1750. Anthropogenic net fluxes are reproduced from [[#Friedlingstein--2020|Friedlingstein et al. (2020)]] . The relative change of gross photosynthesis since pre-industrial times is based on 15 DGVMs used in [[#Friedlingstein--2020|Friedlingstein et al. (2020)]] . The corresponding emissions by total respiration and fire are those required to match the net land flux, exclusive of net land-use change emissions which are accounted for separately. The cumulative change of anthropogenic carbon in the terrestrial reservoir is the sum of carbon cumulatively lost by net land-use change emissions, and net carbon accumulated since 1750 in response to environmental drivers (warming, rising CO <sub>2</sub> , nitrogen deposition). The adjusted gross natural ocean–atmosphere CO <sub>2</sub> flux was derived by rescaling the value in Figure 1 of [[#Sarmiento--2002|Sarmiento and Gruber (2002)]] of 70 PgC yr <sup>–1</sup> by the revised estimate of the bomb radiocarbon ( <sup>14</sup> C) inventory in the ocean. The original bomb <sup>14</sup> C inventory yielded an average global gas transfer velocity of 22 cm hr <sup>–1</sup> ; the revised estimate is 17cm hr <sup>–1</sup> leading to 17/22*70=54. Dissolved organic carbon reservoir and fluxes from [[#Hansell--2009|Hansell et al. (2009)]] . Dissolved inorganic carbon exchanges between surface and deep ocean, subduction and obduction from [[#Levy--2013|Levy et al. (2013)]] . Export production and flux from ( [[#Boyd--2019|Boyd et al., 2019]] ). Net primary production (NPP) and remineralization in surface layer of the ocean from [[#Kwiatkowski--2020|Kwiatkowski et al. (2020)]] ; [[#Séférian--2020|Séférian et al. (2020)]] . Deep ocean reservoir from [[#Keppler--2020|Keppler et al. (2020)]] . Anthropogenic carbon reservoir in the ocean is from [[#Gruber--2019b|Gruber et al. (2019b)]] extrapolated to 2015. Fossil fuel reserves are from [[#BGR--2020|BGR (2020)]] ; fossil fuel resources are 11,490 PgC for coal, 6,780 PgC for oil and 365 PgC for natural gas. Permafrost region stores are from [[#Hugelius--2014|Hugelius et al. (2014)]] ; [[#Strauss--2017|Strauss et al. (2017)]] ; [[#Mishra--2021|Mishra et al. (2021)]] (see also Box 5.1) and soil carbon stocks outside of permafrost region from [[#Batjes--2016|Batjes (2016)]] ; [[#Jackson--2017|Jackson et al. (2017)]] . Biomass stocks (range of seven estimates) are from [[#Erb--2018|Erb et al. (2018)]] . Sources for the fluxes of the land–ocean continuum are provided in main text and adjusted within the ranges of the various assessment to balance the budget ( [[#5.2.1.5|Section 5.2.1.5]] ).

Since AR5 ( [[#Ciais--2013|Ciais et al., 2013]] ), a number of improvements have led to the more constrained carbon budget presented here. Some new additions include: (i) the use of independent estimates for the residual carbon sink on natural terrestrial ecosystems ( [[#Le%20Quéré--2018a|Le Quéré et al., 2018a]] ); (ii) improvements in the estimates of emissions from cement production ( [[#Andrew--2019|Andrew, 2019]] ) and the sink associated with cement carbonation ( [[#Cao--2020|Cao et al., 2020]] ); (iii) improved and new emissions estimates from forestry and other land use ( [[#Hansis--2015|Hansis et al., 2015]] ; [[#Gasser--2020|Gasser et al., 2020]] ); (iv) the use of ocean observation-based sink estimates and a revised river flux partition between hemispheres ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ); and (v) the expansion of constraints from atmospheric inversions, based on surface networks and the use of satellite retrievals.

The budget, based on the annual assessment by the GCP ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ), uses independent estimates of all major flux components: fossil fuel and carbonate emissions (E <sub>FOS</sub> ), CO <sub>2</sub> fluxes from land use, land-use change, and forestry (E <sub>LULUCF</sub> ), the growth rate of CO <sub>2</sub> in the atmosphere (G <sub>atm</sub> ), and the ocean (S <sub>ocean</sub> ) and natural land (S <sub>land</sub> ) CO <sub>2</sub> sinks. An imbalance term (B <sub>Imb</sub> ) is required to ensure mass balance of the source and sinks that have been independently estimated: E <sub>FOS</sub> + E <sub>LULUCF</sub> = G <sub>atm</sub> + S <sub>ocean</sub> + S <sub>land.</sub> + B <sub>Imb.</sub> All estimates are reported with 1 standard deviation (±1 σ , 1 sigma) representing a likelihood of 68%.

Over the past decade (2010–2019), 10.9 ± 0.9 PgC yr <sup>–1</sup> were emitted from human activities, which were distributed between three Earth system components: 46% accumulated in the atmosphere (5.1 ± 0.02 PgC yr <sup>–1</sup> ), 23% was taken up by the ocean (2.5 ± 0.6 PgC yr <sup>–1</sup> ) and 31% was stored by vegetation in terrestrial ecosystems (3.4 ± 0.9 PgC yr <sup>–1</sup> ) (Table 5.1). There is a budget imbalance of 0.1 PgCyr <sup>–1</sup> which is within the uncertainties of the other terms. Over the industrial era (1750–2019), the total cumulative CO <sub>2</sub> fossil fuel and industry emissions were 445 ± 20 PgC, and the LULUCF flux (= net land-use change in Figure 5.12) was 240 ± 70 PgC ( ''medium confidence'' ). The equivalent total emissions (685 ± 75 PgC) was distributed between the atmosphere (285 ± 5 PgC), oceans (170 ± 20 PgC) and land (230 ± 60 PgC; Table 5.1), with a budget imbalance of 20 PgC. This budget (Table 5.1) does not explicitly account for source/sink dynamics due to carbon cycling in the land–ocean aquatic continuum comprising freshwaters, estuaries, and coastal areas. Natural and anthropogenic transfers of carbon from soils to freshwater systems are significant (2.4–5.1 PgC yr <sup>–1</sup> ) ( [[#Regnier--2013|Regnier et al., 2013]] ; [[#Drake--2018|Drake et al., 2018]] ). Some of the carbon is buried in freshwater bodies (0.15 PgC) ( [[#Mendonça--2017|Mendonça et al., 2017]] ), and a significant proportion returns to the atmosphere via outgassing from lakes, rivers and estuaries ( [[#Raymond--2013|Raymond et al., 2013]] ; [[#Regnier--2013|Regnier et al., 2013]] ; [[#Lauerwald--2015|Lauerwald et al., 2015]] ). The net export of carbon from the terrestrial domain to the open oceans is estimated to be 0.80 PgC yr <sup>–1</sup> ( ''medium confidence)'' , based on the average of ( [[#Jacobson--2007|Jacobson et al., 2007]] ; [[#Resplandy--2018|Resplandy et al., 2018]] ) and corrected to account for 0.2 PgC buried in ocean floor sediments. These terms are included in Figure 5.12. Inclusion of other smaller fluxes could further constrain the carbon budget ( [[#Ito--2019|Ito, 2019]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ).

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