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

==== 5.4.5.4 Coupled Climate–Carbon Cycle Projections ====

<div id="h3-37-siblings" class="h3-siblings"></div>

Land and ocean carbon uptake are driven primarily by increases in atmospheric CO <sub>2</sub> (Figure 5.25). As a result, the evolution of land and ocean carbon sinks differs significantly between the SSP scenarios. Under scenarios that have greater increases in atmospheric CO <sub>2</sub> (such as SSP5-8.5 and SSP3-7.0) the absolute values of the sinks are larger, but the fraction of implied emissions taken up by the sinks declines through the 21 <sup>st</sup> century. By contrast, scenarios that assume CO <sub>2</sub> stabilization in the 21 <sup>st</sup> century (such as SSP1-2.6 or SSP2-4.5), have smaller absolute sinks, but these sinks take up an increasing fraction of the implied emissions (Figure 5.25d). These general principles apply to the ocean and land carbon sinks.

The concentration-driven CMIP6 ESMs agree well on the evolution of the global ocean carbon sink through the 21st century for four SSP scenarios (Figure 5.25). The five-year ensemble mean ocean sink declines to 0.6 ± 0.2 GtC yr <sup>–1</sup> by 2100 under SSP1-2.6, and peaks around 2080 at 5.4 ± 0.4 GtC yr <sup>–1</sup> under SSP5-8.5. Cumulative ocean carbon uptake from 1850 is projected to saturate at approximately 290 ± 30 GtC under SSP1-2.6, and to reach 520 ± 40 GtC by 2100 under SSP5-8.5 (Figure 5.25e).

The ensemble mean changes in land and ocean sinks are qualitatively similar, but the land shows much higher interannual variability in carbon uptake (Figure 5.25c) and also a much larger spread in the model projections of cumulative land carbon uptake (Figure 5.25f). The five-year ensemble mean net land carbon sink is projected to decline to 0.4 ± 1.0 GtC yr <sup>–1</sup> by 2100 under SSP1-2.6, and to reach around 5.6 ± 3.7 GtC yr <sup>–1</sup> under SSP5-8.5 (Figure 5.25c). Cumulative net land carbon uptake from 1850 is projected to saturate at approximately 150 ± 35 GtC under SSP1-2.6, and to reach 310 ± 130 GtC by 2100 under SSP5-8.5. Significant uncertainty remains in the future of the global land carbon sink, but there has been a notable reduction in the model spread from CMIP5 to CMIP6.

<div id="_idContainer070" class="_idGenObjectStyleOverride-1"></div>

[[File:5b10d45d4115f53c9bd7f695963d249b IPCC_AR6_WGI_Figure_5_25.png]]

'''Figure 5.25 |''' '''Modelled evolution of the global land and ocean carbon sinks for 1900 to 2100 in concentration-driven CMIP6 Earth system model (ESM) scenario runs.''' (SSP1-2.6: blue; SSP2-4.5: orange; SSP3-7.0: red; SSP5-8.5: brown): '''(a)''' prescribed atmospheric CO <sub>2</sub> concentrations; '''(b)''' five-year running mean ocean carbon sink (GtC yr <sup>–1</sup> ); '''(c)''' five-year running mean net land carbon sink (GtC yr <sup>–1</sup> ); '''(d)''' inferred cumulative sink fraction of emissions from 1850; '''(e)''' change in ocean carbon storage from 1850 (GtC); '''(f)''' change in land carbon storage from 1850 (GtC). Thick lines represent the ensemble mean of the listed ESM runs, and the error bars represents ± 1 standard deviation about that mean. The grey wedges represent estimates from the global carbon project (GCP), assuming uncertainties in the annual mean ocean and net land carbon sinks of 0.5 GtC yr <sup>–1</sup> and 1 GtC yr <sup>–1</sup> respectively, and uncertainties in the changes in carbon stores (ocean, land and cumulative total emissions) of 25 GtC. The net land carbon sink is taken as net biome productivity (NBP) and so includes any modelled net land-use change emissions. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

Geographical patterns of carbon changes for four SSP scenarios are shown in Figure 5.26, with cleared areas (no diagonal lines) showing agreement on the sign of the change by at least 80% of the models. In all scenarios the ocean sink is strongest in the Southern Ocean and North Atlantic. The land carbon sink occurs primarily where there are present-day forests. In the mid- and high-northern latitudes, a carbon sink is projected as a result of the combined impacts of increasing CO <sub>2</sub> and warming ( [[#5.4.5.5|Section 5.4.5.5]] ). Changes in land carbon storage in the tropics also depend strongly on the assumed rate of deforestation which varies in magnitude across the SSPs, from relatively low rates in SSP1-2.6 to relatively high rates in SSP3-7.0.

<div id="_idContainer072" class="_idGenObjectStyleOverride-1"></div>

[[File:2a259b3269d37076a2119b78309d917a IPCC_AR6_WGI_Figure_5_26.png]]

'''Figure 5.26 |''' '''Maps of net carbon changes under four Shared Socio-economic Pathway (SSP) scenarios, as evaluated from nine CMIP6 Earth system models''' . Uncertainty is represented using the simple approach (see Cross-Chapter Box Atlas.1 for more information). No overlay indicates regions with high model agreement, where ≥80% of models agree with the ensemble mean on the sign of change. Diagonal lines indicate regions with low model agreement, where &lt;80% of models agree with the ensemble mean on the sign of change. On land, this is calculated as the time integral of net biome productivity (NBP), for the ocean it is the time-integral of air – sea carbon dioxide (CO <sub>2</sub> ) <sub></sub> gas flux anomalies relative to the pre-industrial. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

In summary, oceanic and terrestrial carbon sinks are projected to continue to grow with increasing atmospheric concentrations of CO <sub>2</sub> , but the fraction of emissions taken up by land and ocean is expected to decline as the CO <sub>2</sub> concentration increases ( ''high confidence'' ). In the ensemble mean, ESMs suggest approximately equal global land and ocean carbon uptake for each of the SSP scenarios. However, the range of model projections is much larger for the land carbon sink. Despite the wide range of model responses, uncertainty in atmospheric CO <sub>2</sub> by 2100 is dominated by future anthropogenic emissions rather than carbon–climate feedbacks ( ''hi'' ''gh confidence'' ).

<div id="5.4.5.5" class="h3-container"></div>

<span id="linear-feedback-analysis"></span>