Editing IPCC:AR6/WGI/Chapter-4 (section)

=== 4.3.2 Cryosphere,Ocean and Biosphere ===

<div id="h2-14-siblings" class="h2-siblings"></div>

<div id="4.3.2.1" class="h3-container"></div>

<span id="arctic-sea-ice"></span>
==== 4.3.2.1 Arctic Sea Ice ====

<div id="h3-3-siblings" class="h3-siblings"></div>

The AR5 assessed from CMIP5 simulations that there will be year-round reductions of Arctic sea ice coverage by the end of this century ( [[#Collins--2013|Collins et al., 2013]] ). These range from 43% under RCP2.6 and 94% under RCP8.5 in September, and from 8% under RCP2.6 and 34% under RCP8.5 in March ( ''medium confidence'' ). Based on a five-member selection of CMIP5 models, AR5 further assessed that for RCP8.5, Arctic sea ice coverage in September will drop below 1 million km <sup>2</sup> and be practically ice free at some point between 2040 and 2060. The SROCC further assessed that the probability of an ice-free Arctic in September for stabilized global warming of 1.5°C and 2.0°C is approximately 1% and 10–35%, respectively ( [[#IPCC--2019|IPCC, 2019]] ).

With regards to the model selection in AR5, model evaluation studies have since identified shortcomings of the CMIP5 models to match the observed distribution of sea ice thickness in the Arctic ( [[#Stroeve--2014|Stroeve et al., 2014]] ; [[#Shu--2015|Shu et al., 2015]] ) and the observed evolution of albedo on seasonal scales ( [[#Koenigk--2014|Koenigk et al., 2014]] ). It was also found that many models’ deviation from observed sea ice cover climatology cannot be explained by internal variability, whereas the models’ deviation from observed sea ice cover trend (over the satellite period) can often be explained by internal variability ( [[#Olonscheck--2017|Olonscheck and Notz, 2017]] ). This hinders a selection of models according to their simulated trends, which additionally has been shown to only have a weak effect on the magnitude of simulated future trends (Stroeve and [[#Notz--2015|Notz, 2015]] ).

Based on results from the CMIP6 models, we conclude that on average the Arctic will become practically ice-free in September by the end of the 21st century under SSP2-4.5, SSP3-7.0, and SSP5-8.5 ( ''high confidence'' ) (Figure 4.2c and Table 4.4). Also, in the CMIP6 models, Arctic SIA in March decreases in the future, but to a much lesser degree, in percentage terms, than in September ( ''high confidence'' ) (Table 4.4). A more detailed assessment of projected Arctic and also Antarctic sea ice change can be obtained in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.3.1).

<div id="_idContainer022"></div>

'''Table 4.4''' '''|''' '''CMIP6 Arctic sea ice area for selected months, time periods, and across five SSPs.''' Displayed are the multi-model averages across the individual models and, in parentheses, the 5 '''–''' 95% ranges. The number of models used in these calculations are shown in Figure 4.2c.

{| class="wikitable"
|-
| colspan="2"| Month and Time Period

| '''SSP1-1.9 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP1-2.6 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP2-4.5 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP3-7.0 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

| '''SSP5-8.5 (10''' <sup>6</sup> '''km''' <sup>2</sup> ''')'''

|-
| rowspan="3"| '''September'''

| 2021–2040

| 2.6 (1.1, 6.5)

| 2.7 (0.6, 6.4)

| 2.8 (0.7, 6.4)

| 3.1 (1.1, 6.4)

| 2.5 (0.4, 5.8)

|-
| 2041–2060

| 2.2 (0.3, 6.5)

| 2.0 (0.2, 6.1)

| 1.7 (0.1, 5.6)

| 1.7 (0.1, 5.7)

| 1.2 (0.0, 5.2)

|-
| 2081–2100

| 2.4 (0.2, 6.2)

| 1.7 (0.0, 6.0)

| 0.8 (0.0, 4.6)

| 0.5 (0.0, 3.3)

| 0.3 (0.0, 2.2)

|-
| rowspan="3"| '''March'''

| 2021–2040

| 14.0 (11.4, 18.7)

| 14.9 (11.9, 25.8)

| 14.9 (11.9, 23.5)

| 15.0 (11.7, 27.3)

| 14.9 (11.9, 24.7)

|-
| 2041–2060

| 13.8 (10.9, 18.3)

| 14.5 (10.9, 25.7)

| 14.3 (11.1, 23.3)

| 14.2 (10.5, 27.1)

| 13.9 (10.2, 24.5)

|-
| 2081–2100

| 13.7 (10.9, 18.5)

| 14.2 (10.6, 25.7)

| 13.1 (9.5, 22.2)

| 11.8 (5.4, 25.5)

| 9.7 (3.1, 21.6)

|}

Studies focusing on the relationship of sea ice extent and changes in external drivers have consistently found a much-reduced likelihood of a practically ice-free Arctic Ocean during summer for global warming of 1.5°C than for 2.0°C ( [[#Screen--2017|Screen and Williamson, 2017]] ; [[#Jahn--2018|Jahn, 2018]] ; [[#Niederdrenk--2018|Niederdrenk and Notz, 2018]] ; [[#Notz--2018|Notz and Stroeve, 2018]] ; [[#Sigmond--2018|Sigmond et al., 2018]] ; [[#Olson--2019|Olson et al., 2019]] ). This is shown here in a large initial-condition ensemble of observationally constrained model simulations where GSAT are stabilized at 1.5°C, 2.0°C and 3.0°C warming relative to 1850–1900 in the RCP8.5 scenario (Figure 4.5). Temperature stabilization is achieved by switching off all the anthropogenic emissions around the time that GSAT first reaches the stabilization thresholds. Simulations have been observationally constrained to correct for a model bias in simulated historical September sea ice extent. In these simulations, Arctic sea ice coverage in September is simulated, on average, to drop below 1 million km <sup>2</sup> around 2040, consistent with the AR5 set of assessed models ( [[#Sigmond--2018|Sigmond et al., 2018]] ). The individual model simulations, for which there are twenty for each stabilized temperature level, show that the probability of the Arctic becoming practically ice free at the end of the 21st century is significantly higher for 2°C warming than for 1.5°C warming above 1850–1900 levels ( ''high confidence'' ).

<div id="_idContainer024" class="Basic-Text-Frame"></div>

[[File:47131a59275ac70c2dcc3d9dfc87e61a IPCC_AR6_WGI_Figure_4_5.png]]

'''Figure 4.5''' '''|''' '''Arctic sea ice extent in September in a large initial-condition ensemble of observationally-constrained simulations of an Earth system model (CanESM2).''' The black and red curves are averages over twenty simulations following historical forcings to 2015 and RCP8.5 extensions to 2100. The other curves are averages of over 20 simulations each after global surface air temperature has been stabilized at the indicated degree of global mean warming relative to 1850–1900. The bars to the right are the minimum to maximum ranges over 2081–2100 ( [[#Sigmond--2018|Sigmond et al., 2018]] ). The horizontal dashed line indicates a practically sea ice-free Arctic. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

<div id="4.3.2.2" class="h3-container"></div>

<span id="global-mean-sea-level"></span>
==== 4.3.2.2 Global Mean Sea Level ====

<div id="h3-4-siblings" class="h3-siblings"></div>

The AR5 assessed from CMIP5 process-based simulations that the rate of GMSL rise during the 21st century will ''very likely'' exceed the rate observed during 1971–2010 for all RCP scenarios due to increases in ocean warming and loss of mass from glaciers and ice sheets ( [[#Church--2013|Church et al., 2013]] ). Further, AR5 concluded that for the period 2081–2100, compared to 1986–2005, GMSL rise is ''likely'' ( ''medium confidence'' ) to be in the 5–95% range of projections from process-based models, which give 0.26–0.55 m for RCP2.6, 0.32–0.63 m for RCP4.5, 0.33–0.63 m for RCP6.0, and 0.45–0.82 m for RCP8.5. For RCP8.5, the rise by 2100 is 0.52–0.98 m with a rate during 2081–2100 of 8–16 mm yr <sup>–1</sup> .

There have been substantial modelling advances since AR5, with most sea level projections corresponding to one of three categories: (i) central-range projections, combining scenario-conditional probability distributions for the different contributions to estimate a central range under different scenarios; (ii) probabilistic projections, which explicitly consider outcomes for a wide range of likelihoods, including low-likelihood, high-impact outcomes; and (iii) semi-empirical projections, based on statistical relationships between past GMSL changes and climate variables, which now calibrate individual contributions and are consistent with physical-model based estimates (Section 9.6.3).

Based on the assessment of the latest modelling information (Figure 4.2d and Section 9.6.3), we conclude that under the SSP3-7.0, the ''likely'' range of GMSL change averaged over 2081–2100 relative to 1995–2014 is 0.46–0.74 m. Under SSP1-2.6, the ''likely'' range over the long-term is 0.30–0.54 m. Further, in SSP2-4.5, SSP3-7.0, and SSP5-8.5, the rise in GMSL is projected to accelerate over the 21st century. A detailed assessment of the processes contributing to these projected rises and accelerations in GMSL, together with a comparison to AR5 and SROCC, can be found in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.6.3). Projected changes in the thermosteric component of GMSL beyond 2300 are assessed in [[#4.7.1|Section 4.7.1]] .

In summary, it is ''virtually certain'' that under any one of the assessed SSPs, there will be continued rise in GMSL through the 21st century.

<div id="4.3.2.3" class="h3-container"></div>

<span id="atlantic-meridional-overturning-circulation"></span>
==== 4.3.2.3 Atlantic Meridional Overturning Circulation ====

<div id="h3-5-siblings" class="h3-siblings"></div>

The AR5 assessed from CMIP5 simulations that the Atlantic Meridional Overturning Circulation (AMOC) will ''very likely'' weaken over the 21st century, and the projected weakening of the AMOC is consistent with CMIP5 projections of an increase of high-latitude temperature and high-latitude precipitation, with both effects causing the surface waters at high latitudes to become less dense and therefore more stable ( [[#Collins--2013|Collins et al., 2013]] ).

Based on CMIP6 models, we find that over the 21st century, AMOC strength, relative to 1995–2014, shows a multi-model mean decrease in each of the SSP scenarios but with a large spread across the individual simulations (Figure 4.6). We also note that the magnitude of the ensemble-mean strength decrease is approximately scenario independent up to about 2060 ( [[#Weijer--2020|Weijer et al., 2020]] ). A more detailed assessment of these projected AMOC changes, and the mechanisms involved, can be found in [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] (Section 9.2.3).

<div id="_idContainer026" class="Basic-Text-Frame"></div>

[[File:bac164475cd281d483ffa55cbe94c78f IPCC_AR6_WGI_Figure_4_6.png]]

'''Figure 4.6 |''' '''CMIP6 annual mean Atlantic Meridional Overturning Circulation (AMOC) strength change in historical and scenario simulations.''' Changes are relative to averages from 1995–2014. The curves show ensemble averages and the shadings the 5–95% ranges across the SSP1-2.6 and SSP3-7.0 ensembles. The circles to the right of the panel show the anomalies averaged from 2081–2100 for each of the available model simulations. The numbers inside the panel are the number of model simulations. Here, the strength of the AMOC is computed as the maximum value of annual mean ocean meridional overturning mass stream function in the Atlantic at 26°N. Results are from concentration-driven simulations. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

In summary, we assess from the CMIP6 models that AMOC weakening over the 21st century is ''very likely'' ; the rate of weakening is approximately independent of the emissions scenario ( ''high confidence'' ).

Based on a large initial condition ensemble of simulations with a CMIP5 model (CanESM2) with emissions scenarios leading to stabilization of global warming of 1.5°C, 2.0°C, or 3.0°C relative to 1850–1900, AMOC continues to decline for 5–10 years after GSAT is effectively stabilized at the given GWL ( [[#Sigmond--2020|Sigmond et al., 2020]] ). This is followed by a recovery of AMOC strength for about the next 150 years to a level that is approximately independent of the considered stabilization scenario. These results are replicated in simulations in a CMIP6 model (CanESM5) with emissions cessation after diagnosed CO <sub>2</sub> emissions reach 750 Gt, 1000 Gt, or 1500 Gt. These emissions levels lead to global warming stabilization at 1.5°C, 2.0°C, or 3.0°C relative to 1850–1900. In summary, in these model simulations the AMOC recovers over several centuries after the cessation of CO <sub>2</sub> emissions ( ''medium confidence'' ).

<div id="4.3.2.4" class="h3-container"></div>

<span id="ocean-and-land-carbon-uptake"></span>
==== 4.3.2.4 Ocean and Land Carbon Uptake ====

<div id="h3-6-siblings" class="h3-siblings"></div>

The AR5 concluded with ''very high confidence'' that ocean carbon uptake of anthropogenic CO <sub>2</sub> will continue under all RCPs through the 21st century, with higher uptake corresponding to higher concentration pathways. The future evolution of the land carbon uptake was assessed to be much more uncertain than for ocean carbon uptake, with a majority of CMIP5 models projecting a continued cumulative carbon uptake.

Based on results from the CMIP6 models, we conclude that the flux of carbon from the atmosphere into the ocean increases continually through most of 21st century in the two highest emissions and decreases continually under the other emissions scenarios (Figure 4.7a). The flux of carbon from the atmosphere to land shows a similar 21st century behaviour across the scenarios but with much higher year-to-year variation than ocean carbon flux (Figure 4.7b). A more in-depth assessment and discussion of the mechanism involved can be found in [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] (Section 5.4.5).

<div id="_idContainer028" class="Basic-Text-Frame"></div>

[[File:09581ff5c63fc03bfc618b5f774488cc IPCC_AR6_WGI_Figure_4_7.png]]

'''Figure''' '''4.7 |''' '''CMIP6 carbon uptake in historical and scenario simulations. (a)''' Atmosphere to ocean carbon flux (PgC yr <sup>–1</sup> ). '''(b)''' Atmosphere to land carbon flux (PgC yr <sup>–1</sup> ). The curves show ensemble averages and the shadings show the 5–95% ranges across the SSP1-2.6 and SSP3-7.0 ensembles. The numbers inside each panel are the number of model simulations. The land uptake is taken as Net Biome Productivity (NBP) and so includes any modelled net land-use change emissions. Results are from concentration-driven simulations. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

In summary, we assess that the cumulative uptake of carbon by the ocean and by land will increase through the 21st century irrespective of the considered emissions scenarios except SSP1-1.9 ( ''very high confidence'' ).

<div id="4.3.2.5" class="h3-container"></div>

<span id="surface-ocean-ph"></span>
==== 4.3.2.5 Surface Ocean pH ====

<div id="h3-7-siblings" class="h3-siblings"></div>

The AR5 assessed from CMIP5 simulations that it is ''virtually certain'' that increasing storage of carbon by the ocean under all four RCPs through to 2100 will increase ocean acidification in the future ( [[#Ciais--2013|Ciais et al., 2013]] ). Specifically, AR5 reported that CMIP5 models project increased ocean acidification globally to 2100 under all RCPs, and that the corresponding model mean and model spread in the decrease in surface ocean pH from 1986–2005 to 2081–2100 would be 0.065 (0.06–0.07) for RCP2.6, 0.145 (0.14–0.15) for RCP4.5, 0.203 (0.20–0.21) for RCP6.0 and 0.31 (0.30–0.32) for RCP8.5.

Based on results from the CMIP6 models we conclude that, except for the lower-emissions scenarios SSP1-1.9 and SSP1-2.6, ocean surface pH decreases monotonically through the 21st century ( ''high confidence'' ) (Figure 4.8).

<div id="_idContainer030" class="Basic-Text-Frame"></div>

[[File:c3df335bb75a1ec547422e0caa92442f IPCC_AR6_WGI_Figure_4_8.png]]

'''Figure''' '''4.8 |''' '''Global average surface ocean pH.''' The shadings around the SSP1-2.6 and SSP5-7.0 curves are the 5–95% ranges across those ensembles. The numbers inside each panel are the number of model simulations. Results are from concentration-driven simulations. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

<div id="4.3.3" class="h2-container"></div>

<span id="modes-of-variability"></span>