Abstract

Biogeosciences

1726-4189

Copernicus Publications

Göttingen, Germany

10.5194/bg-18-2221-2021

Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble

Future Arctic Ocean acidification in CMIP6 models

Terhaar

Jens

jens.terhaar@climate.unibe.ch

https://orcid.org/0000-0001-9377-415X

Torres

Olivier

Bourgeois

Timothée

https://orcid.org/0000-0002-9367-464X

Kwiatkowski

Lester

1Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland 2Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland 3LMD/IPSL, Ecole Normale Supérieure/PSL Université, CNRS, Ecole Polytechnique, Sorbonne Université, Paris, France 4NORCE Norwegian Research Centre and Bjerknes Centre for Climate Research, Bergen, Norway 5LOCEAN/IPSL, Sorbonne Université, CNRS, IRD, MNHN, Paris, France

Jens Terhaar (jens.terhaar@climate.unibe.ch)

1April2021

18 6 22212240 4December2020 23February2021 22February2021 17December2020

2021

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Abstract

The uptake of anthropogenic carbon (Cant) by the ocean leads to ocean acidification, causing the reduction of pH and the saturation states of aragonite (Ωarag) and calcite (Ωcalc). The Arctic Ocean is particularly vulnerable to ocean acidification due to its naturally low pH and saturation states and due to ongoing freshening and the concurrent reduction in total alkalinity in this region. Here, we analyse ocean acidification in the Arctic Ocean over the 21st century across 14 Earth system models (ESMs) from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6). Compared to the previous model generation (CMIP5), models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of Cant into the Arctic Ocean interior, with simulated historical increases in Cant in improved agreement with observational products. Moreover, in CMIP6 the inter-model uncertainty of projected changes over the 21st century in Arctic Ocean Ωarag and Ωcalc averaged over the upper 1000 m is reduced by 44–64 %. The strong reduction in projection uncertainties of Ωarag and Ωcalc can be attributed to compensation between Cant uptake and total alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean Cant over the 21st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in Cant simulate a relatively strong freshening and concurrent total alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21st century, the increase in Cant remains the dominant driver. Even under the low-emissions Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), basin-wide averaged Ωarag undersaturation in the upper 1000 m occurs before the end of the century. While under the high-emissions pathway SSP5-8.5, the Arctic Ocean mesopelagic is projected to even become undersaturated with respect to calcite. An emergent constraint identified in CMIP5 which relates present-day maximum sea surface densities in the Arctic Ocean to the projected end-of-century Arctic Ocean Cant inventory is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in Ωarag and Ωcalc is not apparent in the new generation of models. This is due to both the reduction in Ωarag and Ωcalc projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean Cant and changes in Ωarag and Ωcalc.

1Introduction

Human activities such as the burning of fossil fuels, cement production, and land use change have released large amounts of carbon into the atmosphere that cause global warming. The ocean mitigates global warming by taking up around one-quarter of this anthropogenic carbon (Cant) . However, the increase of carbon in the ocean causes ocean acidification, a process that decreases pH, carbonate ion (CO32-) concentrations, and in consequence the calcium carbonate (CaCO3) saturation states of calcite and aragonite minerals . The Arctic Ocean is particularly vulnerable to ocean acidification due to its naturally high dissolved inorganic carbon concentrations, its low carbonate ion concentrations, and its thus naturally low saturation states .

In contrast to most of the global ocean, Arctic Ocean acidification is caused not solely by increasing Cant concentrations but also by freshening . Fresh water from rivers, precipitation, and sea ice typically has much lower total alkalinity (AT) and total dissolved inorganic carbon (CT) concentrations than the ocean and therefore, in the absence of indirect impacts on other fluxes, dilutes both marine AT and CT . As freshwater AT and CT concentrations are generally similar, freshwater fluxes into the ocean typically act to reduce the difference between AT and CT, decreasing marine CO32- concentrations and ocean pH . In the Arctic Ocean, projected freshening over the 21st century is larger than in most other ocean regions due to ongoing sea ice melt, positive precipitation minus evaporation, and large river runoff .

Due to freshening and increasing Cant concentrations, the Arctic Ocean is projected to be the first large-scale ocean region to become undersaturated with respect to the metastable CaCO3 polymorph aragonite (Ωarag<1) . Under the Representative Concentration Pathway 8.5 (RCP8.5) high-emissions scenario, Arctic Ocean mesopelagic waters may even become undersaturated with respect to the more stable CaCO3 polymorph calcite (Ωcalc<1) before 2100 . Aragonite and calcite undersaturation is likely to affect the growth, reproduction, and survival of calcifying organisms, such as sea butterflies and foraminifera , and could have ramifications for the wider Arctic ecosystem , including some of its most iconic predators, such as grey whales and walruses .

Projections of Arctic Ocean acidification over the 21st century had considerable subsurface uncertainties in the simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5) , with projected end-of-century basin-wide Ωarag in mesopelagic waters ranging from 0.61 to 1.05. This large uncertainty has been attributed to multiple factors, including variable inflow of Atlantic waters and their subsequent subduction; difficulties resolving the narrow passages between the Arctic Ocean and its surrounding basins; and differences in brine rejection during sea ice formation, which is critical to the formation of dense Arctic waters .

1.1Emergent constraints on Arctic Ocean carbon uptake and acidification

Emergent constraints are a suite of statistical techniques that relate observable trends or sensitivities across multi-model ensembles to differences in model projections in order to reduce future uncertainties . An elegant early application that demonstrates many of the principles of emergent constraints is that of . In their study of models that contributed to the Intergovernmental Panel on Climate Change's Fourth Assessment Report (AR4), Hall and Qu found a strong positive correlation between the magnitude of a model's snow albedo feedback on present-day seasonal timescales and under future climate change. They concluded that relevant model biases were consistent across these contrasting timescales and therefore observations of the seasonal snow albedo feedback could be used to constrain the ensemble range of the projected snow albedo feedback under climate change. Since the publication of Hall and Qu (2006), emergent constraint approaches have been applied extensively within the Earth sciences to constrain, amongst other things, projections of climate sensitivity , Arctic sea ice extent , precipitation extremes , carbon cycle feedbacks , and marine primary production .

Recently showed that an emergent constraint could be applied to CMIP5 projections of the Arctic Ocean Cant inventory and coincident acidification over the 21st century. As the Cant increase in the Arctic Ocean is mainly driven by the inflow of Cant-rich waters from the Atlantic and their subsequent subduction in the Barents Sea , the capability of each model to form dense surface waters in the Barents Sea was shown to strongly influence the future Arctic Ocean Cant inventory. By constraining simulated surface water densities with observations, uncertainties related to the end-of-century Arctic Ocean Cant inventory in 2100 were reduced by around one-third, and the best estimate under RCP8.5 was increased by 20 % to 9.0±1.6 Pg C . Along with the projected Cant inventory, uncertainties in the projected associated basin-wide Arctic Ocean acidification could also be reduced in CMIP5. It should be noted, however, that in CMIP5 projected freshening and reductions in alkalinity were of minor importance for Arctic Ocean acidification over the 21st century. Moreover, the models have been shown to underestimate historical freshwater fluxes (1992–2012) in the Arctic Ocean by around 50 % , which suggests they might also have underestimated freshwater fluxes over the 21st century.

Given that emergent constraints in many cases conflict with one another and can even be derived from data-mined pseudocorrelations , it is critical to test published constraints, and the mechanisms that underpin them, across Earth system model (ESM) generations . The CMIP6 simulations provide such an opportunity .

1.2From CMIP5 to CMIP6 models and simulations

During the transition from CMIP5 to CMIP6, ESMs have generally improved the simulation of ocean dynamics and marine biogeochemistry . Across most ESMs, the horizontal and/or vertical resolution of ocean models has increased, which potentially has large effects on the representation of Arctic Ocean circulation, sea ice dynamics , and the carbon cycle . Ocean biogeochemical model components in CMIP6 also tend to have a more complex representation of the carbon and nutrient cycles than in CMIP5. In particular, the treatment of organic matter carbon cycling has generally evolved, with remineralization of particles in sediments now simulated in 10 out of 14 ESMs. These developments will likely have a large effect on simulating the Arctic Ocean biogeochemistry given that 50 % of the Arctic Ocean is made up of shelf seas , where sedimentation and sediment remineralization are crucial components of the carbon and nutrient cycle . Furthermore, the external carbon and nutrient sources from glaciers, atmospheric deposition, and rivers are represented in more models in CMIP6 (Table ) . Riverine inputs in particular have been shown to be of importance for present-day Arctic Ocean acidification and its future changes .

Table 1

The CMIP6 ESMs used in this study with their ocean–sea ice and marine-biogeochemical (MBG) model components plus their biogeochemical (BGC) riverine input.

Model Ocean–sea ice MBG Riverine BGC fluxes Data DOI ACCESS-ESM1.5 MOM5, CICE4 WOMBAT None CanESM5 NEMO 3.4.1-LIM2 CMOC None CanESM5-CanOE NEMO 3.4.1-LIM2 CanOE None (; ) CESM2 POP2-CICE5 MARBL-BEC C, N, P, Fe, Si, AT CESM2-WACCM POP2-CICE5 MARBL-BEC C, N, P, Fe, Si, AT CNRM-ESM2-1 NEMOv3.6-GELATOv6 PISCESv2-gas C, P, AT GFDL-CM4a MOM6, SIS2 BLINGv2 C, N, P, Fe, AT (; ) GFDL-ESM4(; ) MOM6, SIS2 COBALTv2 C, P, N, AT , IPSL-CM6A-LR NEMOv3.6-LIM3 PISCESv2 C, N, P, Fe, Si, AT MIROC-ES2L COCO OECO2 N, P , MPI-ESM1.2-HR(; ) MPIOM HAMOCC6 None , MPI-ESM1.2-LR MPIOM HAMOCC6 None MRI-ESM2b MRICOM4 NPZD None UKESM1-0-LL NEMO v3.6, CICE MEDUSA-2 None ,

a Only SSP2-4.5 and SSP5-8.5. b Only SSP5-8.5.

In this study, we extend recent CMIP6 ocean biogeochemical assessments e.g. and previous attempts to constrain projected Arctic Ocean Cant uptake by

assessing projections of the Arctic Ocean Cant inventory over the 21st century in CMIP6 simulations

exploring the role of Cant inventory increases and freshening in driving concurrent basin-wide ocean acidification in the Arctic Basin

revaluating previous emergent constraints on the Arctic Ocean Cant inventory and associated acidification using the CMIP6 model ensemble and multiple future-emissions scenarios.

2Methods 2.1Arctic Ocean

The Arctic Ocean was defined as the water north of the Fram Strait, the Barents Sea Opening, the Bering Strait, and the Baffin Bay following . This is consistent with the previously published emergent constraint on projected Arctic Ocean Cant and acidification .

2.2Earth system models

An ensemble of 14 ESMs from CMIP6 (Table ) was used with one ensemble member per model. All models follow the biogeochemical protocols outlined in . Riverine input of CT and AT is included in six ESMs (Table ). The absence of CT and AT in riverine freshwater input causes an overly strong reduction of CO32- concentrations and thus low-biased saturation states in coastal regions but is of minor importance on the pan-Arctic scale . The spin-up length for each model varied between 500 years (IPSL-CM6A-LR) and 12 000 years (MPI-ESM1-2-LR) .

For each model, monthly 3D fields of dissolved inorganic carbon, total alkalinity, dissolved inorganic phosphorus and silicon, temperature, and salinity were used. All 3D fields were regridded to the regular 1∘×1∘ grid with 33 depth levels used in the GLobal Ocean Data Analysis Project Version 2 (GLODAPv2) observational product to add simulated changes of these variables over the 21st century to observations of the present-day mean state (see below).

Cant was defined as the difference between annual means of dissolved inorganic carbon in the historical (1850–2014) simulations merged with the respective shared socioeconomic pathway (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5; 2015–2100; ) and the concurrent pre-industrial control simulations of each model. Output from 13 models was available for SSP1-2.6, output from 12 models for SSP2-4.5 and SSP3-7.0, and output from 14 models for SSP5-8.5 (Table ).

Changes in AT over the 21st century were calculated by subtracting changes in the pre-industrial control simulations from changes in the respective SSP. To quantify the effect of freshening on changes in AT, the AT anomalies for each model were further decomposed into changes resulting from freshening and from the combined effect of other biogeochemical processes by calculating the temporal evolution of salinity-corrected alkalinity with a reference salinity of 35 following . A zero-alkalinity endmember was assumed for fresh water. This assumption is correct for models with no alkalinity in fresh water but an overestimation for models with finite alkalinity concentrations in freshwater inputs (Table ). Unfortunately, information on alkalinity concentrations in fresh water is not available for all models. Moreover, with the available model output, we cannot quantify the individual contributions of land ice melt, sea ice melt, precipitation minus evaporation, and riverine input to freshwater changes. Thus, for simplicity a zero-alkalinity endmember was assumed for all models.

Ocean carbon chemistry variables in ESMs commonly exhibit mean state biases . Therefore, observations of CT, AT, dissolved inorganic phosphorus and silicon, temperature, and salinity from GLODAPv2 , which is normalized to the year 2002, were used to calculate present-day Ωarag and Ωcalc, pH, and pCO2 using the mocsy2.0 routine and the equilibrium constants recommended for best practices . Future Ωarag and Ωcalc, pH, and pCO2 were calculated for each model as the sum of the simulated changes in CT, AT, dissolved inorganic phosphorus and silicon, temperature, and salinity from 2002 onwards and the observed quantities in 2002.

The present-day maximum sea surface density in the Arctic Ocean was calculated from monthly climatologies over 1986–2005, constructed from the respective salinity and temperature outputs of each model. Maximum sea surface density was calculated, as in , as the mean density of the densest 5 % of Arctic surface waters (95th-percentile waters) over all 12 months of the year.

2.3Simulations

The simulations performed within CMIP5 were not forced with the same atmospheric CO2 concentrations as the simulations performed under CMIP6. In CMIP5, historically observed atmospheric CO2 concentrations were used from 1850 to 2005 . From 2006 onwards, the CO2 concentrations follow the different RCPs. In CMIP6, the historical period was extended until 2014, and thereafter CO2 concentrations follow the different SSPs .

The different land and energy use assumptions in the SSPs compared to the RCPs lead to higher atmospheric CO2 trajectories over the 21st century for the Tier 1 SSPs compared to their RCP counterparts , which results in globally greater surface ocean acidification in CMIP6 compared to CMIP5 . Historical atmospheric CO2 concentrations were also refined with additional data available since CMIP5 . This refinement did not change the average atmospheric CO2 concentration from 1850 to 2005 (ΔpCO2atm=0±1 ppm) but did change annual CO2 concentration for single years by up to ±2 ppm. Furthermore, global CO2 concentrations were additionally provided as monthly latitudinally resolved concentrations, with model groups free to choose the forcing files they use .

2.4<inline-formula><mml:math id="M95" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">ant</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula> scaling

The different atmospheric CO2 trajectories over the 21st century between CMIP5 and CMIP6 complicates a comparison of simulated Arctic Ocean Cant inventories between model generations. To nevertheless compare the simulated Arctic Ocean Cant, we used the commonly applied scaling approach that assumes that the change in marine Cant is proportional to the atmospheric Cant concentration . Under this assumption, the Cant inventory in 2100 for each scenario (RCP8.5, SSP1-2.6, SSP2-4.5, SSP3-7.0) is linearly rescaled to that of SSP5-8.5 by multiplying the simulated Arctic Ocean Cant inventory under the respective scenario by the ratio of the mean atmospheric Cant concentration from 1850 to 2100 in SSP5-8.5 and the respective scenario.

This approximation is likely imprecise when very different scenarios are compared, such as SSP1-2.6 and SSP5-8.5, as the effects of circulation changes, sea ice melt, and warming are not considered. However, when comparing scenarios with the same radiative forcing, such as SSP5-8.5 and RCP8.5, it permits a first-order comparison.

2.5Observational constraints

As for the ESMs, the maximum sea surface density was calculated based on a monthly sea surface density climatology on a regular 1∘×1∘ grid, which was constructed from observed monthly salinity and temperature climatologies in the World Ocean Atlas 2018 .

The density uncertainty was calculated from the temperature and salinity uncertainties that were reported by the World Ocean Atlas following standard propagation of uncertainty. The total uncertainty is a combination of (1) the standard deviations for sea surface density derived from published standard deviations of sea surface temperature and salinity for each grid cell and each month in the World Ocean Atlas 2018, and (2) the standard deviation from the weighted mean of the 95th-percentile-density waters .

2.6Emergent constraints and probability density functions of <inline-formula><mml:math id="M105" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">ant</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula>

To calculate the emergent constraint, first an ordinary least-squares regression was calculated between the simulated present-day maximum sea surface density and the Arctic Ocean Cant inventory in 2100 for each ESM of the CMIP6 model ensemble. The uncertainty range was estimated using the 1σ prediction interval. In a second step the probability density function (PDF) from the observations was convoluted with the PDF from the linear regression, assuming a Gaussian distribution in both cases. The convolution of both PDFs is the constrained projection of the Arctic Ocean Cant inventory following previous studies . The PDFs for unconstrained projections of the Cant inventory were calculated using equal weights for each model and assuming a Gaussian distribution.

Extending the analysis of , PDFs for the constrained projections of the Cant inventory were calculated not only for the year 2100 but for each year from 2002 to 2100 and not only for the highest-emission scenario (RCP8.5) but for the four SSPs (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5).

3Results 3.1Arctic Ocean <inline-formula><mml:math id="M111" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">ant</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula> inventory 3.1.1Multi-model mean

Over the historical period from 1850 to 2005, the CMIP6 ESMs simulate an Arctic Ocean Cant increase of 2.1±0.6 Pg C (inter-model standard deviation; Fig. a). Thus, the CMIP6 ESMs simulate an Arctic Ocean Cant inventory in 2005 that is 36 % below the data-based estimate for the period from 1765 to 2005 of 3.3±0.3 Pg C .

Figure 1

Arctic Ocean Cant inventory over the 21st century. (a) Time series of multi-model mean Arctic Ocean Cant inventory over the 21st century for SSP1-2.6 (blue), SSP2-4.5 (green), SSP3-7.0 (orange), and SSP5-8.5 (red) with the respective standard deviation across the model ensemble (shading) (n=12–14). The bars on the right side of panels indicate the inter-model standard deviation in the year 2100. (b) Time series of Arctic Ocean Cant inventory for each scenario after the emergent constraint is applied.

Over the 21st century the Arctic Ocean Cant inventory increases depending on the SSP. Following the low-emission pathway SSP1-2.6 leads to a projected Arctic Ocean Cant inventory in 2100 of 6.2±1.3 Pg C. With increasing atmospheric Cant concentrations from SSP1-2.6 to SSP5-8.5, the projected Arctic Ocean Cant inventory in 2100 also increases, resulting in 8.0±1.7 for SSP2-4.5, 9.6±1.6 for SSP3-7.0, and 10.3±2.2 Pg C for SSP5-8.5. The relatively large uncertainties across the model ensemble (17–21 %) result in an overlap of the simulated Cant inventories in 2100 for SSP2-4.5, SSP3-7.0, and SSP5-8.5 within ±1 SD.

3.1.2Constrained results

As was shown for RCP8.5 in the CMIP5 model ensemble , a linear relationship between maximum sea surface density and the Arctic Ocean Cant inventory in 2100 is found across the CMIP6 model ensemble for all four of the SSPs (r2=0.63–0.72; Fig. a, c, e, and g). By deriving a similar relationship for the projected Cant inventory in all years from 2000 to 2100 and combining this with observations of present-day sea surface density, the uncertainty of the projected Arctic Ocean Cant inventory can be reduced throughout the 21st century (Fig. b). The emergent relationship is significant over all years from 2015 to 2100 (p<0.05) and the r2 increases from 0.39 in 2014 to 0.63–0.72 in 2100 depending on the scenario. In the year 2100, this results in Arctic Ocean Cant inventory estimates of 6.3±0.9 (SSP1-2.6), 8.2±1.2 (SSP2-4.5), 9.8±1.1 (SSP3-7.0), and 10.7±1.4 Pg C (SSP5-8.5) (Fig. b, d, f, and h). Thus, the emergent constraint is shown to slightly increase the CMIP6 multi-model mean projected Cant inventory for each SSP (+2 % to +4 %) and substantially reduce associated uncertainties (-29 % to -31 %) resulting in greater separation of the SSPs (Fig. ).

Figure 2

Emergent constraint on Arctic Ocean Cant inventory in 2100. Maximum Arctic Ocean sea surface density (95th percentile) and Arctic Ocean Cant inventory in 2100 for 12–14 ESMs for (a) SSP1-2.6 (blue), (c) SSP2-4.5 (green), (e) SSP3-7.0 (orange), and (g) SSP5-8.5 (red). The ordinary least-squares regressions (dashed lines) and the ±1σ prediction intervals (shaded area) are shown for each SSP. Probability density functions before and after applying the emergent constraint for (b) SSP1-2.6, (d) SSP2-4.5, (f) SSP3-7.0, and (h) SSP5-8.5.

3.2Ocean acidification 3.2.1Multi-model mean

Over the 21st century, ocean acidification leads to a reduction in Ωarag and Ωcalc (Fig. ). Until around 2040, the reduction of both saturation states averaged over the upper 1000 m is independent of the SSP. From 2040 onwards, the rate of the reduction depends on the pathway, and by 2100 basin-wide Ωarag averaged over the upper 1000 m reaches 1.00±0.06 (SSP1-2.6), 0.82±0.05 (SSP2-4.5), 0.66±0.04 (SSP3-7.0), and 0.59±0.05 (SSP5-8.5), while Ωcalc reaches 1.56±0.09 (SSP1-2.6), 1.29±0.08 (SSP2-4.5), 1.06±0.06 (SSP3-7.0), and 0.92±0.05 (SSP5-8.5).

Figure 3

Basin-wide Arctic Ocean calcium carbonate saturation states over the 21st century. Time series of multi-model mean basin-wide saturation states of aragonite from 2000 to 2014 (black) and from 2015 to 2100 for SSP1-2.6 (blue), SSP2-4.5 (green), SSP3-7.0 (orange), and SSP5-8.5 (red) averaged (a) from 0 to 1000 m, (c) from 0 to 50 m, (e) from 50 to 500 m, and (g) from 500 to 1000 m with ±1 SD (n=12–14) shown as a shaded area. The differences when changing from historical simulations to the SSPs result from the different number of ensemble members that are available for each simulation. (b, d, f, h) The same time series but for calcite.

The upper 1000 m of the Arctic Ocean is projected to be on average undersaturated with respect to aragonite by 2100 under all of the SSPs, with Ωarag in SSP1-2.6 just below 1 (0.995). Vertically, the entire water column will be undersaturated with respect to aragonite in 2100 for all pathways but SSP1-2.6 (Fig. ). Under this low-emissions pathway, Arctic Ocean waters above 500 m are projected to remain supersaturated with respect to aragonite, while those below 500 m are projected to be undersaturated. For the more stable calcium carbonate polymorph calcite, basin-wide Arctic Ocean undersaturation is only projected under SSP5-8.5. Under this high-emissions pathway, basin-wide undersaturation is projected for the water masses between 50 and 800 m.

Figure 4

Vertical profile of basin-wide Arctic Ocean calcium carbonate saturation states, pH, and pCO2 in 2100. Multi-model mean vertical profiles of (a) aragonite and (b) calcite saturation states, (c) pH, and (d) pCO2 in 2100 for SSP1-2.6 (blue), SSP2-4.5 (green), SSP3-7.0 (orange), and SSP5-8.5 (red) averaged (a) with ±1 SD (n=12–14) shown as a shaded area. Observed profiles in 2002 from GLODAPv2 are shown in black .

Alongside declines in calcium carbonate saturation states, pH and pCO2 are also projected to change over the 21st century. Basin-wide averaged pH in the upper 1000 m decreases from 8.06 in 2002 to 7.88±0.02 (SSP1-2.6), 7.79±0.03 (SSP2-4.5), 7.68±0.03 (SSP3-7.0), and 7.61±0.03 (SSP5-8.5), while basin-wide averaged pCO2 in the upper 1000 m increases to 739±38 (SSP1-2.6), 918±75 (SSP2-4.5), 1209±92 (SSP3-7.0), and 1428±124 µatm (SSP5-8.5). The lowest pH values and highest pCO2 values are projected to occur between 100 and 1000 m from the surface (Fig. ).

3.2.2Emergent constraints on acidification

The emergent relationship between observed maximum sea surface density and end-of-century Arctic Ocean acidification that was previously identified in CMIP5 models does not exist anymore in the new CMIP6 model generation (Fig. ). Despite the emergent constraint approach still functioning for projections of the Arctic Ocean Cant inventory in CMIP6, this no longer translates into an emergent constraint for Arctic Ocean Ωarag and Ωcalc. However, if all changes in ocean biogeochemistry with the exception of alkalinity, which is kept constant at observed levels from 2002 , are considered when estimating changes in Ωarag and Ωcalc, emergent relationships are still found between maximum sea surface density and Ωarag across all SSPs (r2=0.54–0.73). When decreases in alkalinity (AT) are also taken into account, Ωarag and Ωcalc decrease further, and the emergent relationship disappears (r2=0.01–0.12).

Figure 5

The CMIP6 ensemble relationship between end-of-century Ωarag and the Cant inventory in the Arctic Ocean. Basin-wide Ωarag averaged over the upper 1000 m with (orange) and without (blue) taking into account concurrent changes in alkalinity (AT) against the Arctic Ocean Cant inventory in the year 2100 for (a) SSP1-2.6, (a) SSP2-4.5, (c) SSP3-7.0, and (d) SSP5-8.5.

3.3Comparison between CMIP5 and CMIP6

Compared to ESMs from CMIP5, the new generation of ESMs (CMIP6) has improved in simulating the maximum Arctic Ocean sea surface density. Specifically, negative density biases have been reduced, and the inter-model range in maximum sea surface density has substantially decreased from 3.6 in CMIP5 to 0.9 kgm-3 in CMIP6 (Fig. ). As a result, the inter-model range of the Cant inventory in the CMIP6 model ensemble is also reduced (Fig. ). Moreover, without the negative maximum-sea-surface-density bias, the simulated multi-model mean Arctic Ocean Cant inventory in 2005 is 61 % higher than the inventory that was simulated by the previous model generation (1.3±0.7 Pg C) .

At the end of the 21st century, the unconstrained simulated Arctic Ocean Cant inventory under SSP5-8.5 is 37 % larger and the uncertainty is 19 % smaller than the unconstrained simulated Cant inventory in 2100 under RCP8.5 (7.5±2.9 Pg C) . After applying the constraint, the Arctic Ocean Cant inventory in 2100 under SSP5-8.5 (10.7±1.4 Pg C) is 19 % larger than the constrained Arctic Ocean Cant inventory under RCP8.5 (9.0±1.6 Pg C). This difference is of the same order of magnitude as the difference in prescribed atmospheric CO2 concentration over the 21st century, which is higher in SSP5-8.5 (CMIP6) than RCP8.5 (CMIP5) and therefore results in greater surface ocean acidification for approximately the same radiative forcing .

To compare the emergent constraint across scenarios with different atmospheric CO2 concentrations, the simulated Arctic Ocean Cant inventory in 2100 for each scenario was rescaled to SSP5-8.5 using the mean atmospheric Cant concentration from 1850 to 2100 as a linear scaling factor (Fig. ). The relationship remains robust (r2=0.63–0.74) for all five analysed scenarios. The slope of the emergent relationship is however substantially steeper in CMIP6 (9.4–12.6 PgCkg-1m3) than in CMIP5 (3.3 PgCkg-1m3). However, the slope in CMIP5 increases to 8.9 PgCkg-1m3 if the three CMIP5 models with particularly low maximum sea surface densities (<27.5 kgm-3) are excluded (dotted line in Fig. ). The resulting constrained estimate for the rescaled Arctic Ocean Cant inventory decreases from the low-emission scenario to the high-emission scenario from 12.3 to 10.7 Pg C. When comparing the two high-emission scenarios, the rescaled Arctic Ocean Cant inventories are 10.7 (SSP5-8.5) and 10.4 Pg C (RCP8.5). The latter remains unchanged if the three CMIP5 models with particularly low maximum sea surface densities (<27.5 kgm-3) are excluded.

Figure 6

Arctic Ocean Cant inventory in 2100, rescaled to SSP5-8.5 atmospheric Cant concentrations (see Methods), against maximum Arctic Ocean sea surface density (95th percentile) for SSP1-2.6 (blue diamonds; r2=0.70), SSP2-4.5 (green squares; r2=0.63), SSP3-7.0 (orange triangles; r2=0.70), SSP5-8.5 (red circles; r2=0.72), and RCP8.5 (purple stars; r2=0.74) (a) over the entire density range and (b) over the density range from 27.5 to 28.3 kgm-3. The simulated Arctic Ocean Cant inventory in 2100 for each scenario (Fig. ) was rescaled to SSP5-8.5 using the mean atmospheric Cant concentration from 1850 to 2100 as a scaling factor. Linear fits use all available models for each scenario (coloured dashed lines). For RCP8.5 an additional fit is shown excluding the three models with density anomalies below 27.5 kgm-3 (b) (dotted purple line; r2=0.79).

4Discussion 4.1Arctic Ocean <inline-formula><mml:math id="M233" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">ant</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula> inventory

Across the CMIP6 model ensemble, the Arctic Ocean Cant storage over the 21st century is highly related to maximum sea surface densities (Fig. ), which predominately occur in the Barents Sea . The inter-model range in maximum sea surface density in the Barents Sea is mainly explained by differences in sea surface salinities (r2=0.93), which are influenced by brine rejection and the strength of inflowing, saltier Atlantic waters through the Barents Sea Opening (Fig. ). Compared to CMIP5 models, the reduced negative bias of simulated maximum sea surface densities (Fig. ) indicates model improvement in simulating the circulation of Atlantic and Arctic surface waters. Despite the reduced inter-model range, the robust relationship between maximum sea surface density and Cant across model generations (Fig. ) supports evidence that inflowing Atlantic waters through the Barents Sea Opening and their transformation into deep and intermediate waters via brine rejection are the dominant process governing Arctic Ocean Cant increases . The application of observational constraints to this emergent multi-model relationship in order to constrain the projected Arctic Ocean Cant inventory (Fig. ) and focus efforts on model development therefore remains promising.

Figure 7

Annual mean sea surface salinity in the Barents Sea and its neighbouring waters from (a) the World Ocean Atlas 2018 and from the CMIP6 model with the (b) highest (UKESM1-0-LL) and (c) lowest (CESM2-WACCM) maximum sea surface density and salinity. The 0 ∘C isotherm indicates approximately the extent of Atlantic waters (black solid line).

However, the slope of the linear relationship between maximum sea surface density and the Arctic Ocean Cant inventory over the 21st century in the CMIP5 model ensemble (3.3±0.6 PgCkg-1m3, scaled to SSP5-8.5 atmospheric Cant concentrations) is 3–4 times less than that in the CMIP6 model ensemble (12.6±2.6 for SSP1-2.6, 12.1±2.8 for SSP2-4.5, 9.4±1.9 for SSP3-7.0, and 11.3±2.0 PgCkg-1m3 for SSP5-8.5) (Fig. ). The reduced slope in the CMIP5 ensemble is mainly caused by three models with maximum surface density anomalies well below 27.5 kgm-3. When these three models are excluded, the remaining CMIP5 models follow a slope of 8.9±2.2 PgCkg-1m3, in broad agreement with the CMIP6 model ensemble (Fig. ). This suggests that the linear emergent relationship does not hold below a certain value of maximum sea surface density below which the impact on deep-water formation and subsequent Cant storage in the Arctic Ocean is limited. However, as the two linear relationships happen to cross the observed maximum sea surface density at nearly the same location (Fig. ), the constrained Cant inventory for the CMIP5 model ensemble remains almost entirely unchanged when the three low-density models are excluded.

Even without the low-density bias in the Barents Sea, the constrained Arctic Ocean Cant inventory in 2005 in CMIP6 remains 36 % below the data-based estimate . This underestimation is partly due to the different definition of Cant in data-based estimates and ESMs. While the historical simulations in CMIP5 and CMIP6 typically start in 1850, data-based estimates account for all Cant since 1765. This leads to an underestimation of the global ocean Cant inventory by ESMs of around 30 % and of around 20 % in the Arctic Ocean . Even if we increased the constrained Arctic Ocean Cant inventory in 2005 by 20 %, an underestimation of around 16 % would remain compared to the data-based estimate. This underestimation of the data-based estimate suggests that all ESMs are missing additional pathways of Cant entry into the Arctic Ocean, other than the principal pathway via the Barents Sea. Indeed, small-scale density flows along continental slopes can be observed in different regions of the Arctic Ocean but cannot be simulated by the coarse resolution of most ESMs. Thus, the constrained estimates of the Arctic Ocean Cant inventory presented here are likely still a lower boundary.

Recent observation of dilution of AT and CT in surface waters of the Amerasian Basin caused by freshening have led to the hypothesis that continuous freshening might turn the Arctic Ocean from a sink of Cant into a source over the 21st century . However, observations in the Eurasian basins, which receive more saline Atlantic water and less freshwater input, still show increases of Cant concentrations over a depth of 1500 m in the last 20 years. The CMIP5 and CMIP6 model ensembles both simulate continuous accumulation of Cant in the Arctic Ocean under all SSPs (Fig. ), suggesting that the subduction of Cant-rich Atlantic waters in the Barents Sea remains larger than any loss of Cant in surface waters over the 21st century. Nevertheless, the reduction of the storage rate of Cant under SSP5-8.5 (Fig. ) in combination with constantly increasing atmospheric CO2 concentrations indicates that dilution may reduce the capacity of the Arctic Ocean to store further Cant as suggested by .

4.2Arctic Ocean acidification 4.2.1<inline-formula><mml:math id="M271" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Ω</mml:mi><mml:mtext>arag</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula> and <inline-formula><mml:math id="M272" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Ω</mml:mi><mml:mtext>calc</mml:mtext></mml:msub></mml:mrow></mml:math></inline-formula>, pH, and <inline-formula><mml:math id="M273" display="inline"><mml:mrow class="chem"><mml:mi>p</mml:mi><mml:msub><mml:mi mathvariant="normal">CO</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> in 2100

Even under the most optimistic scenario assessed (SSP1-2.6), the Arctic Ocean will become on average undersaturated with respect to aragonite, with possible consequences for calcifying organisms and the food chain . In this scenario only water masses above 500 m remain supersaturated this century. These findings are in good agreement with idealized estimates of Ωarag and Ωcalc that project the future Cant inventory based on the transient time distribution method and observed CFC-12 concentrations .

In addition to widespread Arctic Ocean undersaturation with respect to aragonite, water masses between 50 and 800 m are even projected to become undersaturated with respect to calcite by the end of the century under SSP5-8.5. Calcite undersaturation in Arctic Ocean subsurface waters will likely further enhance the pressure on the Arctic Ocean ecosystem as calcite-forming organisms, such as foraminifera and coccolithophores , experience potential impacts on growth and survival.

Furthermore, the projected increases in pCO2 (Fig. c and d) alongside projected increases in its seasonal amplitude are likely to lead to hypercapnic conditions that might directly affect the growth and survival of Arctic fish under high-emissions scenarios.

4.2.2Driving processes of acidification

The CMIP6 model ensemble shows astonishingly good agreement with respect to projections of Arctic Ocean acidification over the 21st century. Compared to the CMIP5 model ensemble, the uncertainties in projected Ωarag averaged over the upper 1000 m in 2100 have been reduced from 0.13 to 0.04–0.06, and those for Ωcalc from 0.21 to 0.05–0.09.

The main reason for the reduced uncertainty appears to be that the ESMs that have a lower maximum sea surface density and that thus take up less Cant in the Arctic Ocean over the 21st century (Fig. ) are the same models that simulate a stronger reduction in AT (Fig. ). To explain this negative correlation between Cant uptake and reductions in AT, we propose the following mechanism: over the 21st century, melting of sea ice, land ice, and increased river runoff are projected to freshen the Arctic Ocean and to reduce AT (Fig. ) . In the CMIP6 ensemble, this freshening and reduction of AT tends to be stronger in models with lower sea surface salinities and densities; i.e. the models that simulate less inflow of saline Atlantic waters, such as CESM2-WACCM (Fig. ), have lower Barents Sea surface salinity and density, weak deep-water formation, and therefore less Cant storage (Fig. ). In contrast, models with a larger inflow of saline Atlantic water have stronger deep-water formation, greater Cant storage, less freshening, and a smaller reduction in AT. This compensation then results in a similar simulated reduction of Ωarag and Ωcalc over the 21st century across all models in the CMIP6 ensemble, through either a reduction in AT or an increase in CT. Other possible drivers, such as changes in temperature and salinity, are likely of minor importance given the relatively good correlation between the projected Cant inventory and basin-wide reduction of Ωarag and Ωcalc over the 21st century when AT is not taken into account (r2=0.54–0.73).

Figure 8

CMIP6 basin-wide changes in alkalinity (AT) averaged over the upper 1000 m from 2002 to 2100 caused by freshwater dilution (blue) and changes in biogeochemistry (green) against the Arctic Ocean Cant inventory for (a) SSP1-2.6, (b) SSP2-4.5, (c) SSP3-7.0, and (d) SSP5-8.5. AT anomalies were decomposed by calculating the temporal evolution of salinity-corrected alkalinity following .

In the CMIP5 model ensemble, this compensation effect did not exist, and projected saturation states and pH were mainly driven by the projected Cant storage in each model , with changes in AT, temperature, and salinity being of minor importance. The relatively small changes in AT in CMIP5 compared to CMIP6 might have been caused by an underestimation of Arctic Ocean freshening over the 21st century given that the CMIP5 models also underestimated historical Arctic Ocean freshening by around 50 % in the Arctic Ocean . In the CMIP6 model ensemble this negative bias with respect to freshwater fluxes and subsequent dilution of AT appears to have been reduced. This is probably a consequence of better representation of riverine fluxes as well as increased model resolution and hence improved circulation .

To estimate the likely impacts of CT increases and AT decreases to Arctic Ocean acidification over the 21st century, one can assess their relative contributions to acidification in constrained estimates of end-of-century Cant (dotted line in Fig. ). In SSP5-8.5 for example, increasing CT along with changes in temperature and salinity reduce Ωarag from 1.38 to 0.75, while decreasing AT further reduces this to 0.59. Thus, the future storage of Cant in the Arctic Ocean still remains the main driver of Arctic Ocean acidification, with acidification further enhanced by decreases in AT.

5Conclusions

Earth system models tend to have improved their performance in the Arctic Ocean from CMIP5 to CMIP6. The negative bias with respect to present-day maximum sea surface density is substantially reduced in CMIP6, and the historical Cant inventory is closer to the data-based estimate . Furthermore, the inter-model range of projected Ωarag and Ωcalc and the Cant inventory have been reduced. Uncertainties in the future Cant inventory under four shared socioeconomic pathways can be further reduced by applying the previously identified emergent constraint between present-day maximum Arctic Ocean sea surface density and the future Cant inventory .

define “confirmed” emergent constraints as fulfilling the following criteria: (1) a plausible mechanism, (2) verification of that mechanism, and (3) out-of-sample testing. The mechanism underpinning the relationship between maximum sea surface densities and the projected Cant inventory in the Arctic Ocean is consistent with observations and has been verified in an ocean-only model at different resolutions . The testing of this emergent constraint in the CMIP6 ensemble can be considered out of sampling, although the extent of model independence between generations remains questionable . Despite the overall emergent constraint on projected Arctic Ocean Cant being similar between CMIP6 and CMIP5, the change in the multi-model emergent relationship indicates that further validation is required. Specifically, it appears that the linear relationship between maximum sea surface densities and the projected Cant inventory breaks down in models with extremely low-biased sea surface densities, highlighting a potential limitation to this emergent constraint that requires further assessment.

While the mechanisms underlying emergent constraints on the future Arctic Ocean Cant inventory appear to be consistent between CMIP5 and CMIP6, this is not the case for the previously identified emergent constraint on ocean acidification variables (Ωarag and Ωcalc, pH, and pCO2). In the CMIP6 model ensemble, projected model uncertainties in Ωarag and Ωcalc, pH, and pCO2 are dramatically reduced compared to the uncertainties in CMIP5 and are driven not only by the increase in Cant but also by a reduction in AT due to pan-Arctic freshening. A weak inflow of saline Atlantic waters in ESMs results not only in lighter surface waters and less Cant storage over the 21st century but also in a stronger reduction of alkalinity caused by a stronger freshening. A strong Atlantic inflow, however, appears to limit freshening and reductions in alkalinity but results in greater Cant storage. Although both Cant storage and alkalinity reductions contribute considerably to Arctic Ocean acidification, our results suggest that Cant remains the dominant process. In the CMIP5 model ensemble, the influence of freshening on emergent constraints on acidification variables might have been of limited importance because (a) freshening was largely underestimated and (b) differences in simulating the inflow of Atlantic waters through the Barents Sea Opening were considerably larger across the model ensemble . Thus, the absence of an emergent constraint on projected Arctic acidification in CMIP6 could be viewed as a consequence of successful model development. Indeed, there is clearly no need for such constraints when models are in broad agreement.

Independent of the driving mechanism, the projected Arctic Ocean acidification over the 21st century has possibly grave consequences for the wider Arctic Ocean ecosystem . Under all socioeconomic pathways, the upper 1000 m of the Arctic Ocean will be on average undersaturated with respect to aragonite. Thus, keystone species like the sea butterfly (aragonitic pteropod Limacina helicina) may lose most to all of their suitable habitat in the Arctic Ocean . However, under the low-emissions pathway SSP1-2.6 aragonite undersaturation is projected to be avoided in the upper 500 m of the water column, providing a potential refuge. Under the high-emissions pathway SSP5-8.5, water masses between 50 and 800 m are additionally projected to become undersaturated with respect to calcite. While isolated regions of the surface Arctic Ocean that are strongly influenced by riverine inputs are already seasonally undersaturated with respect to calcite , subsurface Arctic waters would be the first to exhibit annually averaged and basin-wide calcite undersaturation over a depth of several hundred metres. Vertically migrating organisms that form calcite shells and skeletons, such as coccolithophores and foraminifera , may lose their natural refugia to which they migrate during the day . These hostile conditions for calcifying organisms will likely cause their decline, with unknown consequences for the wider Arctic food web and its iconic species .

Data availability

The Earth system model output used in this study is available via the Earth System Grid Federation (https://esgf-node.ipsl.upmc.fr/projects/esgf-ipsl/, last access: 31 March 2021). For further information, please see Table 1. Observations from the World Ocean Atlas 2018 (https://www.nodc.noaa.gov/OC5/woa18/, last access: 3 January 2020; ) and GLODAPv2 (https://www.nodc.noaa.gov/ocads/oceans/GLODAPv2_2019/, ) are available via the National Oceanic and Atmospheric Administration.

Author contributions

The study was led by JT, who made the figures and wrote the initial manuscript. TB and LK provided help for the analyses and interpretation of the results. OT processed the CMIP6 model data (download, regridding). All authors contributed to the final manuscript.

Competing interests

The authors declare that they have no conflict of interest.

Disclaimer

The work reflects only the authors' view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.

Acknowledgements

We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP. For CMIP the US Department of Energy's Program for Climate Model Diagnosis and Intercomparison provided coordinating support and led the development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We also thank the IPSL modelling group for the software infrastructure, which facilitated CMIP6 analysis.

Financial support

This research has been supported by the Horizon 2020 (4C (grant no. 821003), CRESCENDO (grant no. 641816), and COMFORT (grant no. 820989)), the Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (grant no. PP00P2_170687), the Agence Nationale de la Recherche (grant no. ANR-18-ERC2-0001-01), and the Norges Forskningsråd (grant no. 275268).

Review statement

This paper was edited by Jean-Pierre Gattuso and reviewed by Scott C. Doney and Claudine Hauri.

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