Twenty-first century ocean warming, acidification, deoxygenation, and upper ocean nutrient decline from CMIP6 model projections

Anthropogenic climate change leads to ocean warming, acidification, deoxygenation and reductions in near-surface nutrient concentrations, all of which are expected to affect marine ecosystems. Here we assess projections of these drivers of environmental change over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) that were forced under the 35 CMIP6 Shared Socioeconomic Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced under the Representative Concentration Pathways (RCPs). 10 CMIP5 and 13 CMIP6 models are used in the two multi-model ensembles. Under the high-emission scenario SSP5-8.5, the model mean change (2080-2099 mean values relative to 1870-1899) in sea surface temperature, surface pH, subsurface (100600 m) oxygen concentration and euphotic (0-100 m) nitrate concentration is +3.48±0.78 °C, -0.44±0.005, 40 13.27±5.28 mmol m and -1.07±0.45 mmol m, respectively. Under the low-emission, high-mitigation scenario SSP1-2.6, the corresponding changes are +1.42±0.32 °C, -0.16±0.002, -6.36±2.92 mmol m and -0.53±0.23 mmol m. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions, consistent with previous studies. The Earth system models in CMIP6 generally project greater surface warming, acidification, deoxygenation and euphotic nitrate reductions than those from 45 CMIP5 under comparable radiative forcing, with no reduction in inter-model uncertainties. Under the highemission CMIP5 scenario RCP8.5, the corresponding changes in sea surface temperature, surface pH, subsurface oxygen and euphotic nitrate concentration are +3.04±0.62 °C, -0.38±0.005, -9.51±2.13 mmol m and -0.66±0.49 https://doi.org/10.5194/bg-2020-16 Preprint. Discussion started: 27 January 2020 c © Author(s) 2020. CC BY 4.0 License.


Ocean warming, acidification, deoxygenation and enhanced nutrient limitation
Since the preindustrial period the global oceans have experienced fundamental change in physical and geochemical conditions as a result of anthropogenic climate change. Although these physicochemical changes reflect the climate services that the oceans provide through heat and carbon storage, they also have major 60 implications for the health of marine ecosystems.
Temperature is a principal determinant of biological metabolism in the ocean (e.g. Eppley, 1972) and plays a major role in shaping the global distribution of marine species (e.g. Thomas et al., 2012;Sunagawa et al., 2015).
The radiative forcing associated with greenhouse gas emissions results in an accumulation of heat in the Earth Marine organisms typically experience changes in multiple physical and geochemical conditions simultaneously, with impacts determined by the interactions between potential stressors. For example, the combined effect of 130 warming and deoxygenation is projected to force poleward and vertical contractions of metabolically viable habitat for marine ectotherms (Deutsch et al., 2015). At the physiological level, experimental studies indicate that synergistic effects between potential marine stressors are common (Gunderson et al., 2016). Compound warming and acidification, has been shown to exacerbate negative impacts on photosynthesis, calcification, reproduction and survival of marine organisms (Harvey et al., 2013), while compound exposure to acidification 135 and low oxygen can also have synergistic effects (McBryan et al., 2013), and may reduce the thermal tolerance of certain species (Pörtner, 2010).
Here we assess future projections of climate-related drivers of marine impacts within the Coupled Model Intercomparison Project Phase 6 (CMIP6; Eyring et al., 2016;O'Neill et al., 2016) simulations, evaluating how 140 these differ from previous CMIP5 (Taylor et al., 2011) simulations. We focus on projected changes in ocean temperature, pH and dissolved O 2 and NO 3 concentration across 13 CMIP6 and 10 CMIP5 Earth system models.

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A comprehensive assessment of changes between CMIP5 and CMIP6 in the ocean biogeochemical components of ESMs and their associated skill is provided in Séférian et al, (in review). Since CMIP5, CMIP6 has seen a general increase in the horizontal grid resolution of physical ocean models and a limited increase in vertical resolution. The latter may be particularly important for ecosystem projections as it directly affects simulated stratification, a key factor influencing changes in ocean impact drivers (Capotondi et al., 2012;Bopp et al., 2013; 150 Laufkötter et al., 2015;Kwiatkowski et al., 2017) and their impact on higher trophic levels (Stock et al., 2014;Chust et al., 2014;Kwiatkowski et al., 2018;Lotze et al., 2019). Updates in the representation of ocean biogeochemical processes between CMIP5 and CMIP6 have generally included increases in model complexity (Séférian et al., in review). Specifically, CMIP6 models provide more widespread inclusion of micronutrients, such as iron, variable stoichiometric ratios, and improved representation of lower trophic levels including concentrated in the oceans and where impacts from climate change are typically greatest. Specifically, we assess projections of surface ocean temperature, surface ocean pH, subsurface dissolved O 2 concentration (averaged 170 between 100-600 m) and upper-ocean NO 3 concentration (averaged between 0-100 m). The choice of vertical integral for O 2 reflects the potential importance of the expansion of oxygen minimum zones, which are most prominent at such depths. The choice of vertical integral for NO 3 reflects its importance as a critical macronutrient supporting primary production in the euphotic zone. Both vertical integrals are chosen to be compatible with the recent assessment of marine drivers in the IPCC Special Report on the Ocean and

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Cryosphere (Bindoff, et al., in press Consequently, we do not assess the role of internal variability in the emergence of climate-related changes in marine ecosystems drivers (e.g. Frölicher et al., 2016;Lovenduski et al., 2016;Krumhardt et al. 2017;Freeman et al., 2018). Two of the CMIP6 models included in our analysis (GFDL-CM4 and ACCESS-ESM1.

From Representative Concentration Pathways to Shared Socioeconomic Pathways
Aside from changes in ESMs, a fundamental difference between CMIP5 and CMIP6 is that they differ in the In this study, we confine our assessment of ocean impact drivers to concentration-driven simulations, focussing on SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5 of CMIP6, which result in end-of-century radiative forcing of 2.6, 4.5, 7.0 and 8.5 W m -2 , respectively. The SSPs have generally higher associated concentrations of 230 atmospheric CO 2 and lower associated atmospheric concentrations of CH 4 and N 2 O relative to their RCP counterparts (Meinshausen et al., 2011;O'Neill et al., 2016;Meinshausen et al., 2019). This is particularly the case for SSP5-8.5, which in comparison to RCP8.5, assumes that coal constitutes a greater proportion of the primary energy mix in the second half of the 21 st century (Kriegler et al., 2017). Given that differences among  (Table 3), respectively. As the changes have no statistical overlap across the two scenarios (with the exception of subsurface oxygen), the CMIP6 projections further demonstrate the effectiveness of intense mitigation strategies in limiting twenty-first century marine ecosystem exposure to potential stress. This is in agreement with assessments of previous multimodel projections (e.g. CMIP5; Bopp et al., 2013).

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Following previous assessments , model structural uncertainty is estimated as the inter-model standard deviation. Although some of this model spread is due to internal variability, this contribution is relatively small for global averages and expected to decline throughout the twenty-first century (Frölicher et al., 2016). Relative to scenario uncertainty, which is estimated as the maximum difference between mean SSP 265 projections, model structural uncertainty is extremely low for surface pH projections, which show distinct separation between the SSPs prior to 2050. The low model structural uncertainty associated with projections of surface ocean pH is well characterised and associated with the identical CO 2 forcing used by all ESMs in concentration-driven SSP and RCP projections , a weak climate-pH feedback (Orr et al., 2005;McNeil and Matear, 2007), limited interannual variability and consistently adopted standards for ESM 270 ocean carbonate chemistry equations (Orr et al., 2017). Surface ocean pCO 2 and corresponding carbonate chemistry generally follows changes in atmospheric CO 2 with a global mean equilibration time of approximately 8 months (Gattuso and Hansson, 2011). The differences between projected surface pH across the SSPs therefore reflect the divergence of prescribed atmospheric CO 2 concentrations, i.e., the different scenarios.

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In contrast, projections of SST exhibit greater model structural uncertainty (Fig. 1). This uncertainty is likely to result from differences in climate sensitivity between models. Historically, such differences have been attributed to diversity in cloud feedbacks and to a lesser extent water vapour and lapse-rate feedbacks (Andrews et al., 2012;Vial et al., 2013). For projections of subsurface oxygen and euphotic-zone nitrate concentrations, model structural uncertainty is greater still and can exceed scenario uncertainty. This greater structural uncertainty is a 280 result of oxygen and nitrate concentrations being strongly influenced by both physical changes (e.g. changes in solubility, circulation and mixing) and changes in biological sources and sinks (Stramma et al., 2012;Fu et al., 2016;Bopp et al., 2017;Oschlies et al., 2018).

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Global scale projections of end-of-century upper-ocean impact drivers (2080-2099 anomalies relative to 1995-2014 mean values) exhibit spatial variability that is both ocean impact driver and SSP dependent (  (Stramma et al., 2008), which may be a result of climate variability (Deutsch et al., 2011;Bindoff et al., in press). They are however, in line with 315 previous projections, including those from CMIP5, which have highlighted that coarse-resolution models struggle to reproduce subsurface ventilation pathways in these regions (Stramma et al., 2012;Andrews et al., 2013;Bopp et al., 2013;Cabré et al., 2015).
For a subset of the CMIP6 models, projected changes in subsurface O 2 concentration under SSP5-8.5 were O 2sat and AOU under SSP5-8.5 are similar to that of the CMIP5 models under RCP8.5 . The general reduction in O 2sat has been shown to be predominantly due to warming driven reductions in solubility,

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while the heightened AOU declines in the North Pacific and North Atlantic have been attributed to reductions in ventilation and an increase in the age of these waters Tjiputra et al., 2018).
CMIP6 model-mean projections of NO 3 concentrations in the euphotic zone (0-100 m) show variable regional declines under SSP1-2.6 and SSP5-8.5 (Fig. 2g,h). These declines are largest in the Arctic Ocean, equatorial This is partly due to the depth integral over which the index is defined but in the CMIP5 models was also attributed to intensified surface westerlies (Swart and Fyfe, 2012), which increases surface-layer mixing and upwelling in the Southern Ocean (Fu et al., 2016). Regions of enhanced stratification are typically projected to 350 experience reductions in euphotic NO 3 concentrations, in agreement with previous projections (Bopp et al., 2001;Cabré et al., 2014;Fu et al., 2016). An exception to this however, is in certain Arctic Seas, where there are reductions in both stratification index and euphotic-zone NO 3 concentrations. This is presumably a consequence of the loss of permanent or semi-permanent sea ice and a corresponding increase in wind-driven mixing. Atlantic is characterised by sensitivity to combined acidification and nutrient stress, while the Arctic Ocean is sensitive to compound warming, acidification and nutrient stress.

CMIP6 vs. CMIP5 projections
While the temporal behaviour of changes in ocean impact drivers is similar across the CMIP5 and CMIP6 model suites (Fig. 1), the CMIP6 Earth system models generally project greater global surface ocean warming, surface ocean acidification, subsurface deoxygenation and euphotic-zone NO 3 reduction than the CMIP5 projections 375 performed with comparable radiative forcing (Fig. 6,  in RCP2.6, RCP4.5 and RCP8.5, respectively (Fig. 6, Table 3). The greater euphotic-zone NO 3 concentration declines in SSPs compared to their RCP analogue is likely a consequence of the enhanced surface warming in CMIP6 models. This warming results in a greater increase in upper-ocean stratification than that projected in CMIP5 models (Cabré et al., 2014;Fu et al., 2016), the result of which is a greater reduction in the supply of 400 nutrient-rich deep waters to the euphotic zone in CMIP6 projections.

Global benthic ocean projections
On average, bottom waters are consistently projected to warm, acidify and deoxygenate across the twenty-first century (Fig. 7). Under SSP1-2.6, the end-of-century model mean changes ( (Table 4). Thus even for bottom waters, CMIP6 projections highlight that intense mitigation strategies can limit ecosystem exposure to potential warming and acidification stress during the twenty-first century (e.g. Tittensor et al., 2010;Levin and Le Bris, 2015).

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The magnitude of projected changes in bottom waters is less than in surface and upper-ocean waters, while bottom-water uncertainties for a given scenario are larger (Fig. 7). This contrast is particularly evident for pH projections with the SSPs, whose ranges of uncertainties fully separate before 2050 in the surface ocean ( Fig. 1) but still overlap in 2080 for bottom waters. This relative increase in model structural uncertainty results from 425 surface ocean chemistry being in equilibrium with the same atmospheric CO 2 concentrations for all models.
Conversely, benthic pH changes are strongly influenced by ocean circulation, which transports anthropogenic carbon in the upper ocean to the seafloor and is variably impacted by climate change across models (e.g.

Regional patterns of benthic ocean change
In bottom waters, the end-of-century spatial distributions of changes in temperature, pH and dissolved O 2 are 440 similar between SSPs (Fig. 8) and in broad agreement with CMIP5 projections (Sweetman et al., 2017). The intensity of warming and acidification is typically greater in SSP5-8.5 than SSP1-2.6, particularly in coastal

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Projected end-of-century acidification is highly limited in most bottom waters, however in the North Atlantic, Arctic Seas and certain continental shelf waters, pH changes can exceed -0.1 in SSP1-2.6 and -0.3 in SSP5-8.5.
For shelf waters, the greater bottom-water pH declines can be the result of coupling between surface waters, which experience large changes in carbonate chemistry, and bottom waters (e.g. through mixing and 455 entrainment), as well as benthic remineralization of organic matter (Bates et al., 2009). In contrast, enhanced bottom-water acidification in the North Atlantic is associated with deep-water formation and high uptake of anthropogenic carbon (Sabine et al., 2004), which rapidly propagates anomalies in surface ocean chemistry to depth. Bottom-water acidification has been previously projected in the North Atlantic by an ensemble of CMIP5 models under RCP8.5 (Gehlen et al., 2014;Sweetman et al., 2017).

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Enhanced acidification in subsurface mode and intermediate waters has been observed at time series stations (Dore et al., 2009;Byrne et al., 2010;Bates et al., 2012) and in CMIP5 model projections Bopp et al., 2013;Watanabe and Kawamiya, 2017). Although observational studies have suggested that this enhancement results from changes in circulation and biological activity (Dore et al., 2009;Byrne et al., 485 2010), model results indicate that it can be explained by the geochemical effect of rising atmospheric CO 2 and the particular carbonate chemistry of these waters (Orr, 2011;Resplandy et al., 2013). Specifically, the enhanced https://doi.org/10.5194/bg-2020-16 Preprint. Discussion started: 27 January 2020 c Author(s) 2020. CC BY 4.0 License.
acidification sensitivity in mode and intermediate waters has been attributed to their lower temperatures and their higher ratio of dissolved inorganic carbon to total alkalinity relative to that found in surface waters of the same regions (Orr, 2011;Resplandy et al., 2013). The seasonal amplitude of global surface ocean temperature is projected to increase by +0.59± 0.21 °C across SSP5-8.5 (Fig. 11). Over most of the ocean, the seasonal amplitude of sea surface temperature is projected to

Conclusions
The latest CMIP6 Earth system models consistently project surface ocean warming and acidification, subsurface The CMIP6 projections of warming, acidification, deoxygenation and nutrient reduction are greater than those of previous CMIP5 models under comparable radiative forcing. The enhanced acidification is a consequence of 555 higher atmospheric CO 2 concentrations in the SSPs than their RCP analogues. The enhanced warming however reflects the greater climate sensitivity of the CMIP6 models. This increased warming results in greater increases in upper-ocean stratification, which contributes to greater reductions in euphotic nitrate and subsurface oxygen concentration.

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Projected changes to the mean state and seasonality of physical and chemical ocean conditions are likely to present major challenges to diverse marine ecosystems from the surface ocean to abyssal depths. Potential organism stress is likely to be exacerbated by simultaneous exposure to multiple physicochemical changes, emphasising the need for extensive emissions reductions.

Data availability
The Earth system model output used in this study is available via the Earth System Grid Federation (https://esgfnode.ipsl.upmc.fr/projects/esgf-ipsl/).

Author contribution
LK and LB conceived and designed this study. LK, LB and OT processed model outputs and performed the analysis. All authors contributed to the ocean biogeochemistry development of the CMIP6 ESMs and/or the manuscript text.

Competing interests
The authors declare that they have no conflict of interest.

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This article reflects only the authors' view -the funding agencies as well as their executive agencies are not responsible for any use that may be made of the information that the article contains.