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

. Anthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation, reductions in near-surface nutrients and changes to primary production, 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 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 multi-model global mean change (2080-2099 mean values relative to 1870-1899) ± the inter- model standard deviation in sea surface temperature, surface pH, subsurface (100-600 m) oxygen concentration, euphotic (0-100 m) nitrate concentration and depth integrated primary production is +3.47±0.78 °C, -0.44±0.005, -13.27±5.28 mmol m -3 , -1.06±0.45 mmol m -3 and -2.99± 9.11 %, respectively. Under the low-emission, high-mitigation scenario SSP1-2.6, the corresponding global changes are +1.42±0.32 °C, -0.16±0.002, -6.36±2.92 mmol m -3 , -0.52±0.23 mmol m -3 and -0.56± 4.12 %. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions, consistent with previous studies. lesser primary production declines than those from CMIP5 under comparable radiative forcing. The increased projected ocean warming results from a general increase in the climate sensitivity of CMIP6 models relative to those of CMIP5. This enhanced warming increases upper ocean stratification in CMIP6 projections, which 50 contributes to greater reductions in upper-ocean nitrate and subsurface oxygen ventilation. The greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher associated atmospheric CO 2 concentrations than their RCP analogues for the same radiative forcing. We find no consistent reduction in inter-model uncertainties, and even an increase in NPP inter-model uncertainties in CMIP6, as compared to CMIP5.


Ocean warming, acidification, deoxygenation, nutrient stress and reduced primary production
Since the preindustrial period the global oceans have experienced fundamental changes in physical and biogeochemical conditions as a result of anthropogenic climate change. Although these changes reflect the 60 climate services that the oceans provide through heat and carbon storage, they also have major implications for the health of marine ecosystems. Ocean ecosystems are affected by the direct and indirect consequences of climate change. Atmospheric warming and rising CO 2 concentrations drives ocean warming and acidification, while these direct factors cause changes that modulate other important components of the ocean system, such as oxygenation, nutrient levels and net primary production.
of ESMs and their associated skill is provided in Séférian et al, (in revision). 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;Laufkötter et al., 2015;Kwiatkowski et al., 2017) and their impact on higher trophic levels (Stock et al., 2014; 160 Chust et al., 2014;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 revision). Specifically, CMIP6 models provide more widespread inclusion of dissolved oxygen, micronutrients, such as iron, variable stoichiometric ratios, and improved representation of lower trophic levels including heterotrophic bacteria and the cycling and sinking of organic matter (Séférian et al., in 165 revision).
Relative to CMIP5, the CMIP6 Earth system models display an improved ability to reproduce the modern meanstate distribution of a number of key biogeochemical tracers (Séférian et al, in revision). Although global scale totals of ocean carbon flux and net primary production estimates have been improved in CMIP6 with respect to 170 CMIP5, the simulated geographical distribution of present-day mean state air-sea CO 2 fluxes and surface chlorophyll concentrations show only moderate improvements between CMIP5 and CMIP6. There are also moderate improvements in the representation of subsurface dissolved oxygen concentrations in most ocean basins. Model skill in the representation of surface macronutrient concentrations in CMIP6 has improved for dissolved silicate but declined slightly for nitrate.

Methodology
The analysis of projected multiple ocean impact drivers presented here focuses on three key depth levels: the upper ocean, the thermocline, and the benthic zone. The surface zone is where most biological activity is 180 concentrated in the oceans and where impacts from climate change are typically the greatest. Specifically, we assess projections of surface ocean temperature, surface ocean pH, subsurface dissolved O 2 concentration (averaged between 100-600 m), upper-ocean NO 3 concentration (averaged between 0-100 m) and net primary production (depth integrated over the full water column). The choice of vertical range for O 2 reflects the potential importance of the expansion of oxygen minimum zones, which are more prominent at such depths. The

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choice of vertical range for NO 3 reflects its importance as a critical macronutrient supporting primary production in the euphotic zone. Both vertical ranges are chosen to be compatible with the recent assessment of marine drivers in the IPCC Special Report on the Ocean and Cryosphere (Bindoff, et al., in press). Additionally, for the CMIP6 models we assess benthic ecosystem drivers, focussing on projections of bottom temperature, pH and O 2 concentration. The benthic level is defined as the bottom ocean model layer at each grid point. As such, its exact 190 depth depends on vertical discretisation and bathymetry, which differs across the CMIP6 ensemble. All benthic model outputs were corrected for potential drift at the grid-cell level (e.g. Gehlen et al., 2014;Séférian et al., 2016) using coincident preindustrial control simulations.

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All ESMs assessed in the CMIP5 and CMIP6 ensembles (Tables 1 and 2) include physical ocean models and coupled ocean biogeochemistry schemes that account for some or all of the potential ocean impact drivers: temperature, pH, O 2 , NO 3 and net primary production. A total of 10 CMIP5 and 13 CMIP6 models are assessed with the model ensemble size differing among scenarios depending on contributions from each model group.

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The CMIP5 ensemble is the same as that used in the comprehensive assessment of projected ocean drivers provided by Bopp et al. (2013). Only one ensemble member per model is used for a given scenario. That is, in CMIP terminology we typically use ensemble member 'r1i1p1' from each CMIP5 model and 'r1i1p1fx' from each CMIP6 model (where 'fx' is the recommended set of external forcings employed by the various modelling groups). Consequently, we do not assess the role of internal variability in the emergence of climate-related ESM1.5) do not include NO 3 as a prognostic tracer. Hence their NO 3 concentrations were calculated from modelled total dissolved inorganic phosphorus assuming a constant Redfield ratio of 16:1.

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To facilitate intercomparison, model output on each native grid was regridded to the same regular 1°x1° horizontal grid using distance weighted average remapping (climate data operators; remapdis). Model outputs were kept on their native vertical grids, with vertical discretisation ranging from 40 (MPI-ESM1.2) to 75 (IPSL-CM6A-LR, CNRM-ESM2-1 and UKESM1-0-LL) levels, except for models using hybrid or isopycnic vertical coordinates for which model outputs were vertically regridded to 35 (GFDL-ESM4, GFDL-CM4) and 70 (NorESM2-LM) levels. Following generally adopted practice (e.g. Bopp et al., 2013), all models were given equal weighting in the respective CMIP6 and CMIP5 ensemble mean. However, within the CMIP6 ensemble two modelling groups contributed two ESMs and within the CMIP5 ensemble three modelling groups contributed two ESMs, which is likely to influence the extent of model independence (Masson and Knutti, 2011;Knutti et al., 2015;Sanderson et al., 2015;Lovenduski et al. 2017).

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The CMIP5  used as a baseline period. Throughout the analysis, the uncertainty associated with global mean projections is assessed using the inter-model standard deviation (given ± uncertainties). At regional scales, projection 230 robustness is evaluated using previously adopted approaches (e.g. Bopp et al., 2013) including whether the magnitude of the multi-model mean anomaly exceeds the inter-model standard deviation or if there is at least 80 % model sign agreement. The interquartile range of regional projections is given in the annexes.

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Aside from changes in ESMs, a fundamental difference between CMIP5 and CMIP6 is that they differ in the future scenarios used for anthropogenic emissions and land-use change. Those scenarios are derived from integrated assessment models and based on plausible future pathways of societal development. In CMIP6, the Shared Socioeconomic Pathways (SSPs) provided via the Scenario Model Intercomparison Project 240 (ScenarioMIP) are used instead of the RCPs that were used in CMIP5 (O'Neill et al., 2016). The SSPs provide revised emission and land-use scenarios relative to the RCPs (Riahi et al., 2017).

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which result in end-of-century approximate total radiative forcing levels of 2.6, 4.5, 7.0 and 8.5 W m -2 , respectively. The SSPs have generally higher associated concentrations of atmospheric CO 2 and lower associated atmospheric concentrations of CH 4 and N 2 O relative to their RCP counterparts (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 twenty-first century 250 (Kriegler et al., 2017). Given that differences among projections of surface ocean acidification are dominated by scenario uncertainty, with relatively little inter-model uncertainty and internal variability (e.g. Bopp et al., 2013;Frölicher et al., 2016), such changes in atmospheric concentrations of CO 2 are expected to have a large impact on projections of ocean pH and related carbonate system variables.

Comparison with historical global trends
Observed historical trends in global mean surface ocean temperature, pH and subsurface oxygen were compared with the multi-model mean of the CMIP6 ensemble over the corresponding years of historical simulations (Table   265 3). Global observations of historical trends in euphotic-zone nitrate concentrations and integrated primary production were deemed insufficiently robust, given the associated interannual-decadal variability, to be assessed in the models (Elsworth et al. 2020  well reproduced by the CMIP6 models, particularly considering that the observed trend is a reconstruction based on discrete surface ocean fCO 2 measurements and alkalinity estimates that are derived from temperature and salinity . Finally, with respect to subsurface deoxygenation, the observed dissolved oxygen trend of -0.30 to -1.52 mmol m -3 decade -1 from 1970 to 2010 (90% confidence range; Bindoff, et al., in press) encompasses the CMIP6 multi-model mean response over the corresponding years. Given the performance of 275 the CMIP6 models at reproducing ocean biogeochemical mean conditions (Séférian et al, in revision) and trends, they are deemed appropriate to project future trends in biogeochemistry under the SSPs.

Global upper-ocean projections
Under all SSPs, global multi-model mean sea surface temperature is projected to increase, while surface pH, subsurface dissolved oxygen concentration, euphotic-zone nitrate concentration and net primary production are projected to decline during the twenty-first century (Fig. 1). The projected change in the five ocean impact drivers increases with associated radiative forcing across the four SSPs. Under the high mitigation SSP1-2.6 scenario, the end-of-century model mean changes (2080-2099 mean values relative to 1870-1899) in sea surface respectively. Under the high emissions scenario SSP5-8.5 the corresponding changes are 3.47±0.78 °C, -0.44±0.005, -13.27±5.28 mmol m -3 , -1.06±0.45 mmol m -3 and -2.99± 9.11 % (Table 4), respectively. Across these two scenarios, the separation between CMIP6 projections of sea surface temperature and pH, and to a 290 lesser extent oxygen and nitrate, 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 multi-model projections (e.g. CMIP5; Bopp et al., 2013).
Following previous assessments , inter-model uncertainty is estimated as the inter-model 295 standard deviation. Although some of this model spread is due to internal variability, this contribution is limited for global averages and its relative contribution to inter-model uncertainty is 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 projections, inter-model uncertainty is extremely low for surface pH projections, which show distinct separation between the SSPs prior to 2050. The low inter-model uncertainty associated with 300 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 ocean carbonate chemistry equations (Orr et al., 2017). Surface ocean pCO 2 and corresponding carbonate chemistry generally follow changes in atmospheric CO 2 with a global mean equilibration time of 305 approximately eight 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.
In comparison to pH, projections of SST exhibit greater inter-model uncertainty (Fig. 1). This uncertainty is 310 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, inter-model uncertainty is greater still and can exceed scenario uncertainty. This greater inter-model uncertainty is a result of oxygen and nitrate concentrations being strongly influenced by both physical changes (e.g. changes 315 in solubility, circulation and mixing) and changes in biological sources and sinks (Stramma et al., 2012;Fu et al., 2016;Oschlies et al., 2018).
The inter-model uncertainty associated with CMIP6 net primary production projections is consistently larger than the scenario uncertainty. Indeed for each SSP, individual models project both increases and decreases in 320 global primary production, with the inter-model standard deviation encompassing positive and negative anomalies ( Fig. 1). This is a consequence of net primary production changes reflecting a diverse and delicately The spatial patterns of CMIP6 projected changes in subsurface 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, while the heightened AOU declines in the North Pacific and North Atlantic have been primarily attributed to reductions in ventilation and an increase in the age of these waters Tjiputra et al., 2018).

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Projections of primary production anomalies are highly diverse across regions (Fig. 2n,o). The global CMIP6 multi-model mean decline in primary production is shown to be primarily driven by declines in the North Atlantic and the western equatorial Pacific, which can exceed 40 gC m -2 y -1 under SSP5-8.5. In the high latitudes, primary production generally increases, with anomalies approaching 20 gC m -2 y -1 in parts of the Arctic and 400 Southern Oceans under SSP5-8.5. Such changes have historically been associated with enhanced stratification as the upper ocean warms (Doney, 2006). In tropical and mid-latitude waters, where phytoplankton are nutrient limited, this tends to reduce the vertical nutrient supply and exacerbate nutrient stress. In contrast, in high latitude waters, where phytoplankton are typically light limited, enhanced stratification can reduce mixing below the euphotic depth and therefore result in reduced light stress. However, the simplicity of this paradigm has been demonstrating the additional importance of changes to the horizontal advection of nutrients (Whitt, 2019) and zooplankton grazing (Laufkötter et al., 2015) in shaping regional primary production responses.

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Although the general pattern of NPP changes is similar in CMIP6, compared to CMIP5, regional declines are reduced in magnitude, less spatially extensive and are typically less robust. This is particularly notable in the Indian Ocean and subtropical North Pacific, which were regions of consistent NPP decline in CMIP5 projections , but exhibit limited robust trends in CMIP6. The projected increase in NPP in the high latitudes is however broadly consistent with previous model intercomparisons (Steinacher et al., 2010;  In these regions, particularly the North Atlantic, the maximum mixed layer depth also shoals. As such, there is strong evidence that reduced vertical mixing and entrainment of nutrients into the upper ocean is, at least partially, responsible for these regional declines in primary production. However, similar increases in stratification and reductions in mixed layer depth occur in regions such as the North Pacific and Indian Ocean, where declines in primary production are largely absent. Therefore further assessment of simultaneous changes 440 in processes such as nutrient advection (e.g. Whitt, 2019), nitrogen fixation (Riche and Christian, 2018), the microbial loop (e.g. Schmittner et al., 2008;Taucher and Oschlies, 2011) and top-down grazing pressure (e.g. Laufkötter et al., 2015) are required to fully understand the regional primary production response in CMIP6.

Compound stressors
The projected occurrence of multiple potential ecosystem stressors in the upper ocean was determined across the SSPs using prescribed thresholds of surface warming (>+2 °C), surface acidification (< -0.2 units), subsurface deoxygenation (< -30 mmol m -3 ) and euphotic-zone NO 3 decline (< -1 mmol m -3 ), with anomalies calculated as 2080-2099 mean values relative to 1995-2014 (Fig. 5). It should be noted that our choice of stressor thresholds, based on the magnitude of biogeochemical anomalies, are somewhat arbitrary. Indeed, it could be argued that absolute biogeochemical thresholds, for example as defined for hypoxia or oligotrophy, may better reflect potential ecosystem stress. Moreover, the thresholds take no account of regional differences in natural temperature, pH, O 2 and NO 3 variability, which may mediate ecosystem responses to changes in mean conditions (e.g. Kroeker et al., 2020). That being said, a single threshold that encompasses the variety of

CMIP6 vs. CMIP5 projections
While the temporal behaviour of changes in ocean impact drivers is similar across the CMIP5 and CMIP6 model 470 ensembles (Fig. 1), the CMIP6 Earth system models generally project greater global surface ocean warming, acidification, subsurface deoxygenation and euphotic zone NO 3 reductions than the CMIP5 projections performed with comparable radiative forcing (Fig. 6, Table 4). The CMIP6 models however, project reduced global primary production declines relative to comparable CMIP5 simulations. There is no consistent reduction in inter-model uncertainty in CMIP6. In fact, with respect to projections of primary production, inter-model 475 uncertainty is substantially increased in CMIP6.

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Therefore although the SSP and RCP simulation pairs have analogous end-of-century radiative forcing, the higher CO 2 levels in the SSPs result in greater acidification for the CMIP6 projections.
The greater euphotic-zone NO 3 concentration declines in SSPs compared to their RCP analogues are likely a consequence of the enhanced surface warming in CMIP6 models. This warming results in a greater increase in 495 upper-ocean stratification than that projected in CMIP5 models (Cabré et al., 2014;Fu et al., 2016). At the global scale, models have been shown to consistently exhibit strong negative correlations between relative stratification anomalies and relative NO 3 anomalies on interannual timescales (Fu et al., 2016). The greater increases in stratification in CMIP6 therefore result in greater reductions in mixing and entrainment of nutrient-rich deep waters into the euphotic zone in comparison with CMIP5.

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The enhanced subsurface deoxygenation in the SSPs relative to comparable RCPs, is likely the consequence of both physical and biogeochemical processes (e.g. Oschlies et al., 2018). The greater warming in CMIP6 projections results in a greater reduction in O 2 solubility, while also affecting the ventilation and transport of O 2 within the ocean interior. In addition, concurrent changes in biological production, export and 505 respiration can either mitigate or exacerbate physically driven subsurface deoxygenation (Oschlies et al., 2018).
The reduced declines in global net primary production projected for the twenty-first century in the SSPs, relative to comparable RCPs, combined with large increases in the associated inter-model uncertainty, is striking ( Fig. 6e). Particularly, given that declines in euphotic zone NO 3 concentrations are typically greater in the SSPs.

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This suggests that the temporal evolution of phytoplankton resource limitation and grazing pressure under climate change may have significantly altered between CMIP5 and CMIP6. In previous CMIP biogeochemistry intercomparisons, all models projected global primary production declines, albeit with large inter-model uncertainty (Steinacher et al., 2010;Bopp et al., 2013). However in CMIP6, four of the models (CESM2, CESM2-WACCM, CNRM-ESM2-1 and IPSL-CM6A-LR) consistently project global increases in primary 515 production across the SSPs and are primarily responsible for both the reduced multi-model mean declines and the large increase in inter-model standard deviation. Global increases in net primary production have been previously documented in other model studies (Schmittner et al., 2008;Taucher and Oschlies, 2011;Laufkötter et al., 2015) and attributed to temperature dependent intensification of the microbial loop increasing regenerated production. Further analysis of the CMIP6 models that project primary production increases is clearly required to determine whether this is also the case, or additional processes (e.g. the temporal evolution of nitrogen fixation or iron limitation) explain why they differ from previous generations of the same Earth system model family.
The magnitude of projected changes in bottom waters is less than in surface and upper-ocean waters, while 535 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 uncertainty fully separate before 2050 in the surface ocean ( Fig. 1) but still have some overlap in 2080 for bottom waters. This relative increase in inter-model uncertainty results from 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

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 similar between SSPs (Fig. 8) and in broad agreement with CMIP5 projections (Sweetman et al., 2017). The intensity of warming, acidification and deoxygenation is generally greater in SSP5-8.5 than SSP1-2.6, in benthic 555 waters above 2000 m, while at greater depths the impact is similar for both SSPs.
The largest projected benthic warming in SSP1-2.6 and SSP5-8.5 occurs in continental shelf waters, the Arctic Seas and the Southern Ocean, where temperature increases can exceed 0.5 °C by the end-of-century (2080-2099 average relative to the 1995-2014 baseline). In contrast, for most of the abyssal benthic ocean projected increases 560 in temperature are less than 0.2 °C. The characteristic North Atlantic "warming hole" present in projections of the surface ocean (Fig. 2) is also evident in benthic layers above 1000 m, such as the mid-Atlantic ridge (Fig.   8b,c). This represents the only major region of multi-model mean benthic cooling across SSP1-2.6 and SSP5-8.5, with however high associated uncertainty. As in the surface ocean, this cooling is likely associated with a slow down of the Atlantic meridional overturning circulation (Drijfhout et al., 2012;Menary and Wood, 2018).
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 -0.2 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 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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In contrast to temperature and pH, projections of benthic dissolved O 2 concentration show changes that are not predominantly confined to shelf waters and specific regions. Most of the global benthic ocean is projected to experience deoxygenation under both SSP1-2.6 and SSP5-8.5, even at depths below 2000 m (Fig. 8). Bottomwater deoxygenation is higher in the Southern Ocean, equatorial Pacific and North Atlantic, where declines in 580 the multi-model mean can exceed 20 mmol m -3 .
It should be noted that Earth system models are not explicitly designed to explore the benthic biogeochemical response to climate change and certain caveats should be considered. Model spin-up simulations, although longer in CMIP6 than CMIP5 (Séférian et al., in revision), are typically insufficient in length to equilibrate 585 biogeochemical conditions in the deep ocean (Séférian et al., 2016) and therefore contemporaneous preindustrial control simulations are required to correct biogeochemical drift. Vertical thickness of bottom ocean layers is also highly variable across the CMIP6 ensemble, although for a given model resolution is typically highest near the surface and decreases dramatically with depth. As such, the extent to which continental shelves are resolved greatly differs and uncertainties associated with resolution are pronounced in the abyssal ocean.

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Moreover, the representation of biogeochemical processes associated with ocean sediments and benthic ecosystems is typically absent or highly limited in ESMs.

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The depth of maximum end-of century pH and [H + ] change is often below the surface, and it varies regionally in CMIP6 projections (Fig. 9). Although the maximum pH change is usually found in surface waters in the high latitudes and upwelling regions, it is typically located between 200-400 m in subtropical mode and intermediate  (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., 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 610 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).

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Changes in the seasonal amplitude of surface ocean temperature, pH and hydrogen ion concentration ([H + ]) were determined after detrending, by subtracting a cubic spline fit from the monthly time series in each grid cell, and then calculating the annual peak-to-peak amplitude for each year of the detrended data set, following the approach of Kwiatkowski and Orr (2018).

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The multi-model mean 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 show limited but robust increases (< +0.5 °C). However, in the North Atlantic, North Ocean, the seasonal amplitude typically increases by > +5 °C.
The CMIP6 projections of the changing seasonal amplitude of SST under SSP5-8.5 are consistent with previous projections from the CMIP5 models (Carton et al., 2015;Alexander et al., 2018). The limited increases in SST seasonal amplitude for most of the global ocean have been attributed to greater relative shoaling of the mixed 650 layer depth in summer than in winter (Alexander et al., 2018). However, in the Arctic Ocean the large increase in SST seasonal amplitude is primarily due to the loss of sea ice. The seasonal melting and refreezing of sea ice accounts for approximately half of the present-day seasonal Arctic Ocean net surface heat flux, buffering seasonal variability in Arctic Ocean heat content and SSTs (Serreze et al., 2007;Fig. 11). The loss of this seasonal melting/freezing cycle under high-emissions scenarios such as RCP8.5 has been shown to account for a 655 doubling of seasonal Arctic Ocean heat content variability. Ice loss further amplifies the seasonal cycle of SSTs by increasing the seasonal cycle of net surface heat fluxes. The net downward radiative flux increases in summer as albedo declines, while the net upward radiative flux increases in winter due to greater evaporative and sensible heat loss (Carton et al., 2015).

Conclusions
The latest CMIP6  In addition to changing mean-state conditions, the CMIP6 models also project changes to the seasonal cycles of temperature and carbonate chemistry under the SSPs. The seasonal amplitude of surface ocean acidity ([H + ]) nearly doubles over the twenty-first century under SSP5-8.5, with a concurrent reduction in the seasonal 675 amplitude of pH. Over the same period, the seasonal amplitude of temperature is projected to increase, particularly in the Arctic Ocean.
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 680 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. The CMIP6 multi-model mean projections of primary production declines are less than those of previous CMIP5 models under comparable radiative forcing however there is a large increase in inter-model 685 uncertainty that requires further assessment.
Projected changes to the mean state and seasonality of biogeochemical 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 biogeochemical changes, emphasising the 690 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://esgf-695 node.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 700 manuscript text.

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

Disclaimer
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.