It is now well understood that the global surface ocean, whose pH has been reduced by ~0.1 in response to rising atmospheric CO2 since industrialization, will continue to become more acidic as fossil fuel CO2 emissions escalate. However, it is unclear how uncertainties in climate sensitivity to future CO2 emissions will alter the manifestation of ocean acidification. Using an earth system model of intermediate complexity, this study performs a set of simulations that varies equilibrium climate sensitivity by 1.0°–4.5°C for a given CO2 emissions scenario and finds two unexpected and decoupled responses. First, the greater the climate sensitivity, the larger the surface mixed layer acidification signal but the smaller the subsurface acidification. However, taken throughout the ocean, the highest climate sensitivity will paradoxically cause greater global warming while buffering whole-ocean pH by up to 24% on centennial time scales. Second, this study finds a large decoupling between pH and carbonate ion concentration in surface waters whereby these chemical properties show opposite effects under variable climate sensitivity. For every 1°C increase in climate sensitivity, the surface ocean pH reduction grows by 4%, while surface ocean carbonate ion reduction shrinks by 2%. The chemical and spatial decoupling found here highlights the importance of distinguishing the biological impacts of pH and aragonite saturation and understanding the spatial extent of important calcifying biomes so as to truly understand the long-term impacts of ocean acidification.
Much of the concern with regard to fossil fuel consumption and emissions of CO2 into the atmosphere is focused on climate change. For example, Working Group 2 of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) has an extensive assessment of adverse effects of enhanced warming and changes in precipitation on human health, native ecosystems, food production, water, and the like (Parry et al. 2007). However, relatively little is discussed about ocean acidification, which occurs as a direct consequence of the oceanic uptake of fossil fuel CO2.
Since human industrialization began in the late eighteenth century, the global ocean has absorbed nearly half of all fossil fuel CO2 emitted to the atmosphere (Sabine et al. 2004). Because CO2 becomes a (weak) acid when hydrated, this net uptake causes the ocean to become more acidic, lowering seawater pH (Caldeira and Wickett 2003). Surface ocean pH largely tracks atmospheric pCO2, because equilibration of CO2 in seawater is abiotically controlled and very fast. Therefore, it is not difficult to quantify the degree of surface ocean acidification that has occurred over the past 200 years given the atmospheric trajectory of pCO2. Previous studies have shown consistently that on average this drop in pH over the postindustrial period is about 0.1 (Caldeira and Wickett 2003; McNeil and Matear 2007; Orr et al. 2005). A drop in pH indicates an increase in the hydrogen ion concentration ([H+]) and a decrease in the carbonate ion concentration ().
Ocean acidification is a cause of concern since it has widely been shown to adversely affect marine organisms. For corals, commercially important shellfish, and other calcifying organisms in the ocean, a drop in  increases the energy costs of calcification at a cost of growth and reproduction and reduces the stability of their CaCO3 shells and skeletons. In the ocean,  at saturation with respect to CaCO3 minerals aragonite or calcite (sat) depends strongly on pressure and temperature. The degree to which seawater  deviates from sat is commonly approximated as their ratio (Ω ≈ /sat, since the seawater [Ca2+] variation is very small). A value of less than 1.0 for Ω indicates an undersaturated state, where CaCO3 dissolution would be favored thermodynamically. Business-as-usual future CO2 levels are expected to bring about these corrosive conditions for aragonite (Ω < 1), the less stable form of CaCO3, in the Southern Ocean (McNeil and Matear 2008) and the Arctic in the next few decades (Steinacher et al. 2009).
Acidification is also expected to impact noncalcifying organisms in the ocean. In the laboratory, the health of some fish juveniles and physiological functions are shown to be adversely affected by acidification (Devine et al. 2012; Domenici et al. 2012; Munday et al. 2010, 2009). The pH also influences the chemical speciation of such elements as iron (Shi et al. 2010), which is a critical micronutrient for marine phytoplankton, so ocean acidification may have far broader consequences for the larger marine food web. Recognizing such potential negative impacts of future ocean acidification, Zeebe et al. (2008) argue that anthropogenic CO2 emissions need to be reduced regardless of climate change.
In contrast to the surface ocean pH response to anthropogenic CO2 emissions, there is significantly more uncertainty regarding the global climate response. Working Group 1 (WG1) of IPCC AR4 (Solomon et al. 2007) defines the equilibrium climate sensitivity as “global mean annual mean surface air temperature change experienced by the climate system after it has attained a new equilibrium in response to a doubling of atmospheric CO2 concentration” (about 550 ppm). As such, it gives no indication of spatial heterogeneity or occurrence of abrupt changes or extreme events. However, it is useful to know how much average surface air temperature would change as radiative forcing (e.g., having more or less CO2 in the atmosphere) changes, because many aspects of climate model scale well with global average temperature. WG1 synthesized estimates of equilibrium climate sensitivity using a combination of historical proxy data and coupled climate models and concluded that the most likely range to be between 1.5° and 4.5°C, while values beyond this range (<1.5° and 4.5°–8°C) should not be completely excluded. The spread in the models’ climate sensitivity estimates is attributed to the difference in climate feedbacks involving water vapor, clouds, and ice albedo (Solomon et al. 2007).
An ensemble of global model simulations suggests that this level of climate sensitivity will become a dominant factor in influencing oceanic uptake of carbon and therefore atmospheric pCO2 in the coming centuries. It shows that under a postindustrial atmospheric CO2 stabilization scenario of 650 ppm, the oceanic storage of anthropogenic CO2 can vary by ~100 Pg C (Pg C = 1015 gram C) (Matsumoto et al. 2010). This climate sensitivity of long-term oceanic anthropogenic CO2 absorption must also translate to a climate sensitivity for ocean acidification, especially below the surface mixed layer. This expectation serves as motivation for this study. We use a global climate model of intermediate complexity to explore how the long-term manifestation of ocean acidification depends on the climate sensitivity for a given CO2 emissions scenario. The main findings include the following: 1) the acidification signal is decoupled spatially between the surface and subsurface waters; and 2) the two common measures of ocean acidification, pH and Ω, become decoupled as the temperature dependence of thermodynamic quantities becomes important.
The model employed here is the first version of the Minnesota Earth System Model for Ocean biogeochemistry (MESMO 1.0), a 3D global model of biogeochemistry and climate of intermediate complexity (Matsumoto et al. 2008). Intermediate complexity models have been shown to yield results comparable to high-resolution, coupled atmosphere–ocean models in terms of global attributes of air temperature, ocean circulation, and responses to atmospheric CO2 perturbations and oceanic freshwater perturbations (Solomon et al. 2007). MESMO is derived from the fast climate model of Edwards and Marsh (2005) and has three modules: a 3D ocean circulation model, a dynamic thermodynamic model of sea ice, and a 2D energy-moisture balance model of the atmosphere. The model is used successfully in a number of process studies of the global carbon cycle of the present and glacial periods (Chikamoto et al. 2008; Lee et al. 2011; Matsumoto et al. 2010; Sun and Matsumoto 2010; Ushie and Matsumoto 2012) as well as in community-wide model intercomparison projects (Archer et al. 2009; Cao et al. 2009).
Since the model details are already provided elsewhere, some of its salient features are noted here. The dynamics of the ocean model is based on the frictional geostrophic equations as described by Edwards and Marsh (2005). It has a 36 × 36 equal area horizontal grid with 10° increments in longitude and is uniform in sine of latitude; latitude spacing increases from about 3° at the equator to about 20° at the poles. There are 16 levels in the vertical with production occurring within the top two layers and 100 m. The biogeochemical module of MESMO includes a simple prognostic export production that depends on Michaelis–Menten nutrient uptake kinetics, light, estimated phytoplankton biomass, temperature, and mixed layer depth. Mixed layer depth is diagnosed using σt = 0.125 (Levitus 1982). Remineralization of particles is based on tuned rates of sinking and temperature-dependent remineralization. Also, seasonal variation in insolation causes seasonal variation in export production, as light availability, mixed layer depth, and vertical nutrient supply change.
Export production of carbon, nitrogen, and phosphorus are related by elemental stoichiometry of C:N:P = 117:16:1 according to Anderson and Sarmiento (1994). The production of CaCO3 occurs in waters supersaturated with carbonate ion with respect to mineral calcite. It is related to organic carbon production by a ratio, which depends on the degree of supersaturation, so that in lower pH waters, CaCO3 production is reduced and thus organic carbon to CaCO3 export ratio becomes higher. Key biogeochemical parameters have been tuned using oxygen and global production as targets (Matsumoto et al. 2008). For example, the global export production of particulate organic carbon and CaCO3 (10.6 and 0.9 Pg C yr−1) compare reasonably well to available estimates (Dunne et al. 2007).
The equilibrium run uses the initial conditions of preindustrial era and has atmospheric pCO2 of 278 ppm. It is well calibrated in terms of biological production and with respect to standard metrics of the modern ocean ventilation, including anthropogenic carbon, chlorofluorocarbon, and natural radiocarbon. For example, the oceanic uptake of anthropogenic carbon and CFC-11 for the year 1994 is 120 Pg-C and 0.68 × 109 moles, respectively, which compare favorably to data based estimates of 118 ±19 Pg C and 0.55 ±0.12 × 109 moles (Matsumoto et al. 2008). The uptake of these transient tracers would compare well against those in the community wide Ocean Carbon Cycle Model Intercomparison Project (Matsumoto et al. 2004).
Here we use a scenario of 5000 Pg C emitted to the atmosphere over a time scale of 500 years according to Zeebe et al. (2008). The scenario is based on historic emissions data until year 2010 and a future-projected emissions curve assuming a Gaussian function. Such a functional form is typically assumed, because the consumption of fossil fuels is expected to follow the so-called Hubbert curve, which predicts that the production rate of a resource will rise and fall symmetrically about a peak. In the chosen scenario, which is taken as an extreme emissions scenario, a peak in emissions of more than 24 Pg C yr−1 occurs in year 2126. Although we do not present these results here, we also performed a suite of simulations using a range of more optimistic emissions scenarios, which are also briefly discussed below. This 5000 Pg C emissions scenario is applied to MESMO with variable equilibrium climate sensitivities, which span the range of sensitivities of IPCC AR4 (Solomon et al. 2007). For CO2 doubling, the eight climate sensitivities are 0.9°, 1.4°, 2.0°, 2.5°, 2.9°, 3.4°, 3.8°, and 4.3°C of warming (see Fig. 1a legend). Values above 4.5°C are very unlikely according to IPPC AR4, and more recent climate sensitivity studies indeed suggest lower sensitivity (Padilla et al. 2011; Schmittner et al. 2011). All runs are 1750 years long starting from the preindustrial state in year 1751 and ending in year 3500.
3. Results and discussion
For the same 5000 Pg C emissions scenario (Fig. 1a), the model responds with different magnitudes of global surface air warming depending on the climate sensitivity (Fig. 1b). Warming exceeds 13°C following the peak emissions in the simulation with the greatest climate sensitivity. These simulations show consistently that the greater the climate change, the higher the atmospheric pCO2 (Fig. 1c), the lower the surface ocean pH (Fig. 1d), and the smaller the oceanic uptake of carbon (Fig. 1e). In all simulations, the net uptake of anthropogenic carbon by the World Ocean causes lower Ω throughout the water column compared to the preindustrial (Fig. 1f).
The change in anthropogenic CO2 storage under varying climate sensitivities is significant. By year 3500, low climate sensitivity will result in an ocean anthropogenic CO2 inventory of about 3000 Pg C, while it drops to 2400 Pg C in high climate sensitivity. Future oceanic uptake of anthropogenic CO2 will be altered by ~20% based on the current span of uncertainty for climate sensitivity under this extreme emissions scenario. We also tested this using a suite of more optimistic emissions scenarios, including a 650-ppm stabilization scenario, which showed a ~12% anthropogenic CO2 climate sensitivity (Matsumoto et al. 2010). We, therefore, can suggest that under the span of potential emissions pathways into the future, the climate sensitivity impact on long-term anthropogenic CO2 uptake by the ocean is 12%–20%. It is the manifestation of this climate sensitivity uncertainty on ocean acidification that we explore, and it is important to note that the decoupling we describe below is independent to the emission scenario used.
The notion that the net anthropogenic carbon uptake by the oceans becomes smaller as global warming becomes stronger is consistent with previous studies of carbon–climate feedbacks (Bopp et al. 2005, 2001; Crueger et al. 2008; Joos et al. 1999; Matear and Hirst 1999; Matsumoto et al. 2010; Plattner et al. 2001; Sarmiento et al. 1998; Zickfeld et al. 2008). A number of reasons, physical, chemical, and biological, contribute to this net result, but the important factors are a reduction in gas solubility and enhancement of upper-ocean stratification, both due to warming. Those factors lead to much greater net outgassing of CO2 by the ocean. Other contributing factors include a gradual reduction of the acid neutralizing capacity of seawater as the ocean absorbs anthropogenic CO2. For a given pulse of carbon emissions, when global warming is greater, a larger fraction of the emitted CO2 remains in the atmosphere, which then leads to a more severe surface ocean acidification. That is, the surface ocean is hit the hardest by climate change and by acidification when climate sensitivity is high. There is thus an additional layer of complexity to the irony that oceanic uptake of anthropogenic carbon, which causes acidification, slows the rise of atmospheric CO2, and alleviates climate change (Zeebe et al. 2008).
Ocean acidification below the surface mixed layer, however, manifests in the opposite sense to the overlying surface layer (Fig. 2). The reduction in the whole ocean pH, which essentially reflects the subsurface change, is the smallest for the simulation with the largest climate change. It reflects the fact that enhanced stratification limits the penetration of anthropogenic CO2 to the surface mixed layer, thus exacerbating acidification there, while sparing the subsurface from severe acidification. The reduction in pH over the entire simulation period is 15% greater at the surface and 24% smaller for the whole ocean in the highest climate sensitivity simulation compared to the lowest climate sensitivity simulation. Because the range in those climate sensitivities is 3.4°C, this translates to pH reduction growing by 4% at the surface and shrinking by 7% for the whole ocean for every 1°C increase in climate sensitivity. The transition between the surface signal and subsurface signal occurs below 100 m, near the base of the surface mixed layer (Fig. 2b).
This spatial decoupling in acidification between surface mixed layer and subsurface layers may have unexpected implications on the vertical distribution, sensitivity, and resilience of certain organisms into the future. Pteropods, for example, are common macrozooplankton throughout the world’s ocean and important dietary components for some other zooplankton and higher trophic species like herring, salmon, whales, and birds. Pteropods are known to vertically migrate between the surface ocean and 300 m (Hunt et al. 2008). The vertical decoupling of ocean acidification we find here may act on this vertical migration, such that for a given CO2 emissions scenario, pteropods may become more preferentially distributed in the depth zone where acidification is more limited. That is, they may become more abundant toward the surface mixed layer if Earth’s true climate sensitivity were low, and more abundant toward the subsurface in opposite case. Other vertically migrating organisms, calcifiers or noncalcifiers, which are also sensitive to changes in pH may have their distributions altered as well. Such changes may lead to unexpected changes in the larger ecosystem or the food web. At the same time, the vertical decoupling in acidification may have important impacts on CaCO3 and alkalinity cycling. In the highest sensitivity simulation, remineralization of export CaCO3 particles occurs rapidly immediately below the mixed layer, presumably reflecting the greater degree of acidification there. As a consequence, the high climate sensitivity runs in general have shorter depth scale of CaCO3 remineralization. A more rigorous analysis of this phenomenon would require a mechanistic consideration of ballasts, which impart dissimilar sinking velocity and microenvironment for dissolution depending on the ballast material (Klaas and Archer 2002).
Finally, aragonite Ω and pH are often taken to be measures of ocean acidification; however, the reduction in Ω is the smallest (Fig. 3a), when the pH change is the largest (Fig. 1d). This chemical decoupling increases with higher climate sensitivity. The reason that Ω and pH become decoupled is that the temperature dependence of the thermodynamic constants that govern carbon speciation in seawater becomes significant (McNeil and Matear 2007). To illustrate this point, at a salinity of 37 PSU, pCO2 of 1500 ppm, and alkalinity of 2400 μmol kg−1, the seawater carbonate ion  is calculated to be 55 μmol kg−1 when temperature is 12°C. However, when the temperature is increased to 25°C while keeping other parameters the same, changes in the CO2 dissociation constants causes  to jump to 91 μmol kg−1, a 65% increase. For pH, the same temperature increase causes it to change minimally from 7.53 to 7.55. In Fig. 3b, the reduction in  over the entire simulation period is 6.8% smaller in the highest (4.3°C) climate sensitivity simulation compared to the lowest (0.9°C), so that the reduction in  shrinks by 2% for every 1°C increase in climate sensitivity.
An important implication of this decoupling between different chemical species is that ecological and physiological studies, which attempt to characterize the biological impact of ocean acidification, need to clarify whether the observed biological impact is caused by changes in [H+] or . For example, our finding is highly relevant to the recent controversy regarding the response of the coccolithophore Emiliania huxleyi to acidification perturbation experiments. Whereas Riebesell et al. (2000) showed a reduction in calcification, Iglesias-Rodriguez et al. (2008a) showed increased organic matter production and calcification under elevated CO2 conditions in a batch laboratory experiment. Each of these experiments created ocean acidification conditions using two different methods. In the first experiment, Riebesell et al. (2000) reduced alkalinity, while keeping total carbon dioxide constant (DIC) by adding an acid in order to mimic future ocean acidification. The second experiment, which more closely matches how the real ocean CO2 perturbation is acting, bubbled CO2 gas into seawater, which increases DIC but keeps alkalinity constant (Iglesias-Rodriguez et al. 2008b). The chemical speciation of pH and carbonate ion under these two different ocean acidification manipulation experiments result in similar, but ~10% difference in speciation under high CO2 conditions among those important species (Schulz et al. 2009).
Irrespective of this experimental debate over the optimal method for laboratory CO2 perturbation experiments, the very different response by this one calcifying species (Emiliania huxleyi) points to an important need that complements our results here and extends throughout the biological field. Because of the nonlinearities in carbon chemistry and the decoupled climate sensitivity of ocean acidification shown here, pH and  (or Ω) are similar but not interchangeable as is commonly done in the field. Given this quasi-independent response, knowledge of how a marine species responds to specific chemical carbon species is important.
It is well understood that emissions of CO2 by humans lead to both global climate change and ocean acidification, although the latter has not received as much attention as the former. Because CO2 becomes an acid when hydrated, its uptake by the global ocean causes it to become acidic. At the same time, oceanic uptake of CO2 slows its accumulation in the atmosphere and alleviates climate change. So it is sometimes viewed that climate change and ocean acidification offset each other with respect to oceanic uptake of CO2: when one is severe, the other is limited. By considering how uncertainty in climate sensitivity alters the manifestation of ocean acidification, this study presents a more intricate view of the ocean in a high-CO2 world. We find that for a given CO2 emissions scenario and when climate sensitivity is high, global warming and surface ocean acidification are both more severe. The offset, however, occurs between global climate change and acidification in the subsurface mixed layer, not in the surface ocean. The different manifestation of ocean acidification by depth has potentially important implications on the geochemical cycling of CaCO3 and alkalinity and on the distribution of organisms sensitive to pH and . We also find that under warming, pH and  become decoupled, as the temperature increase alters the chemical equilibrium of the carbon species in seawater. This would suggest that studies of biological impacts due to acidification would be more useful if they can attribute the impacts to specific chemical species.
This collaborative work was made possible through funding via an Australian Research Council Discovery Grant (DP110104955), a UNSW Visiting Fellowship and the University of Minnesota sabbatical support to KM, and by an ARC QEII Fellowship to BM. The carbon emissions scenarios were provided by R. Zeebe. Numerical computation was carried out using resources at the University of Minnesota Supercomputing Institute.