Ocean acidification in emission-driven temperature stabilization scenarios: the role of TCRE and non-CO₂ greenhouse gases

The AERA estimates a future trajectory of CO₂-fe emissions [2, 3, 10] that allows to stabilize the global atmospheric surface warming at a prescribed temperature level in three steps solely based on past annually averaged trajectories of (a) CO₂ emissions, (b) atmospheric CO₂, (c) the radiative forcing of all non-CO₂ forcing agents, and (d) global mean surface temperatures [53].

First, the AERA estimates the anthropogenic warming from the time series of radiative forcing and temperatures [58] using an impulse response function [59]. Second, the determined anthropogenic warming and the cumulative CO₂-fe emissions (sum of CO₂ emissions from fossil fuels and land use change and CO₂-fe from non-CO₂ radiative agents [3]) are used to determine the transient climate response to cumulative CO₂-fe emissions (TCRE) [2, 3, 5, 8]. The TCRE then allows to estimate the remaining allowable emission budget (REB) of CO₂-fe before the chosen temperature target is reached. In the third step, the CO₂-fe in the REB are distributed over the next decades so that an overshoot in temperature is tried to be avoided and that year-to-year changes in CO₂-fe emission reductions remain as small as possible. A detailed explanation of the AERA is provided by Terhaar et al [53].

Bern-3D-LPX is an Earth system model of intermediate complexity that simulates dynamically the physics, chemistry, and biology of the land biosphere, the ocean, and sea ice, as well as their coupling to the atmosphere [60, 61]. The model is used at nine different setups that cover TCREs from 1.35 °C (EgC)⁻¹ to 2.16 °C (EgC)⁻¹, which resembles the CMIP6 TCRE range of 1.32 °C–2.30 °C (EgC)⁻¹ [62] and the observation-constrained range of 1.3 °C–2.3 °C (EgC)⁻¹ [50] (EgC = 1000 PgC = 10¹⁸ gC). The range of TCREs is obtained by combinations of three different ocean diapycnal mixing parameters (3 × 10⁻⁵, 2 × 10⁻⁵, 1 × 10⁻⁵ m² s⁻¹) and three different climate feedback parameters (−0.1, −0.5, −1.0 Wm⁻² K⁻¹) that account for feedbacks in the Earth system that are not explicitly or potentially not correctly simulated by Bern3D-LPX. These setups were chosen as their range of ECSs from 2.3 to 4.6 °C covers the 5%–95% likelihood range of the latest ECS assessment based on multiple lines of evidence [63] and because the ocean mixing parameters result in a circulation that represents observed distributions of CFCs, O₂, or Δ¹⁴C [61, 64].

The simulations with Bern3D-LPX start in 1765 and are until 2025 all prescribed with the same CO₂ emissions, non-CO₂ radiative forcing, and land use area change. CO₂ emissions are from the Global Carbon Budget until 2020 [65] and develop from 2021 to 2025 proportionally to the national determined contributions (NDC) as quantified by the Climate Action Tracker [66]. Non-CO₂ radiative forcing is based on the RCP database until 2000 [67–71] and from 2000 to 2025 on the most recent assessment from chapter 7 in the IPCC AR6 WG1 [72] report using SSP2-4.5 as an extension of the historical period after 2014 [44] to better reflect historic CH₄ and N₂O emissions over the last years. Land-use change is prescribed from 1850 to 2100 based on SSP1-2.6 [73] and associated emissions are dynamically calculated by Bern3D-LPX. SSP1-2.6 was chosen for land-use change as CMIP simulations following this scenario result in temperature are on average between 1.5 °C and 2.0 °C [47], the temperature targets that are closest to the aims of the Paris Agreement to limit warming well below 2 °C and to pursue efforts to reduce it to 1.5 °C [1]. Volcanic radiative forcing is based on the Ice-core Volcanic Index 2 [74, 75] until 2000 and zero afterwards.

From 2025 to 2300, the emissions of CO₂, N₂O, CH₄, CO, NO_x , and VOC as well as trajectories of other non-CO₂ radiative agents, such as aerosols, evolve dynamically to match the prescribed CO₂-fe emission curve, which is updated every five years. The AERA is first applied in 2025 and then every five years to mimic the stock take process foreseen in the Paris Agreement that includes a new submission of world-wide NDCs every five years with the next NDC submission being in 2025. The emissions of N₂O, CH₄, CO, NO_x , and VOC are used by a reduced form atmospheric chemistry model to calculate their respective atmospheric concentrations and the associated radiative forcing and CO₂-fe emissions. A detailed explanation of the reduced atmospheric chemistry model and parameters is provided by Terhaar et al [53]. The ozone forcing and the aerosol forcing in the atmospheric chemistry model was here changed to 0.47 Wm⁻² and −1.06 Wm⁻² in 2019, respectively, to match the non-CO₂ radiative forcing from the IPCC AR6 WG1 report [72].

As many combinations of the different GHGs and radiative agents exist that would lead to the same prescribed CO₂-fe emission curve, prior assumptions must be made for the evolution of the non-CO₂ radiative agents. Three sets of simulations were made to represent a wide range of future choices (supplementary table 1). In the first set of simulations, called ‘baseline’, CH₄ and N₂O emissions evolve after 2020 according to SSP2-4.5, under which global warming will likely be limited to 2 °C warming [76], and CH₄ and N₂O emissions are constant from 2100 onwards (supplementary figure 1). Although SSP1-2.6 represents the scenario that results in temperatures closest to the Paris Agreement targets (see above), we chose SSP2-4.5 to provide a set of simulations were CO₂ reductions are relatively higher so that the three sets of simulations span a range of CO₂ emissions. In addition, the aerosol radiative forcing decreases exponentially (80% with a lifetime of 100 years, and 20% with a lifetime of 50 years). CO₂ emissions evolve dynamically so that the total CO₂-fe emission match the AERA-prescribed CO₂-fe emissions. In the second set of simulations, called ‘high-CO₂ ’, aerosols also decrease exponentially as in the ‘baseline’ simulations and CH₄ and N₂O emissions evolve parallel to CO₂ emissions. The parallel evolution causes CH₄ and N₂O emissions to decrease faster than under SSP2-4.5 and result accordingly in smaller CO₂-fe emissions from CH₄ to N₂O. Reductions in CO₂ emissions can therefore be weaker to still equal the same CO₂-fe emission than in the baseline simulation, which yields comparatively higher CO₂ emissions. In the third set of simulation, called ‘constant aerosol’, CO₂, CH₄, and N₂O emissions also evolve proportionally but the aerosol radiative forcing remains at 2025 levels. Therefore, even less stringent CO₂ emission reductions are necessary in this set of simulations to reach the temperature targets and CO₂ emissions and, in turn, atmospheric CO₂ can remain higher than in the ‘baseline’ and ‘high-CO₂’ simulations. Land-use change remains unchanged across all simulations.

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Throughout the manuscript, we focus on the ‘baseline’ and ‘high-CO₂’ simulations that rely only on emission reductions, which are the only sustainable to reach a prescribed temperature target [77]. As opposed to these mitigation scenarios, the ‘constant aerosol’ simulations also rely on strong aerosol injection via solar radiation modification to keep the aerosol forcing constant. Solar radiation modification is only an emergency solution as it comes with high risks, e.g. an abrupt rise in temperatures if the solar radiation modification should fail in the future for technical or political reasons [78], further commitments and unintended side effects like shifting monsoon patterns, changes in meridional temperature gradients, atmospheric and oceanic circulation, and the modes of climate variability [79]. By presenting solar radiation modification briefly in the main manuscript and more detailed in the Supplementary Material, we transparently show all theoretical options but focus on the sustainable and more likely options.

In all simulation, CH₄ and N₂O emissions have lower limits of 30 Tg CH₄ yr⁻¹ and 5.3 Tg N₂O yr⁻¹ due to non-abatable emissions in agriculture and livestock sectors. Simulations for the three set of simulations were made for four temperature targets of 1.5°C, 2.0 °C, 2.5 °C, and 3.0 °C with each of the nine setups of Bern3D-LPX representing ECS’ from 2.3 °C to 4.7 °C, resulting in 108 simulations (3 set of simulations ×4 temperature targets ×9 Bern3D-LPX setups). As Bern3D-LPX underestimates the inter-annual variability, each of these 108 setups was made with eight temporally varying superimposed inter-annual surface atmospheric temperature variabilities that are derived from observations [53] and hence yield 864 simulations in total (3 set of simulations ×4 temperature targets ×9 Bern3D-LPX setups ×8 temperature variabilities; supplementary table 1). The presented simulation results are openly available [83].

As all setups with different ECS simulate a different historical warming until 2020 compared to the 1850–1900 period (0.83 °C–1.39 °C), the remaining emissions and increases in atmospheric CO₂ until a chosen temperature target is reached depends sensitively on this past warming. To remove this uncertainty, the temperature target is always defined relative to the observation-based anthropogenic warming in year 2020, which is estimated to be 1.23 ± 0.20 °C [53]. For the 1.5 °C target, this means that an allowable warming of 0.27 °C remains. This allowable warming is then added to the anthropogenic warming in each setup. This Δ-approach is the same that is regularly used for ocean acidification studies where different models have difficulties to simulate the baseline biogeochemistry in the interior ocean at present [23, 37, 39, 40].

The simulated historical temperature anomaly with respect to 1850–1900 under prescribed CO₂ emissions increases at a similar pace as the observed temperature over all nine model setups (±0.25 °C), despite varying climate sensitivities and ocean carbon sink strengths (figure 1(a)). All model setups do not only capture the historical temperature trajectory, but also the observed trajectory of atmospheric CO₂ [80] (figure 1(d)), the observation-based estimate of the cumulative ocean carbon uptake [65] (figure 1(e)), and the globally averaged pH over the last decades (figure 1(f)).

After the AERA is switched on in 2025, all trajectories converge by 2150 within ±0.05 °C to the prescribed 1.5 °C temperature target because fossil fuel CO₂ emissions evolve freely so that the combined CO₂-fe emissions from fossil fuels, land use change, and non-CO₂ radiative agents match the prescribed CO₂-fe emissions by the AERA. The total CO₂-fe emissions from 2021 to 2150 that allow to meet the 1.5° target vary from −59 Pg C to +203 Pg C (range of nine setups after averaging over all eight realizations with varying inter-annual superimposed variability) under the ‘baseline’ scenario (figure 1(b)). The uncertainty ranges (−35 Pg C to +239 Pg C under the ‘high-CO₂’ scenario; −27 Pg C to +255 Pg C under the ‘constant aerosol’ scenario) differ between the three scenarios due to the good but imperfect transformation of non-CO₂ radiative forcing to CO₂-fe emissions. In simulations with the AERA that converge to a temperature target, the uncertainty from the TCRE transfers into the CO₂-fe budget, whereas traditional projections used in the IPCC reports [47] based on shared socioeconomic pathways (SSPs) [44, 45, 81] have an inter-model temperature range for a given CO₂-fe trajectory (for example, the 5%–95% range for global warming in 2100 under SSP1-2.6 is 1.3 °C–2.8 °C) [47].

The range of cumulative CO₂-fe emissions from 2021 to 2150 that allow to meet the 1.5 °C target (−59 Pg C to +203 Pg C for the ‘baseline’ scenario) propagates directly into different fossil fuel CO₂ emission trajectories. The remaining allowable fossil fuel CO₂ emissions depend on AERA derived CO₂-fe emissions and the non-CO₂ emissions and the CO₂ emissions from land-use change. In the ‘baseline’ scenario, CH₄ and N₂O emissions and aerosols follow prescribed trajectories, resulting in CO₂-fe emissions of all non-CO₂ radiative agents from 2021 to 2150 of 160 Pg C and land-use change emissions of 17 Pg C. Therefore, the remaining allowable fossil-fuel CO₂ emissions from 2021 to 2150 under this scenario range from −236 Pg C to +25 Pg C (figure 1(c)).

The range of CO₂ emission trajectories affects the projected atmospheric CO₂ and the rates and magnitude of ocean carbon uptake (figures 1(d) and (e), supplementary figure S2). By 2150, possible atmospheric CO₂ in a 1.5 °C world under the ‘baseline’ scenario range from 320 ppm to 368 ppm after having peaked at 433–439 ppm between 2030 and 2035, and the cumulative ocean carbon sink from 1765 to 2150 varies from 190 Pg C to 271 Pg C. In comparison, the SSPs that are used by the IPCC have by definition no uncertainty in the future atmospheric CO₂ as atmospheric CO₂ is prescribed as a boundary condition. While the uncertainty of the atmospheric CO₂ in 2100 is by definition non-existent, the range of the cumulative carbon sink is also smaller when atmospheric CO₂ is prescribed, for example 274–332 Pg C for SSP1-2.6 across the CMIP6 ensemble [56]. Thus, when CO₂ emissions evolve freely to converge to a temperature target, the range of the future sink is ∼40% larger than under fixed atmospheric CO₂.

The wide range of possible atmospheric CO₂ translates into different surface ocean pH projections. At the ocean surface changes in pH are almost entirely driven by changing dissolved inorganic carbon [57], which in turn closely follows the changes in atmospheric CO₂ due to the air-sea CO₂ flux that tends to equilibrate differences in CO₂ partial pressure between the atmosphere and ocean. Hence, the projected atmospheric CO₂ range from 320 ppm to 368 ppm in 2150 results in an almost perfectly anti-correlated projected range in surface ocean pH from 8.079 to 8.129 (after a minimum of 8.024–8.033 between 2031 and 2037) (figures 1(d)–(f)). This range is an order of magnitude larger than the projected standard deviation in projected ocean surface pH of ±0.002 under SSP1-2.6 in 2100 [19]. Furthermore, the annually averaged surface area that is projected to have saturation states of aragonite—a mineral form of calcium carbonate produced by marine organisms—below one remains almost non-existent (<10⁶ km²) due to the rapidly decreasing CO₂ emissions. However, saturation states may still drop below one on diurnal [84, 85] or seasonal [31, 86] timescales or in the 126 m below the surface (figure 3(f), supplementary figure 3), where calcifying organisms vertically migrate [87].

As opposed to surface ocean acidification, long-term ocean acidification below the surface depends not only on the increasing surface ocean dissolved inorganic carbon but also on the quantity of additional dissolved inorganic carbon at the ocean surface that is transported below the ocean surface [31, 37, 39, 40, 56, 88] and distributed within the ocean [42, 89]. The decrease in the global ocean volume that is projected to remain saturated towards aragonite (Ω_A > 1) from 2002 to 2150 ranges from 47 to 86 × 10⁶ km³ (17%–32% of supersaturated volume in 2002) (figure 2(a)), with the central estimate (setup with ECS = 3.2 °C) being 71 × 10⁶ km³ (26%) and the standard deviation across all nine setups being 12 × 10⁶ km³ (4%). Similarly, the range in volumes changes with Ω_A between 1 and 2, 2 and 3, and above 3 vary strongly across the different model setups. Under prescribed atmospheric CO₂ in SSP1-2.6, the simulated decrease in volume that is projected to remain saturated towards aragonite (Ω_A > 1) in 2100 is 89 ± 6 × 10⁶ km³ [56]. Thus, the uncertainty of future interior ocean acidification rates under a prescribed temperature target when using the AERA is around twice as large as under prescribed atmospheric CO₂. The difference in uncertainty under prescribed temperature targets and prescribed atmospheric CO₂ is smaller in the ocean interior (factor 2) than at the ocean surface (factor 10), because most water masses are not in direct contact with the atmosphere so that the differences in simulated ocean circulation and deep-water formation affects both kinds of simulations [37, 39, 40, 56, 90, 91].

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The evolution of non-CO₂ radiative forcing agents does not influence the temperature and CO₂-fe trajectory (figures 1(a) and (b)) but limits, for a given temperature target, the range of possible future trajectories of CO₂ emissions, atmospheric CO₂, ocean carbon uptake, surface ocean pH, and interior ocean aragonite saturation states (figures 1(c)–(f) and 2). Under the ‘baseline’ scenario, non-CO₂ radiative agents are prescribed following SSP2-4.5 and the radiative forcing of major non-CO₂ radiative agents (N₂O, CH₄, and aerosols) under SSP2-4.5 do not get strongly reduced. Thus, the reduction in CO₂-fe emissions as prescribed by the AERA is achieved via reductions in CO₂ emissions. As opposed to the ‘baseline’ scenario, the radiative forcing or emissions of the major non-CO₂ radiative agents are reduced strongly under the ‘high-CO₂’ scenario so that CO₂ emissions remain higher from 2025 to 2075, yielding a remaining CO₂ budget from 2021 to 2150 of 54–286 Pg C, significantly larger than the −236 Pg C to +25 Pg C range under the ‘baseline’ scenario. As CO₂ emissions are larger under the ‘high-CO₂’ scenario, so are atmospheric CO₂ (371–420 ppm in 2150 after having peaked at 436–445 ppm between 2034 and 2043), and the cumulative carbon uptake by the ocean (281–352 Pg C in 2150).

The larger atmospheric CO₂ in the ‘high-CO₂’ scenario and the larger cumulative carbon uptake by the ocean result in higher ocean acidification. The range of projected surface ocean pH across the nine different model setups with stronger reduction in non-CO₂ radiative agents is 8.033–8.080 in 2150 (after a minimum of 8.018–8.029 between 2042 and 2047) (figures 1(d)–(f)). The decrease in volume that is projected to remain saturated towards aragonite (Ω_A > 1) in 2150 under the ‘high-CO₂’ scenario ranges from 86 to 111 × 10⁶ km³ (31%–41%) (figure 2(a)). Overall, the choice of non-CO₂ radiative forcing agents hence increases the possible range of ocean acidification rates in a 1.5 °C world by a factor of two and makes the uncertainty about the future ocean acidification rates in a 1.5 °C world thus even larger than it already was due to the different TCRE’s (see above). The entire range of globally averaged surface ocean pH under both scenarios of 0.096 is thus as large as the difference in surface ocean pH between SSP1-2.6 (warming of 1.3 °C–2.8 °C [47]) and SSP2-4.5 (warming of 2.1 °C–4.0 °C) and ∼50 times as large as the inter-model difference for each scenario across the range of CMIP6 Earth system models [19].

In addition to the ‘high-CO₂’ scenario, the ‘constant aerosol’ scenario allows to assess a potential future in which the necessary GHG reductions are compensated by continuing aerosol emissions so that the cooling effect of aerosols in the atmosphere does not reduce over the 21st century. In that scenario, the CO₂ emissions can remain even higher, yielding higher atmospheric CO₂ in 2150 (411–465 ppm), higher ocean cumulative CO₂ uptake from 1765 to 2150 (335–405 Pg C), and much more severe ocean acidification with globally averaged pH values in 2150 ranging from 7.998 to 8.045 (supplementary figure 4). This example demonstrates the importance of multiple climate targets [17] in addition to temperature targets to reduce the damaging effect of anthropogenic emissions on Earth ecosystems.