Many scientific efforts towards understanding the sensitivity of marine calcifiers to ocean acidification have focused on the ‘sentinel’ Thecosomata pteropod, a pelagic zooplankton, also known as “sea butterflies” that form delicate shells of aragonite, a metastable mineral form of calcium carbonate. Pteropods are found throughout the global ocean 16 and play an important role in marine ecosystems and biogeochemical cycling. They consume phytoplankton in the ocean’s euphotic zone17, serve as an important food source for upper trophic levels including salmon and herring17,18, and contribute significantly to regional sequestration of carbon at the ocean’s surface via enhanced global export of carbon to depth 19–21.
Pteropods demonstrate acute sensitivity to the relative chemical availability (saturation) of aragonite in the local marine environment. Omega aragonite (Ωarag), the saturation state of aragonite, expresses the favorability of sea-water for formation of aragonite. When Ωarag is greater than unity (Ωarag> 1) , the formation of aragonite is favorable, when less than unity (Ωarag< 1) dissolution is favorable. Laboratory experiments and observations in the Southern Ocean and California Current suggest that brief exposure to low-Ωarag waters, lasting only days to weeks, adversely impacts pteropod fitness and survival. Calcification rates slow when Ωarag< 1.4, and when Ωarag< 1.0 calcification ceases and habitat restriction is evident 7,22–24.
In this study we use projections from a suite of Initial Condition Large Ensemble simulations 25–27 of an Earth system model14,15 (ESM; Supplementary Discussion 1 and Extended Data Figs. 1-2) to assess the benefits of climate mitigation efforts for reducing and delaying the progression of conditions corrosive to aragonite (i.e. Ωarag< 1). Pteropods vertically migrate about the seasonally-varying euphotic zone depth (EZD), feeding above the EZD during night and migrating below the EZD during day to escape visual predation22. We therefore evaluate Ωarag at the EZD, thereby capturing the effects of the seasonal migratory patterns of our representative organism upon its vulnerability to ocean acidification (Methods, Extended Data Fig. 3). Using the Large Ensemble framework, we quantify the influence that internal climate variability has on the onset of aragonite undersaturation , the pace of transition between saturated and predominately undersaturated conditions 11, and on the robustness of mitigation efforts for curtailing aragonite undersaturation at the EZD.
Progression of aragonite undersaturation
We find undersaturated waters and associated habitat compression at the EZD are projected to progressively invade vast areas of the Arctic, Antarctic, subpolar gyres and equatorial upwelling regions by year 2050 (Fig. 1a). Under a high emissions scenario (RCP8.5), the affected regions expand between 2050 and 2100 to include the subtropical gyres of the North and South Pacific, the Eastern North Atlantic and the Equatorial Atlantic and Pacific. The transition period between the onset of month-long mean-state undersaturation and predominant undersaturation varies regionally from less than 5 years to more than 25 years (Fig. 1b). The transition period is significantly shorter in the Southern Ocean, North Atlantic and Subtropical North Pacific ( <5 years) than in the Equatorial and Subpolar North Pacific (> 25 years, Fig. 1b). The influence of natural variability on the onset year of undersaturated conditions and length of the transition period is largest in the Atlantic and Pacific Eastern Boundary Currents and SubPolar North Pacific, where both the onset year and transition duration vary by over 25 years between ensemble members (Fig. 1c).
The spatial progression of undersaturated conditions (Fig. 1a) can be understood through the superposition of the historical or pre-industrial Ωarag and the global invasion of anthropogenic carbon (Cant; Extended Data Figs. 5-7 ). Regions where the mean-state historical saturation horizon (the depth at which Ωarag = 1) is already approaching the EZD, such as the Subpolar North Pacific and Eastern Boundary Upwelling Regions (Extended Data Fig. 6a) are susceptible to earlier onset of undersaturation events. Regions such as the Arctic and Southern Ocean, with already-low surface Ωarag and weak vertical gradients in Ωarag allow small additions of Cant to drive large upward displacement of the undersaturation horizon and rapid transition to predominate undersaturation (Fig. 1b; Extended Data Figs. 5, 6b-c).
Impacts of mitigation on progression of undersaturation
Without mitigation efforts, undersaturated waters are projected to comprise 40-42% (range of ensemble members) of the EZD by 2100, representing potentially wide-spread habitat loss for aragonite calcifiers (Fig. 2a). However, saturation horizon shoaling is limited with moderate mitigation to the blue and green regions (16-19%), and with aggressive mitigation, to the blue regions (6-7%). Moderate mitigation averts undersaturated conditions in the yellow regions, which include the subpolar and circumpolar, Arctic, equatorial, and eastern subtropical oceans. For the northern subpolar regions, the effects of mitigation are generally stronger in the east than west, reflecting the higher upper-ocean dissolved inorganic carbon (DIC) concentrations in the west (Extended Data Figs. 5,6). High DIC pre-conditions the west to more rapid undersaturation that outpaces mitigation benefits. The anthropogenic declines in Ωarag and the sensitivity of these declines to mitigation is mediated by the magnitude of the invasion flux of Cant, and not by anthropogenically-driven changes in circulation (Extended Data Fig. 7; Supplementary Discussion 2). This is important, as it suggests that our main results are not contingent on specifics of model sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) stability but rather on the skill with which the model represents the upper ocean carbon cycle.
Mitigation also affords longer transition times in nearly all regions, but particularly in the Southern Ocean, where mitigation increases transition times from < 5 years on average to > 25 years (Fig. 2c). This reveals that in addition to reducing the spatial extent of habitat compression, aggressive mitigation also significantly increases the timescales available for migration 22, adaptation 28, and trophic adjustments to the reduction or local extinction of pteropods 29.
Many of the regions for which mitigation averts undersaturation are home to the world’s Large Marine Ecosystem (LMEs), biogeographically defined provinces which cumulatively contribute 80% to the global fisheries yield 30 and contain the highest species richness and serve as biodiversity hotspots 31. Mitigation more effectively averts aragonite undersaturation in LMEs located on the eastern side of the extra-tropical northern Pacific and Atlantic Oceans (Fig. 3). This contrast is exemplified in the North Pacific, where committed emissions ensure undersaturation by 2050 in the West Bering Sea and Oyashio Current whereas it is avoidable in the Gulf of Alaska and California Current.
Theoretical and empirical evidence suggest that compression of intermediate trophic levels, such as those occupied by pteropods, can result in deleterious effects at higher trophic levels 29. The reduced abundance or absence of pteropods could have potentially large impacts on reliant species such as salmon and herring, and thus global fisheries 3. Our results indicate that the timing of such impacts could differ by many decades between the eastern and western parts of each basin, and that near-term reductions in emissions will not mitigate the risk of undersaturation-related impacts in the western parts of the basins. This work motivates expansion of ongoing efforts to monitor plankton distributions and gut contents of key species as a key aspect of fisheries management for early warning of population vulnerability.
Impacts of vertical seasonal migration
The globally-integrated progression of one-month-per-year undersaturation is consistent across multiple depth horizons in the surface ocean (Fig. 4), with ~40% of each horizon experiencing month-long undersaturation events by year 2100 without mitigation, 20% with moderate mitigation and less than 10% with aggressive mitigation. Despite the progression of onset being similar, the area impacted over the seasonal cycle is much less severe at the seasonally-varying EZD horizon (Fig. 4a-c) than at the fixed horizons (Fig. 4e-f, 4i-k). Differences in the expression of the seasonal cycle at the different horizons result in transition periods at the seasonally-varying EZD being longer (by more than 10 years on average) and more responsive to mitigation than at the 100 meters considered in previous studies 11,12 (Fig. 4l) or the annual-mean EZD (Fig. 4h).
During winter months, a shallower EZD offers a protectionary effect when the undersaturation horizon is also the shallowest in most regions (Extended Data Figs. 3b, 8). This protectionary effect of seasonal habitat movement delays the onset of predominant undersaturation therefore extending the transition period at the seasonally-varying EZD relative to the annual-mean EZD and 100-meter depth horizon (Fig. 4d, 4h and Extended Data Figs. 9-10), and consequently provides more leverage for mitigation efforts to increase transition durations. The distribution of transition times at the EZD reveals a stark contrast between the rapid transition period for RCP8.5, and successively longer adaptation timescales for RCP4.5 and then RCP2.6 (Fig. 4d), whereas the transition period defined at the annual-mean EZD and 100m depth for the three scenarios reveal nearly coincident peaks at less than 5 years (Fig. 4h, 4l).
Another critical difference between the progression of undersaturation at the EZD versus fixed-depth horizons is the time of year in which undersaturation begins to occur (Extended Data Fig. 10). At the seasonally-varying EZD, undersaturation arrives first in the summer months, whereas at 100 meters, undersaturation arrives first during winter months. This distinction may have important implications for spawning cycles and other seasonally-varying ecosystem drivers3.
Conceptually, the length of the transition period is related to the ratio between the pace of anthropogenic-declines in Ωarag and the amplitude of the seasonal cycle at a given static (100m) or dynamic (EZD) depth horizon (Extended Data Fig. 6d-i). The larger the amplitude of the seasonal cycle (or interannual variability in the case of Equatorial regions) in Ωarag relative to anthropogenic declines in Ωarag, the longer the transition period, i.e. more annual cycles are required for the majority of the year to fully surpass the undersaturation threshold. The amplitude of the seasonal and interannual variability in Ωarag is also related to the vertical gradient in Ωarag, with weak gradients producing weaker seasonal cycles (Extended Data Fig. 6c). Weak vertical gradients accelerate the transition, as the saturation horizon rapidly invades the euphotic zone. This mechanism gives insight into why transition periods differ: (i) between the EZD and fixed horizons (Extended Data Fig. 6g-i) – the anthropogenic declines in Ωarag are similar but the amplitude of the Ωarag seasonal cycle is larger when considered at the seasonally varying depth; (ii) across space – where the vertical gradient of Ωarag varies by an order of magnitude between the poles and tropics (Extended Data Fig. 6b); and (iii) between different emissions scenarios, where mitigation reduces the rate of the anthropogenic declines in Ωarag and therefore increases the transition period (Fig. 4; Extended Data Fig. 6g-i).