Ocean Acidification: Another Planetary Boundary Crossed

Abstract

Ocean acidification has been identified in the Planetary Boundary Framework as a planetary process approaching a boundary that could lead to unacceptable environmental change. Using revised estimates of pre-industrial aragonite saturation state, state-of-the-art data-model products, including uncertainties and assessing impact on ecological indicators, we improve upon the ocean acidification planetary boundary assessment and demonstrate that by 2020, the average global ocean conditions had already crossed into the uncertainty range of the ocean acidification boundary. This analysis was further extended to the subsurface ocean, revealing that up to 60% of the global subsurface ocean (down to 200 m) had crossed that boundary, compared to over 40% of the global surface ocean. These changes result in significant declines in suitable habitats for important calcifying species, including 43% reduction in habitat for tropical and subtropical coral reefs, up to 61% for polar pteropods, and 13% for coastal bivalves. By including these additional considerations, we suggest a revised boundary of 10% reduction from pre-industrial conditions more adequately prevents risk to marine ecosystems and their services; a benchmark which was surpassed by year 2000 across the entire surface ocean.

Keywords: biodiversity; carbonate chemistry; climate change; conservation; marine; ocean acidification; planetary boundary.

FIGURE 1

Ocean Acidification planetary boundary. (a) Percentage (%) reduction between present day and pre‐industrial aragonite saturation state for the surface global ocean and the seven ocean regions, also comparing to the Richardson et al. (2023) planetary boundary assessment (blue circle and blue line). The red circles represent the multi‐model ensemble median with the associated propagated errors for the multi‐model ensemble standard deviation and the pre‐industrial uncertainties. The 20% boundary value is presented as the dark grey lines with their associated uncertainties shown by the light grey banding. Regions are defined as: Arctic Ocean (Arctic) region north of 65° N; north Pacific Ocean (Pacific‐N) between 40° N and 65° N; north Atlantic Ocean (Atlantic‐N) between 40° N and 65° N; central Pacific Ocean (Pacific‐C) between 40° S and 40° N; central Atlantic Ocean (Atlantic‐C) between 40° S and 40° N; Indian Ocean (Indian) between 40° S and 25° N; and the Southern Ocean (Southern) ocean south of 40° S. (b) Regional assessment of ocean acidification in year 2020, relative to the boundary of 20% reduction from pre‐industrial aragonite saturation state, as in (a), with grey bars representing the boundary uncertainties, and colours depicting whether that boundary has been crossed (red) or not (green). (c) Map showing the percentage difference in surface Ω_Arag between pre‐industrial (1750) and year 2020. The black contour line on the map represents a 20% reduction from pre‐industrial values.

FIGURE 2

Surface water aragonite saturation state (Ω_Arag) in the Arctic Ocean between 1750 and 2020. Maps show average conditions for the respective decade (marked at the top of each map). Numbers given at the bottom of each map shows the percentage (multi‐model median ± propagated error using multi‐model SD) of the area between 60° and 90° N that has Ω_Arag < 1. Maps are created using the hindcast data product from Jiang et al. (Jiang et al. 2022).

FIGURE 3

Maps of surface ocean aragonite saturation state (Ω_Arag), highlighting the 3.5 contour to show the regions that can be considered marginal conditions for coral systems, with coral reefs distribution overlaid on each map in purple dots. (a) Pre‐industrial Ω_Arag, (b) year 2020 Ω_Arag, (c) Ω_Arag conditions at 10% reduction from pre‐industrial levels, and (d) Ω_Arag conditions at 20% reduction from pre‐industrial levels.

FIGURE 4

Regional assessment of ocean acidification in year 2020, relative to the boundary of 20% reduction from pre‐industrial aragonite saturation state for each depth layer. (a) surface, 0 m, (b) 50 m, (c) 100 m and (d) 200 m. The grey bands represent the boundary, the extent of each wedge represents how far each region has changed. Data is multi‐model medians from Jiang et al. (2022) and Jiang (2024) and includes ± propagated errors both on the 2020 value (black lines on each wedge) and the boundary (grey band). Regions are defined as in Figure 1.