Biotic and human vulnerability to projected changes in ocean biogeochemistry over the 21st century

Abstract

Ongoing greenhouse gas emissions can modify climate processes and induce shifts in ocean temperature, pH, oxygen concentration, and productivity, which in turn could alter biological and social systems. Here, we provide a synoptic global assessment of the simultaneous changes in future ocean biogeochemical variables over marine biota and their broader implications for people. We analyzed modern Earth System Models forced by greenhouse gas concentration pathways until 2100 and showed that the entire world's ocean surface will be simultaneously impacted by varying intensities of ocean warming, acidification, oxygen depletion, or shortfalls in productivity. In contrast, only a small fraction of the world's ocean surface, mostly in polar regions, will experience increased oxygenation and productivity, while almost nowhere will there be ocean cooling or pH elevation. We compiled the global distribution of 32 marine habitats and biodiversity hotspots and found that they would all experience simultaneous exposure to changes in multiple biogeochemical variables. This superposition highlights the high risk for synergistic ecosystem responses, the suite of physiological adaptations needed to cope with future climate change, and the potential for reorganization of global biodiversity patterns. If co-occurring biogeochemical changes influence the delivery of ocean goods and services, then they could also have a considerable effect on human welfare. Approximately 470 to 870 million of the poorest people in the world rely heavily on the ocean for food, jobs, and revenues and live in countries that will be most affected by simultaneous changes in ocean biogeochemistry. These results highlight the high risk of degradation of marine ecosystems and associated human hardship expected in a future following current trends in anthropogenic greenhouse gas emissions.

Figure 1. Accuracy and precision on future ocean biogeochemical projections.

Plots A–D refer to sea-surface parameters; plots E–H to seafloor parameters. These plots illustrate the number of 1 km² cells by their projected change to the year 2100 under the RCP45 (blue lines), RCP85 (red lines), errors in accuracy (green lines), and precision (purple lines). Accuracy was defined as the difference between multimodel average projections and actual data and precision as the standard deviation among the projections of all models. Comparison of these frequency distributions illustrates that errors in accuracy and precision are insufficient to offset projected changes in surface temperature, oxygen, and pH. Note that in those cases, accuracy (green lines) is centered to zero, meaning that for the great majority of cells the multimodel average prediction was identical to actual observations. Errors in precision were often larger, but they are added to both sides of the projections, meaning that they will broaden expected projections but will not reverse them. However, in the case of surface productivity and all parameters at the seafloor, errors in accuracy and precision were larger than the projected change, highlighting the need for caution in those cases. Further details are presented in Table S2; the performance of individual models is shown in Table S1.

Figure 2. Future biogeochemistry change in the world's oceans.

Plots A–D show the spatial difference between future (i.e., the average from 2091 to 2100) and contemporary (i.e., the average from years 1996 to 2005) values under the RCP85 scenario (decadal averages were chosen to minimize aliasing by interannual variability; beside each color scale we provide the absolute change, whereas the numbers on top indicate the rescaled values; complete results for the RCP85 and RCP45 for the ocean surface and floor are shown in Figure S2). Plots E–H show the global average change relative to contemporary values under the RCP45 and RCP85 at the ocean surface and seafloor; semitransparent lines are the projections for individual models.

Figure 3. Co-occurring ocean biogeochemical changes to the year 2100 under the RCP85.

For these plots, we separated absolute changes shown in Figure 2A–D between those that will be positive (i.e., cooling, basification, oxygenation, and productivity increase; Plots A–E) and negative (i.e., warming, acidification, oxygen depletion, and primary food reduction; Plots F–J). Resulting absolute changes were scaled between 0 and 1 (Plots B–E, G–J), 0 being zero absolute change and 1 being the extreme 97.5% observed value globally. The resulting scaled scores from each variable were added to provide a global composite map of co-occurring positive (Plot A) and negative (Plot F) changes in ocean biogeochemistry. These cumulative change maps ranged from 4 (i.e., the maximum predicted change in all four parameters occurred in that cell) to 0 (i.e., no negative or positive change in any of the four parameters occurred in that cell). The results for the RCP45 at the ocean surface and both RCPs for the seafloor are presented in the Supporting Information section.

Figure 4. Exposure of the world's oceans to co-occurring changes in ocean biogeochemistry to the year 2100.

(A–B) are the cumulative percentage of cells globally exposed to the composite score of co-occurring ocean biogeochemistry changes (see Figure 3 for details). (A) is for negative and (B) for the positive biogeochemistry changes. (C–D) is the discrimination of total ocean cells globally exposed to negative (C) and positive (D) changes in each variable and the composite score.

Figure 5. Future ocean biogeochemistry change on marine habitats and biodiversity hotspots.

Here we show the mean (horizontal dashes) and standard deviation (curved lines) of the absolute change in each parameter projected to the year 2100 for each marine habitat (Plot A) and biodiversity hotspot for individual taxa (Plot B). A hotspot is defined as the top 10% most diverse (in number of species) areas on Earth where the given taxa are found . In both plots, values for each parameter are color-coded according to the left-hand axes. Values to the left and right of each habitat or hotspot indicate the expected results according to RCP85 and RCP45, respectively. Data on marine habitats were obtained mainly from Halpern et al. ; additional sources are indicated in the Table S6; data on biodiversity hotspots were obtained from Tittensor et al. . Complete results of the exposure of each habitat and hotspots to all parameters as well as the sources of error due to accuracy and precision are presented in Table S4. Particulate organic carbon flux (or simply carbon flux in the legend) applies only to seafloor habitats.

Figure 6. Vulnerability of humans to projected ocean biogeochemistry change.

This plot illustrates the total number of people likely to be vulnerable through exposure to ocean biogeochemistry change according to RCP45 (Plot A) and RCP85 (Plot B). Numbers in the plot are in billions (summations may not be exact owing to rounding). Categorization of people according to their levels of exposure to biogeochemical changes, dependency on ocean goods and services, and social adaptability is described in the main text.