Abstract
Abstract. Ocean ecosystems are increasingly stressed by human-induced changes of their physical, chemical and biological environment. Among these changes, warming, acidification, deoxygenation and changes in primary productivity by marine phytoplankton can be considered as four of the major stressors of open ocean ecosystems. Due to rising atmospheric CO2 in the coming decades, these changes will be amplified. Here, we use the most recent simulations performed in the framework of the Coupled Model Intercomparison Project 5 to assess how these stressors may evolve over the course of the 21st century. The 10 Earth system models used here project similar trends in ocean warming, acidification, deoxygenation and reduced primary productivity for each of the IPCC's representative concentration pathways (RCPs) over the 21st century. For the "business-as-usual" scenario RCP8.5, the model-mean changes in the 2090s (compared to the 1990s) for sea surface temperature, sea surface pH, global O2 content and integrated primary productivity amount to +2.73 (±0.72) °C, −0.33 (±0.003) pH unit, −3.45 (±0.44)% and −8.6 (±7.9)%, respectively. For the high mitigation scenario RCP2.6, corresponding changes are +0.71 (±0.45) °C, −0.07 (±0.001) pH unit, −1.81 (±0.31)% and −2.0 (±4.1)%, respectively, illustrating the effectiveness of extreme mitigation strategies. Although these stressors operate globally, they display distinct regional patterns and thus do not change coincidentally. Large decreases in O2 and in pH are simulated in global ocean intermediate and mode waters, whereas large reductions in primary production are simulated in the tropics and in the North Atlantic. Although temperature and pH projections are robust across models, the same does not hold for projections of subsurface O2 concentrations in the tropics and global and regional changes in net primary productivity. These high uncertainties in projections of primary productivity and subsurface oxygen prompt us to continue inter-model comparisons to understand these model differences, while calling for caution when using the CMIP5 models to force regional impact models.
Highlights
In recent decades, the ocean has undergone large physical and biogeochemical modifications in response to humaninduced global change, as revealed by a variety of in situ and remote sensing observations
We focus on four stressors: temperature, pH, O2 and NPP, while comparing four different representative concentration pathways (RCPs) scenarios across 10 different Earth system models (ESMs), all including a marine biogeochemical component
Using here 10 ESMs participating to the recent Coupled Model Intercomparison Project 5, we assess how the major stressors of marine ecosystems, namely ocean temperature, pH, dissolved oxygen concentrations and primary productivity, may evolve over the course of the 21st century and under several atmospheric CO2 pathways
Summary
The ocean has undergone large physical and biogeochemical modifications in response to humaninduced global change, as revealed by a variety of in situ and remote sensing observations. L. Bopp et al.: Multiple stressors of ocean ecosystems in the 21st century ocean surface warming, changes in ocean salinity, modifications of density structure and stratification, as well as an increase in dissolved inorganic carbon concentrations and a decrease in seawater pH in response to ocean uptake of anthropogenic carbon (Doney, 2010). Bopp et al.: Multiple stressors of ocean ecosystems in the 21st century ocean surface warming, changes in ocean salinity, modifications of density structure and stratification, as well as an increase in dissolved inorganic carbon concentrations and a decrease in seawater pH in response to ocean uptake of anthropogenic carbon (Doney, 2010) These physical and chemical modifications (Bindoff et al, 2007) have the potential to affect marine organisms and ecosystems (Doney et al, 2012). Rising ocean temperatures lead to increased ocean stratification and changes in ocean mixing and ventilation that could induce indirect effects, which in turn reduce subsurface dissolved O2 concentrations (Plattner et al, 2001) and supply of nutrients to the euphotic layer, increasing nutrient stress for phytoplankton (Bopp et al, 2001)
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