Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models

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 CO 2 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 "businessas-usual" scenario RCP8.5, the model-mean changes in the 2090s (compared to the 1990s) for sea surface temperature, sea surface pH, global O 2 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 O 2 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 O 2 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 intermodel comparisons to understand these model differences, while calling for caution when using the CMIP5 models to force regional impact models.

Major impacts of climate change on deep-sea benthic ecosystems

“…Warming of the oceans will enhance thermal stratification and density gradients, which will reduce vertical mixing. Combined with a reduction in O 2 solubility in warmer water, increased thermal stratification is predicted to create widespread ocean deoxygenation (Keeling et al, 2010;Long et al, 2016), with the greatest effect in intermediate waters (100-1000 m; Stramma et al, 2012;Bopp et al, 2013). Already, distinct deep-water masses in the Southern Ocean (Helm et al, 2011), eastern North Atlantic (e.g., Sub-polar Mode Water, the Intermediate Water and the Mediterranean Outflow Water; Stendardo et al, 2015), and in the West Pacific (Levin, 2003;Helly and Levin, 2004).…”

Obtaining Phytoplankton Diversity from Ocean Color: A Scientific Roadmap for Future Development

“…To simplify the representation of the vast planktonic diversity, plankton have been grouped into plankton functional types according to their biogeochemical functions (e.g., Le Quéré et al, 2005). Biogeochemical models now commonly include 3-10 plankton functional types (e.g., Bopp et al, 2013;Laufkötter et al, 2015), with a few models including up to 100 or more types (Follows et al, 2007;Dutkiewicz et al, 2015;Masuda et al, 2017). Since in situ observations on plankton biogeography and abundance are scarce and many vast oceanic regions are too remote to be routinely monitored, biogeochemical modelers rely on surface ocean estimates of phytoplankton composition from satellite observations to evaluate model simulations and help to develop and validate their models.…”

Marine projections of warming and ocean acidification in the Australasian region

“…Aragonite saturation state ΩA and pH values were calculated using the simulated fields of dissolved inorganic carbon, total alkalinity, sea surface temperature and the dissociation constants of Lueker et al (2000), following Bopp et al (2013). The values of pH were calculated using the total scale, following the recommendations of Riebesell et al (2010).…”