Abstract
Abstract. Anthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation, reductions in near-surface nutrients, and changes to primary production, all of which are expected to affect marine ecosystems. Here we assess projections of these drivers of environmental change over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) that were forced under the CMIP6 Shared Socioeconomic Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced under the Representative Concentration Pathways (RCPs). A total of 10 CMIP5 and 13 CMIP6 models are used in the two multi-model ensembles. Under the high-emission scenario SSP5-8.5, the multi-model global mean change (2080–2099 mean values relative to 1870–1899) ± the inter-model SD in sea surface temperature, surface pH, subsurface (100–600 m) oxygen concentration, euphotic (0–100 m) nitrate concentration, and depth-integrated primary production is +3.47±0.78 ∘C, -0.44±0.005, -13.27±5.28, -1.06±0.45 mmol m−3 and -2.99±9.11 %, respectively. Under the low-emission, high-mitigation scenario SSP1-2.6, the corresponding global changes are +1.42±0.32 ∘C, -0.16±0.002, -6.36±2.92, -0.52±0.23 mmol m−3, and -0.56±4.12 %. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions, consistent with previous studies. The ESMs in CMIP6 generally project greater warming, acidification, deoxygenation, and nitrate reductions but lesser primary production declines than those from CMIP5 under comparable radiative forcing. The increased projected ocean warming results from a general increase in the climate sensitivity of CMIP6 models relative to those of CMIP5. This enhanced warming increases upper-ocean stratification in CMIP6 projections, which contributes to greater reductions in upper-ocean nitrate and subsurface oxygen ventilation. The greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher associated atmospheric CO2 concentrations than their RCP analogues for the same radiative forcing. We find no consistent reduction in inter-model uncertainties, and even an increase in net primary production inter-model uncertainties in CMIP6, as compared to CMIP5.
Highlights
1.1 Ocean warming, acidification, deoxygenation, nutrient stress, and reduced primary productionSince the pre-industrial period the global oceans have experienced fundamental changes in physical and biogeochemical conditions as a result of anthropogenic climate change
We find no consistent reduction in inter-model uncertainties, and even an increase in net primary production inter-model uncertainties in Coupled Model Intercomparison Project Phase 6 (CMIP6), as compared to CMIP5
We focus on projected changes in ocean temperature, pH, dissolved O2 and NO−3 concentration, and net primary production across 13 CMIP6 and 10 CMIP5 Earth system models
Summary
1.1 Ocean warming, acidification, deoxygenation, nutrient stress, and reduced primary production. Since the pre-industrial period the global oceans have experienced fundamental changes in physical and biogeochemical conditions as a result of anthropogenic climate change. These changes reflect the climate services that the oceans provide through heat and carbon storage, they have major implications for the health of marine ecosystems. Ocean ecosystems are affected by the direct and indirect consequences of climate change. Atmospheric warming and rising CO2 concentrations drives ocean warming and acidification, while these direct factors cause changes that modulate other important components of the ocean system, such as oxygenation, nutrient levels, and net primary production. Averaged sea surface temperature (SST) has increased by +0.7 ◦C over the last 100 years (Bindoff et al, 2007), with observations indicating that the heat content trend in the upper 2000 m of the ocean has increased from 0.55 to 0.68 J m−2 s−1 since 1991 (Cheng et al, 2019)
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