[A-CG45] Biogeochemical linkages between the surface ocean and atmosphere

[ACG45-09] Response of the ocean carbon and oxygen cycles to climate change and eutrophication from the atmosphere and rivers

*Akitomo Yamamoto¹, Tomohiro Hajima¹, Dai Yamazaki², Maki Noguchi Aita¹, Akinori Ito¹, Michio Kawamiya¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Institute of Industrial Sciences, The University of Tokyo)

Keywords:eutrophication, climate change, oceanic primary productivity, deoxygenation

Nutrient inputs from the atmosphere and rivers to the ocean are increased substantially by human activities. These increasing inputs of nutrients from human activities promote oceanic NPP, potentially partially counteracting decreases caused by climate change. Then, increases in export of organic matter to the ocean interior and its decomposition consumes dissolved oxygen. Therefore, nutrient inputs to the ocean promote carbon uptake and amplify climate-driven ocean deoxygenation. However, the previous generation of Earth system models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5), which contributed substantially to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, did not account for anthropogenic nutrient inputs to the ocean. Several CMIP phase 6 (CMIP6) Earth system models do consider anthropogenic nutrient inputs to the ocean for the historical period, but their impact on ocean biogeochemical cycles has not been fully assessed, even for individual Earth system models.


In this study, using historical simulations by one of the CMIP6 models (MIROC-ES2L) that considers anthropogenic nutrient inputs from atmosphere and rivers, we demonstrate that the contribution of anthropogenic nutrient inputs to past changes in global oceanic productivity, carbon uptake, and deoxygenation is of similar magnitude to the effect of climate change. In particular, two noteworthy results are obtained: (1) that anthropogenic fertilization could more than counteract the expected decrease in NPP caused by ocean warming and stratification for the historical period, and (2) that it could accelerate climate-driven deoxygenation in the upper ocean, helping to close the gap between models and observations. Additionally, current estimation of the imbalance in the carbon budget could be explained partially by increase in oceanic carbon uptake associated with anthropogenic nutrient inputs to the ocean. These improvements provide support regarding the significant contribution of anthropogenic nutrient inputs to global changes in ocean biogeochemistry. Considering the effects of both nutrient inputs and climate change is crucial in assessing anthropogenic impacts on ocean biogeochemistry.

In the presentation, we will also show the relationship between these two effects on ocean biogeochemical cycles under scenarios of climate change.