[A-CG35] Projection and detection of global environmental change

[ACG35-10] The response of ocean biogeochemical cycles to global warming and anthropogenic nutrient inputs from atmosphere and rivers

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

Keywords:Atmospheric nutrient inputs, Riverine nutrient inputs, Global warming, Oceanic primary production, Ocean deoxygenation, Earth System model

Increased ocean stratification due to global warming reduces oceanic productivity and oxygen content. In contrast, nutrient (i.e., nitrogen, phosphate, iron) inputs from human activities (such as combustion of fossil fuels, land-use change, increased food production and application of fertilizer) to the ocean stimulate oceanic productivity, accelerating ocean CO₂ uptake and deoxygenation. However, little is known about the combined effects of these factors on ocean biogeochemistry. Here, we use historical simulations conducted by an Earth System model MIROC-ES2L to show that productivity increase by anthropogenic nutrient inputs from atmospheric deposition and rivers offsets most of the productivity decrease by global warming. The main contributor to this net primary production (NPP) increase is anthropogenic nitrogen input from the atmosphere. Although the increases in atmospheric and riverine nitrogen inputs from preindustrial to present-day are comparable, the contribution of anthropogenic nitrogen and phosphorus inputs from rivers to global NPP increase is about half of the contribution of anthropogenic nitrogen deposition. This is because most of the increase in riverine nitrogen input is consumed by enhanced denitrification, which limits the accumulation of nitrogen in the ocean.

Anthropogenic nutrient inputs to the ocean also cause ocean deoxygenation through increased decomposition of organic matter. Oxygen depletion induced by anthropogenic nutrient inputs is comparable with the climate-driven deoxygenation in the upper ocean. While current models tend to underestimate observed deoxygenation rate, our simulations demonstrate that accounting for anthropogenic nutrients input improves the reproducibility of the observed global trend. Since nutrient inputs from human activities increase or remain relatively constant during the 21st century under some scenarios, these results suggest that anthropogenic nutrients input is also crucial for future projections of ocean biogeochemical cycles.