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Ocean deoxygenation due to anthropogenic warming represents a major threat to marine ecosystems and fisheries. Challenges remain in simulating the modern observed changes in the dissolved oxygen (O₂). Here, we present an analysis of upper ocean (0-700m) deoxygenation in recent decades from a suite of the Coupled Model Intercomparison Project phase 6 (CMIP6) ocean biogeochemical simulations. The physics and biogeochemical simulations include both ocean-only (the Ocean Model Intercomparison Project Phase 1 and 2, OMIP1 and OMIP2) and coupled Earth system (CMIP6 Historical) configurations. We examine simulated changes in the O₂inventory and ocean heat content (OHC) over the past 5 decades across models. The models simulate spatially divergent evolution of O₂trends over the past 5 decades. The trend (multi-model mean and spread) for upper ocean global O₂inventory for each of the MIP simulations over the past 5 decades is 0.03 ± 0.39×1014 [mol/decade] for OMIP1, −0.37 ± 0.15×10¹⁴[mol/decade] for OMIP2, and −1.06 ± 0.68×10¹⁴[mol/decade] for CMIP6 Historical, respectively. The trend in the upper ocean global O₂inventory for the latest observations based on the World Ocean Database 2018 is −0.98×10¹⁴[mol/decade], in line with the CMIP6 Historical multi-model mean, though this recent observations-based trend estimate is weaker than previously reported trends. A comparison across ocean-only simulations from OMIP1 and OMIP2 suggests that differences in atmospheric forcing such as surface wind explain the simulated divergence across configurations in O₂inventory changes. Additionally, a comparison of coupled model simulations from the CMIP6 Historical configuration indicates that differences in background mean states due to differences in spin-up duration and equilibrium states result in substantial differences in the climate change response of O₂. Finally, we discuss gaps and uncertainties in both ocean biogeochemical simulations and observations and explore possible future coordinated ocean biogeochemistry simulations to fill in gaps and unravel the mechanisms controlling the O₂changes.

 

Surface ocean biogeochemistry and photochemistry regulate ocean–atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or p CO 2 ) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N 2 O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes. 

Abstract. In the current era of rapid climate change, accuratecharacterization of climate-relevant gas dynamics – namely production,consumption, and net emissions – is required for all biomes, especially thoseecosystems most susceptible to the impact of change. Marine environmentsinclude regions that act as net sources or sinks for numerous climate-activetrace gases including methane (CH4) and nitrous oxide (N2O). Thetemporal and spatial distributions of CH4 and N2O are controlledby the interaction of complex biogeochemical and physical processes. Toevaluate and quantify how these mechanisms affect marine CH4 andN2O cycling requires a combination of traditional scientificdisciplines including oceanography, microbiology, and numerical modeling.Fundamental to these efforts is ensuring that the datasets produced byindependent scientists are comparable and interoperable. Equally critical istransparent communication within the research community about the technicalimprovements required to increase our collective understanding of marineCH4 and N2O. A workshop sponsored by Ocean Carbon and Biogeochemistry (OCB)was organized to enhance dialogue and collaborations pertaining tomarine CH4 and N2O. Here, we summarize the outcomes from theworkshop to describe the challenges and opportunities for near-futureCH4 and N2O research in the marine environment. 

The ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N₂O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N₂O fluxes, we report new relationships between oxygen concentration and rates of N₂O production from nitrification and denitrification directly measured with¹⁵N tracers in the Eastern Tropical Pacific. Highest N₂O production rates occurred near the oxic‐anoxic interface, where there is strong potential for N₂O efflux to the atmosphere. The dominant N₂O source in oxygen minimum zones was nitrate reduction, the rates of which were 1 to 2 orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N₂O from both nitrification and denitrification. These experimental data provide new constraints to a multicomponent global ocean biogeochemical model, which yielded annual oceanic N₂O efflux of 1.7–4.4 Tg‐N (median 2.8 Tg‐N, 1 Tg = 10¹² g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N₂O production pathway in the marine environment.