Is deoxygenation detectable before warming in the thermocline?

Abstract

Abstract. Anthropogenic greenhouse gas emissions cause ocean warming and oxygen depletion, with adverse impacts on marine organisms and ecosystems. Warming is one of the main indicators of anthropogenic climate change, but, in the thermocline, changes in oxygen and other biogeochemical tracers may emerge from the bounds of natural variability prior to warming. Here, we assess the time of emergence (ToE) of anthropogenic change in thermocline temperature and thermocline oxygen within an ensemble of Earth system model simulations from the fifth phase of the Coupled Model Intercomparison Project. Changes in temperature typically emerge from internal variability prior to changes in oxygen. However, in about a third (35±11 %) of the global thermocline deoxygenation emerges prior to warming. In these regions, both reduced ventilation and reduced solubility add to the oxygen decline. In addition, reduced ventilation slows the propagation of anthropogenic warming from the surface into the ocean interior, further contributing to the delayed emergence of warming compared to deoxygenation. Magnitudes of internal variability and of anthropogenic change, which determine ToE, vary considerably among models leading to model–model differences in ToE. We introduce a new metric, relative ToE, to facilitate the multi-model assessment of ToE. This reduces the inter-model spread compared to the traditionally evaluated absolute ToE. Our results underline the importance of an ocean biogeochemical observing system and that the detection of anthropogenic impacts becomes more likely when using multi-tracer observations.

Emerging Global Ocean Deoxygenation Across the 21st Century

“…The ToE spatial patterns of deoxygenation in surface zones in this work are consistent with previous results from multimodel projections (Hameau et al, 2020;Henson et al, 2017;Rodgers et al, 2015). This study shows time constraints of changes in oxygen concentration from the ocean's surface to its bottom and provides insights into the extent of deoxygenation in marine ecosystems under climate change.…”

The crucial contribution of mixing to present and future ocean oxygen distribution

“…The last Intergovernmental Panel on Climate Change Special Report on the Ocean and Cryosphere in a Changing Climate [IPCC, 2019] assessment reports that over this period, the upper 1000 m of the ocean has lost about 2% of its oxygen reservoir [Ito et al, 2017;Schmidtko et al, 2017]. Deoxygenation is projected to continue during the 21 st century unless greenhouse gas emissions are rapidly curtailed, as in the RCP2.6 scenario (Figure 8) [Bopp et al, 2013;Cabré et al, 2015;Bopp et al, 2017;Hameau et al, 2020;Kwiatkowski, 2020;Li et al, 2020]. For marine animals living in oceanic environments just at the edge of their metabolic oxygen needs, the declining oxygen reservoir represents an immediate threat to survival [Vaquer-Sunyer and Duarte, 2008].…”

Anthropogenic Attribution of the Increasing Seasonal Amplitude in Surface Ocean pCO2

“…The detection and attribution of observational trends to anthropogenic activities are often not straightforward as natural variability obscures anthropogenic signals (Barnett et al, 2005;Hameau et al, 2019;Hegerl et al, 2010). For example, Keller et al (2014) find the time of emergence for anthropogenic trends in annual-mean surface pCO 2 to be around a decade, but 45-90 years for sea surface temperature and even longer timescales are identified for anthropogenic deoxygenation (Hameau et al, 2020). It remains therefore unclear whether the observed increase in SA over the past few decades can be attributed to human-induced emissions or whether the changes are currently still within the noise of natural internal and externally-forced variability.…”