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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-339
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-339
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 03 Sep 2019

Submitted as: research article | 03 Sep 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Biogeosciences (BG).

Is deoxygenation detectable before warming in the thermocline?

Angélique Hameau1,2, Thomas L. Frölicher1,2, Juliette Mignot3, and Fortunat Joos1,2 Angélique Hameau et al.
  • 1Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland
  • 2Oeschger Centre for Climate Change Research, Bern, Switzerland
  • 3LOCEAN/IPSL, Sorbonne Université (SU)-CNRS-IRD-MNHN, Paris, France

Abstract. Multiple lines of evidence from observation- and model-based studies show that anthropogenic greenhouse gas emissions cause ocean warming and oxygen depletion, with adverse impacts on marine organisms and ecosystems. Temperature is considered as one of the main indicators of climate change, but, in the thermocline, anthropogenic changes in biogeochemical tracers such as oxygen may emerge from the bounds of natural variability before changes in temperature. Here, we compare the local time of emergence (ToE) of anthropogenic temperature and oxygen changes in the thermocline within an ensemble of Earth system model simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Anthropogenic deoxygenation emerges from natural internal variability before warming in 35 ± 11 % of the global thermocline. Earlier emergence of oxygen than temperature change is simulated by all models in parts of the subtropical gyres of the Pacific and the Southern Ocean. Earlier detectable changes in oxygen than temperature are typically related to decreasing trends in ventilation. The supply of oxygen-rich surface waters to the thermocline is reduced as evidenced by an increase in apparent oxygen utilisation over the simulations. Concomitantly, the propagation of the warming signal is hindered by slowing ventilation, which delays the warming in the thermocline. As the magnitudes of internal variability and simulated temperature and oxygen changes, which determine ToE, vary considerably among models, we compute the local ToE relative to the global mean ToE within each model. This reduces the inter-model spread in the relative ToE 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.

Angélique Hameau et al.
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Status: final response (author comments only)
Status: final response (author comments only)
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Angélique Hameau et al.
Angélique Hameau et al.
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Short summary
Marine deoxygenation and anthropogenic ocean warming are observed and projected to aggravate under continued greenhouse gas emissions. Whereas temperature is considered the main climate change indicator, we show that in certain regions of the thermocline, deoxygenation may be detectable before warming.
Marine deoxygenation and anthropogenic ocean warming are observed and projected to aggravate...
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