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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-2018-523
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-523
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Jan 2019

Research article | 10 Jan 2019

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

Assessment of time of emergence of anthropogenic deoxygenation and warming: insights from a CESM simulation from 850 to 2100 CE

Angélique Hameau1,2, Juliette Mignot3, and Fortunat Joos1,2 Angélique Hameau et al.
  • 1Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland
  • 2Oeschger Centre for Climate Change Research, Bern, Switzerland
  • 3LOCEAN Laboratory, Sorbonne Universités, Paris, France

Abstract. Marine deoxygenation and anthropogenic ocean warming are observed and projected to aggravate under continued greenhouse gas emissions. These changes potentially adversely affect the functioning and services of marine ecosystems. A key question is whether marine ecosystems are already or will soon be exposed to environmental conditions not experienced during the last millennium. We find that anthropogenic deoxygenation and warming in the thermocline have today already left the bounds of natural variability in respectively 60 % and 90 % of total ocean area in a forced simulation with the Community Earth System Model (CESM) over the period 850 to 2100. Natural variability is assessed from last millennium (850–1800) results considering forcing from explosive volcanic eruptions, solar irradiance, and greenhouse gases in addition to internal, chaotic variability. Control simulations are typically used to estimate variability. However, natural variability in oxygen (O2) and temperature (T) are systematically larger than internal variability (e.g. the latter amounts to 20 % for T and 60 % for O2 in the thermocline), rendering such estimates of natural variability to be biased low. Results suggest that anthropogenic change in apparent oxygen utilisation (AOU) and in O2 solubility (O2,sol) are earlier detectable by measurements than in O2 in the tropical thermocline, where biological and solubility-driven O2 changes counteract each other. Both natural variability and change in AOU are predominantly driven by variations in circulation with a smaller role for productivity. Ventilation becomes more vigorous in the tropical thermocline by the end of the 21st century, whereas ideal age in deep waters increases by more than 200 years until 2100. Different methodological choices are compared and the time for an anthropogenic signal to emergence (ToE) is earlier in many thermocline regions when using variability from a short period, the control, or estimates from the industrial period instead variability from the last millennium. Our results highlight that published methods lead to deviations in ToE estimates, calling for a careful quantification of variability and that realised anthropogenic change exceeds natural variations in many regions.

Angélique Hameau et al.
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Angélique Hameau et al.
Angélique Hameau et al.
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Short summary
The observed decrease of oxygen and warming in the ocean may adversely affect marine ecosystems and their services. We analyse results from an Earth System Model for the last millennium and the 21st century. We find changes in temperature and oxygen due to fossil fuel burning and other human activities to exceed natural variations in many ocean regions already today. Natural variability is biased low in earlier studies neglecting forcing from past volcanic eruptions and solar change.
The observed decrease of oxygen and warming in the ocean may adversely affect marine ecosystems...
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