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Earth System Dynamics An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/esd-2017-45
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
24 May 2017
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Earth System Dynamics (ESD).
Irreversible ocean thermal expansion under negative CO2 emissions
Dana Ehlert and Kirsten Zickfeld Department of Geography, Simon Fraser University, 8888 University Drive, Burnaby, B.C., Canada V5A 1S6
Abstract. In the Paris Agreement in 2015 countries agreed on holding global mean surface air warming well below 2 °C but the emission reduction pledges under that agreement are not ambitious enough to meet this target. Therefore, the question arises whether restoring temperature to this target after exceeding it by artificially removing CO2 from the atmosphere is possible. One important aspect regards the reversibility of ocean heat uptake and the associated sea level rise, which have very long (centennial to millennial) response time scales. In this study the response of sea level rise due to thermal expansion (TSLR) to a 1 % yearly increase of atmospheric CO2 up to a quadrupling of the pre-industrial concentration followed by a 1 % yearly decline back to the pre-industrial CO2 concentration is examined using the University of Victoria Earth System Climate Model (UVic ESCM). We find that TSLR continues for several decades after atmospheric CO2 starts to decline and that sea level does not return to pre-industrial levels for over thousand years after atmospheric CO2 is restored to pre-industrial concentrations. This finding is independent of the strength of vertical sub-grid scale ocean mixing implemented in the model. Furthermore, TSLR rises faster than it declines in response to a symmetric rise and decline in atmospheric CO2 concentration partly because the deep ocean continues to warm for centuries after atmospheric CO2 returns to the pre-industrial concentration. Both TSLR rise and decline rate increase with increasing vertical ocean mixing. Exceptions from this behaviour arise if the overturning circulations in the North Atlantic and Southern Ocean intensify beyond pre-industrial levels in model versions with lower vertical mixing, which leads to rapid cooling of the deep ocean.

Citation: Ehlert, D. and Zickfeld, K.: Irreversible ocean thermal expansion under negative CO2 emissions, Earth Syst. Dynam. Discuss., https://doi.org/10.5194/esd-2017-45, in review, 2017.
Dana Ehlert and Kirsten Zickfeld
Dana Ehlert and Kirsten Zickfeld
Dana Ehlert and Kirsten Zickfeld

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Short summary
This study uses a global climate model to explore the extent to which sea level rise due to thermal expansion of the ocean is reversible if the atmospheric concentration of carbon dioxide (CO2) declines. It is found that sea level continues to rise for several decades after atmospheric CO2 starts to decline and does not return to the pre-industrial level for over thousand years after atmospheric CO2 is restored to the pre-industrial concentration.
This study uses a global climate model to explore the extent to which sea level rise due to...
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