Simulated effect of calcification feedback on atmospheric CO2 and ocean acidification.

Han Zhang,Long Cao

doi:10.1038/srep20284

Abstract

Ocean uptake of anthropogenic CO2 reduces pH and saturation state of calcium carbonate materials of seawater, which could reduce the calcification rate of some marine organisms, triggering a negative feedback on the growth of atmospheric CO2. We quantify the effect of this CO2-calcification feedback by conducting a series of Earth system model simulations that incorporate different parameterization schemes describing the dependence of calcification rate on saturation state of CaCO3. In a scenario with SRES A2 CO2 emission until 2100 and zero emission afterwards, by year 3500, in the simulation without CO2-calcification feedback, model projects an accumulated ocean CO2 uptake of 1462 PgC, atmospheric CO2 of 612 ppm, and surface pH of 7.9. Inclusion of CO2-calcification feedback increases ocean CO2 uptake by 9 to 285 PgC, reduces atmospheric CO2 by 4 to 70 ppm, and mitigates the reduction in surface pH by 0.003 to 0.06, depending on the form of parameterization scheme used. It is also found that the effect of CO2-calcification feedback on ocean carbon uptake is comparable and could be much larger than the effect from CO2-induced warming. Our results highlight the potentially important role CO2-calcification feedback plays in ocean carbon cycle and projections of future atmospheric CO2 concentrations.

Highlights

Ocean uptake of anthropogenic CO2 reduces pH and saturation state of calcium carbonate materials of seawater, which could reduce the calcification rate of some marine organisms, triggering a negative feedback on the growth of atmospheric CO2
Between 1959 and 2011 total anthropogenic CO2 emission is 436 PgC (1PgC = 1015 gram carbon = 1 billion ton carbon) with 44% of the emission stayed in the atmosphere, while 29% and 27% of emissions were absorbed by the terrestrial biosphere and the ocean, respectively[2]
This study aims to further our understanding in the role of the CO2-calcification feedback on the ocean carbon cycle and atmospheric CO2, which is important for a reliable projection of future atmospheric CO2 concentrations and climate change

Summary

Introduction

Ocean uptake of anthropogenic CO2 reduces pH and saturation state of calcium carbonate materials of seawater, which could reduce the calcification rate of some marine organisms, triggering a negative feedback on the growth of atmospheric CO2. Inclusion of CO2-calcification feedback increases ocean CO2 uptake by 9 to 285 PgC, reduces atmospheric CO2 by 4 to 70 ppm, and mitigates the reduction in surface pH by 0.003 to 0.06, depending on the form of parameterization scheme used. A warmer ocean could accelerate respiration/remineralization rate of organic carbon and cause a reduction in the vertical flux of particulate organic carbon (POC) to the abyssal ocean This reduced vertical transport of POC would weaken the ocean biological pump and decrease the ocean’s ability to take up carbon, providing a positive feedback to the growth of atmospheric CO26. The rise of hydrogen ion concentration would lower carbonate ion concentration ([CO32−])and in turn causes a reduction of seawater CaCO3 (aragonite or calcite) saturation state, which is defined as

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