Implications of climate mitigation by ocean-based negative emission technologies need sound field-based evaluation before application, but also some model-based approximation of how useful they might be. For this effort the CO2 reduction model inter-comparison (CDRMIP) has been established with defined protocols on which scenarios should be tested with both state-of-the-art climate models, but also with simpler model for an approximation of long-term effects. I here analyze ocean alkalinization in a high CO2 world (emission scenario SSP5-85-EXT++ and CDR-ocean-alk within CDRMIP) for the next millennia using the carbon cycle model BICYCLE, whose long-term feedbacks will be calculated for the next 1 million years. The applied model has recently been extended by a process-based sediment module, now being able to also handle solid Earth processes, such as volcanic CO2 outgassing, continental weathering and deep ocean CaCO3 accumulation and dissolution. In the applied negative emission experiment 0.14 Pmol/yr of alkalinity — comparable to the dissolution of 5 Pg of olivine per year — is entering the surface ocean starting in year 2020 for either 50 years or until full neutralization of the anthropogenic emissions is achieved. I find that the cumulative emissions of 6740 PgC emitted until year 2350 19 lead to a peak atmospheric CO2 concentration of nearly 2400 ppm in year 2326, which is reduced by only 200 ppm by the alkalinization experiment. Atmospheric CO2 is brought down to 400 or 300 ppm after 2730 or 3480 years of alkalinization, respectively. Such low CO2 concentrations are reached without ocean alkalinization only after several tens of thousands of years, when the feedbacks from weathering and sediments bring the part of the anthropogenic emissions that stays in the atmosphere (the so-called airborne fraction) below 10%. Although the applied model is very simple, it is in its global response to emissions comparable to more complex models. I analytically explain why in the simulation results a linear relationship in the transient climate response to cumulative emissions (TCRE) is found for low emissions (similarly as for more complex climate models), which evolves for high emissions to a non-linear relation.
Köhler, Peter (in review): Anthropogenic CO2 of high emission scenario compensated after 3500 years of ocean alkalinization with an annually constant dissolution of 5 Pg of olivine. Frontiers in Climate
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