Anthropogenic CO2 of High Emission Scenario Compensated After 3500 Years of Ocean Alkalinization With an Annually Constant Dissolution of 5 Pg of Olivine

Abstract

The CO2 removal model inter-comparison (CDRMIP) has been established to approximate the usefulness of climate mitigation by some well-defined negative emission technologies. 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 a revised version of the carbon cycle model BICYCLE, whose long-term feedbacks are calculated for the next 1 million years. The applied model version not only captures atmosphere, ocean, and a constant marine and terrestrial biosphere, but also represents solid Earth processes, such as deep ocean CaCO3 accumulation and dissolution, volcanic CO2 outgassing, and continental weathering. 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 or 5000 years. I find that the cumulative emissions of 6,740 PgC emitted until year 2350 lead to a peak atmospheric CO2 concentration of nearly 2,400 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 hundreds 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 4%. The efficiency of carbon sequestration by this alkalinization approach peaks at 9.7 PgC per Pmol of alkalinity added during times of maximum anthropogenic CO2 emissions and slowly declines to half this value 2000 years later due to the non-linear marine chemistry response and ocean-sediment processes. In other words, ocean alkalinization sequesters carbon only as long as the added alkalinity stays in the ocean. To understand the basic model behavior, I analytically explain why in the simulation results a linear relationship in the transient climate response (TCR) to cumulative emissions is found for low emissions (similarly as for more complex climate models), which evolves for high emissions to a non-linear relation.

Alkalinization Scenarios in the Mediterranean Sea for Efficient Removal of Atmospheric CO2 and the Mitigation of Ocean Acidification

“…These values are substantially lower than the upper bound of 1 mol CO 2 (mol TA) −1 given by the consumption of aqueous CO 2 in Equation (1), due to two processes: the redistribution of inorganic dissolved carbon species toward a new equilibrium of the carbonate system and dynamic exchanges with the underlying water bodies further reducing the efficiency. They are also lower than the estimate of 0.8 mol CO 2 (mol TA) −1 in Renforth et al (2013), who however did not involve the latter process and the estimate of 0.7 mol CO 2 (mol TA) −1 in Keller et al (2014), but are consistent with the efficiency estimates given recently by Köhler (2020) for the near future in a multimillenial alkalinization experiment.…”

Potential CO2 removal from enhanced weathering by ecosystem responses to powdered rock

“…Here, we consider the enhanced weathering from BD to accelerate the reaction of atmospheric CO 2 with the silicates contained in basalt 12 . The silicate grains chemically react with CO 2 to form bicarbonate dissolved ions; these are transported by rivers to the oceans and potentially stored for hundreds years and longer, depending on calcium carbonate sedimentation processes 13 . In addition to this abiotic carbon dioxide removal (CDR) pathway, amending soils with BD enhances soil fertility by releasing nutrients, buffering low soil pH, stabilizing soil organic matter 14 , and can improve soil water retention 15 , thereby promoting plant growth and CDR in agriculture [16][17][18] and forestry 11 .…”

Atmospheric CO2 Concentration Based on Boron Isotopes Versus Simulations of the Global Carbon Cycle During the Plio‐Pleistocene

“…In its strong form, the weathering feedback had been compared, and agreed well with results based on the interpretation of typically weathering proxies, such as the isotopes of Sr, Li, Os, over the Cenozoic (Caves et al., 2016). Furthermore, the strong weathering feedback has already been applied to long‐term future emission scenarios calculated with BICYCLE‐SE (Köhler, 2020).…”