Rapidly increasing carbon dioxide emissions over the past decades and the possibility of further increases in coming decades has motivated global efforts to remove and sequester carbon from atmosphere. Among recent proposals for marine-based Carbon Dioxide Removal (mCDR), ocean iron-fertilization has been revisited, although its efficacy on a global scale remains uncertain. We thus assessed the carbon uptake efficacy in small (~GgC) and large (~PgC) scales in the global coupled earth system model, GFDL-ESM4.1. Our simulations indicate that large-scale fertilization yields greater carbon uptake efficacies than small-scale fertilization. Efficacies in large-scale fertilization are from 179% to 225% higher than small-scale fertilization in the Equatorial Pacific and Southern Ocean, but were more comparable in the North Pacific. Higher carbon uptake efficacies in large-scale fertilization are attributed to greater accumulation of dissolved iron within the fertilized region. While our results emphasize the carbon uptake efficacy benefits of a large-scale approach over the fertilized area, some of additional carbon uptake is compensated by reduced uptake in the non-fertilized oceans. This study finds that 80% and 85% of the carbon taken up in the Equatorial and North Pacific are compensated by decreases in carbon uptake elsewhere, implying that outside of the Southern Ocean major nutrients would have been taken up in non-fertilized regions, leaving little carbon additionality. This study puts into serious question the net benefits and further exposes risks iron fertilization, such as reduction of productions in the Equatorial Pacific, as a large scale mCDR approach outside of the Southern Ocean.

A document by Yechul Shin. Click on the document to view its contents.

In most emissions scenarios consistent with the temperature targets of the Paris Agreement, carbon dioxide removal (CDR) would be required to achieve net negative emissions. The efficiency of CDR depends on the behavior of the natural carbon reservoirs, land and ocean, that regulate atmospheric CO2 concentrations, but their change in response to negative emissions is highly uncertain. Here, we investigate the response of the terrestrial and oceanic carbon cycle to negative emissions based on an idealized emission-driven simulation using a state-of-the-art Earth system model. The terrestrial and oceanic carbon sinks become carbon sources ~30 years after the onset of negative emissions. Thereafter, although the atmospheric CO2 concentration returns to its initial level, the terrestrial ecosystem and the ocean continue to release carbon. The ocean recovers as a carbon sink within a few decades, while the terrestrial ecosystem remains a carbon source until the end of the simulation. The prolonged carbon emissions from the land are due to the delayed response of respiration to the earlier increase in terrestrial carbon uptake. As a result, the total carbon stock on land gradually decreases but still remains higher than its initial state. The ocean carbon inventory shows an irreversible change within a few centuries due to the accumulation of anthropogenic CO2 in the deep ocean, which would take more than centuries to be removed naturally.

A Lagrangian plankton model (LPM) is developed, in which the motion of a large number of Lagrangian particles, representing a plankton community, is calculated under the turbulence field simulated by large eddy simulation. A spring phytoplankton bloom is realized using the LPM, and the mechanism for its generation is investigated. Mixing by convective eddies during the night helps to maintain the uniform concentration of phytoplankton within the mixed layer, even if the daily mean surface heat flux is positive in spring. Accordingly, the spring bloom can be predicted by the critical depth hypothesis, if the mixing layer is used instead of the mixed layer. The shoaling of the mixing layer occurs immediately after the start of surface heating, but the shoaling of the mixed layer is delayed. A new criterion for the spring bloom is proposed, which predicts that spring blooms are more likely to occur at higher latitudes, even if the atmospheric forcing is the same. Furthermore, various statistics of Lagrangian particles, such as the vertical migration of plankton, the residence time of plankton within the euphotic zone, and the growth of plankton are investigated by taking advantage of the LPM.

In recent decades, Antarctic ice-sheet/shelf melting has been accelerated, releasing freshwater into the Southern Ocean. It has been suggested that the meltwater flux could lead to cooling in the Southern Hemisphere, which would retard global warming and further induce a northward shift of the Inter-Tropical Convergence Zone (ITCZ). In this study, we use experimental ensemble climate simulations to show that Antarctic meltwater forcing has distinct regional climate impacts over the globe, leading in particular to regional warming in East Asia. It is suggested that Antarctic meltwater forcing leads to a negative precipitation anomaly in the Western North Pacific (WNP) via cooling in the tropics and the northward shift of the ITCZ. This suppressed convection in WNP induces an anticyclonic flow over the North Pacific, which leads to regional warming in East Asia. This hypothesis is supported by analyses of inter-ensemble spread and long-term control simulations.