We assess the Southern Ocean CO2 uptake (1985-2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project phase 2 (RECCAP2). The Southern Ocean acted as a sink for CO2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75±0.28 PgCyr-1) and pCO2-observation-based products (0.73±0.07 PgCyr-1). This sink is only half that reported by RECCAP1. The present-day net uptake is to first order a response to rising atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO2-products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26±0.06 and 0.11±0.03 PgCyr-1 decade-1 respectively). This is despite nearly identical pCO2 trends in GOBMs and pCO2-products when both products are compared only at the locations where pCO2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non-thermal processes of biology and mixing/circulation. Ocean interior accumulation of Cant points to an underestimate of Cant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push towards better process understanding of drivers of the carbon cycle.

Anthropogenic climate change will drive extensive mass loss across both the Antarctic (AIS) and Greenland Ice Sheets (GrIS), with the potential for feedbacks on the global climate system, especially in polar regions. Historically, the high latitude North Atlantic and Southern Ocean have been the most critical regions for global anthropogenic heat and carbon uptake, but our understanding of how this uptake will be altered by future freshwater discharge is incomplete. Here, we assess each ice sheet’s impact on the global ocean storage of anthropogenic heat and carbon for a high-emission scenario over the 21$^{\textrm{st}}$ century using a coupled Earth system model. Notably, combined AIS and GrIS freshwater engenders distinct anthropogenic heat and carbon storage anomalies as the two diagnostics respond disparately in the high latitude Southern Ocean and North Atlantic. We explore the impact of contemporaneous mass loss from both ice sheets on anthropogenic heat and carbon storage and quantify the linear and nonlinear contributions of each ice sheet. We find that GrIS mass loss exerts a primary control on the 21$^{st}$-century evolution of both global oceanic heat and carbon storage, with AIS impacts appearing after the 2080s. Non-linear impacts of simultaneous ice sheets’ discharge have a non-negligible contribution to the evolution of both heat and carbon storage. Further, anthropogenic heat changes are realized more quickly in response to ice sheet discharge than anthropogenic carbon. Our results highlight the need to incorporate both ice sheets actively in climate models in order to accurately project future global climate.

Multidecadal satellite observations indicate that the Antarctic Ice Sheet (AIS) is losing mass at an accelerating rate, potentially impacting many aspects of the coupled climate system. While previous studies have demonstrated the importance of AIS freshwater discharge for regional and global climate processes using climate model experiments, many have applied unrealistic freshwater forcing. Here, we explore the potential Southern Ocean impacts of realistic AIS mass loss over the 21$^{st}$ century in the Community Earth System Model version 2 (CESM2) by applying observation-based historical and ice sheet model-based future AIS freshwater forcing. The added freshwater reduces wintertime deep convective area by 72\% while retaining 83\% more sea ice. Congruent with other studies, we find the increased freshwater discharge extensively impacts local and remote Southern Ocean surface and subsurface temperature and stratification. These results demonstrate the necessity of accounting for AIS mass loss in global climate models for projecting future climate.

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ~60% larger in models (-0.72 vs. -0.44 PgC/yr, 1998-2018, coastal ocean area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.