Rising global temperatures pose significant risks to marine ecosystems, biodiversity and fisheries. Recent comprehensive assessments suggest that large-scale mitigation efforts to limit warming below crucial thresholds are falling short, and all feasible future climate projections, including those that represent ideal emissions reductions, exceed the Paris Agreement’s aspirational <1.5{degree sign}C warming target, at least temporarily. As such, there are a number of proposed climate interventions that aim to deliberately manipulate the environment at large scales to counteract anthropogenic global warming. Yet, there is a high level of uncertainty in how marine ecosystems will respond to these interventions directly as well as how these interventions may impact marine ecosystems’ responses to climate change. Due to the key role the ocean plays in regulating Earth’s climate and ensuring global food security, understanding the effects that these interventions may have on marine ecosystems is crucial. This review provides an overview of proposed intervention methodologies for solar radiation modification and marine carbon dioxide removal and outlines the potential trade-offs and knowledge gaps associated with their impacts on marine ecosystems. Climate interventions have the potential to reduce warming-driven impacts, but could also substantially alter marine food systems, biodiversity and ecosystem function. Impact assessments are thus crucial to quantify trade-offs between plausible intervention scenarios and to identify and discontinue scaling efforts or commercial implementation for those with unacceptable risks.

Air-sea exchange of carbon dioxide (CO$_2$) in the Southern Ocean plays an important role in the global carbon budget. Previous studies have suggested that flow around topographic features of the Southern Ocean enhances the upward supply of carbon from the deep to the surface, influencing air-sea CO$_2$ exchange. Here, we investigate the role of seafloor topography on the transport of carbon and associated air-sea CO$_2$ flux in an idealized channel model. We find elevated CO$_2$ outgassing downstream of a seafloor ridge, driven by anomalous advection of dissolved inorganic carbon. Argo-like Lagrangian particles in our channel model sample heterogeneously in the vicinity of the seafloor ridge, which could impact float-based estimates of CO$_2$ flux.

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.

Orbital precession has been linked to glacial cycles and the atmospheric carbon dioxide (CO2) concentration, yet the direct impact of precession on the carbon cycle is not well understood. We analyze output from an Earth system model configured under different orbital parameters to isolate the impact of precession on air-sea CO2 flux in the Southern Ocean – a component of the global carbon cycle that is thought to play a key role on past atmospheric CO2 variations. Here, we demonstrate that periods of high precession are coincident with anomalous CO2 outgassing from the Southern Ocean. Under high precession, we find a poleward shift in the southern westerly winds, enhanced Southern Ocean meridional overturning, and an increase in the surface ocean partial pressure of CO2 along the core of the Antarctic Circumpolar Current. These results suggest that orbital precession may have played an important role in driving changes in atmospheric CO2

Anthropogenic carbon emissions and associated climate change are driving rapid warming, acidification, and deoxygenation in the ocean, which increasingly stress marine ecosystems. On top of long-term trends, short term variability of marine stressors can have major implications for marine ecosystems and their management. As such, there is a growing need for predictions of marine ecosystems on monthly, seasonal, and multi-month timescales. Previous studies have demonstrated the ability to make reliable predictions of the surface ocean physical and biogeochemical state months to years in advance, but few studies have investigated forecasts of multiple stressors simultaneously or assessed the forecast skill below the surface. Here, we use the Community Earth System Model (CESM) Seasonal to Multiyear Large Ensemble (SMYLE) along with novel observation-based biogeochemical and physical products to quantify the predictive skill of dissolved inorganic carbon, dissolved oxygen, and temperature in the surface and subsurface ocean. CESM SMYLE demonstrates high physical and biogeochemical predictive skill multiple months in advance in key oceanic regions and frequently outperforms persistence forecasts. We find up to 10 months of skillful forecasts, with particularly high skill in the Northeast Pacific (Gulf of Alaska and California Current Large Marine Ecosystems) for temperature, surface DIC, and subsurface oxygen. Our findings suggest that dynamical marine ecosystem prediction could support actionable advice for decision making.

Pinatubo erupted during the first decadal survey of ocean biogeochemistry, embedding its climate fingerprint into foundational ocean biogeochemical observations and complicating the interpretation of long-term biogeochemical change. Here, we quantify the influence of the Pinatubo climate perturbation on externally forced decadal and multi-decadal changes in key ocean biogeochemical quantities using a large ensemble simulation of the Community Earth System Model designed to isolate the effects of Pinatubo, which cleanly captures the ocean biogeochemical response to the eruption. We find increased uptake of apparent oxygen utilization and preindustrial carbon over 1993-2003. Nearly 100\% of the forced response in these quantities are attributable to Pinatubo. The eruption caused enhanced ventilation of the North Atlantic, as evidenced by deep ocean chlorofluorocarbon changes that appear 10-15 years after the eruption. Our results help contextualize observed change and contribute to improved constraints on uncertainty in the global carbon budget and ocean deoxygenation.

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.

Reducing uncertainty in the global carbon budget requires better quantification of ocean CO2 uptake and its temporal variability. Several methodologies for reconstructing air-sea CO2 exchange from sparse pCO2 observations indicate larger decadal variability than estimated using ocean models. We develop a new application of multiple Large Ensemble Earth system models to assess these reconstructions’ ability to estimate spatiotemporal variability. With our Large Ensemble Testbed, pCO2 fields from 25 ensemble members each of four independent Earth system models are subsampled as the observations and the reconstruction is performed as it would be with real- world observations. The power of a testbed is that the perfect reconstruction is known for each of the 100 original model fields; thus, reconstruction skill can be comprehensively assessed. We find that a commonly used neural-network approach can skillfully reconstruct air-sea CO2 fluxes when and where it is trained with sufficient data. Flux bias is low for the global mean and Northern Hemisphere, but can be regionally high in the Southern Hemisphere. The phase and amplitude of the seasonal cycle are accurately reconstructed outside of the tropics, but longer-term variations are reconstructed with only moderate skill. For Southern Ocean decadal variability, insufficient sampling leads to a 39% [15%:58%, interquartile range] overestimation of amplitude, and phasing is only moderately correlated with known truth (r=0.54 [0.46:0.63]). Globally, the amplitude of decadal variability is overestimated by 21% [3%:34%]. Machine learning, when supplied with sufficient data, can skillfully reconstruct ocean properties. However, data sparsity remains a fundamental limitation to quantification of decadal variability in the ocean carbon sink.

We use a statistical emulation technique to construct synthetic ensembles of global and regional sea-air carbon dioxide (CO2) flux from four observation-based products over 1985-2014. Much like ensembles of Earth system models that are constructed by perturbing their initial conditions, our synthetic ensemble members exhibit different phasing of internal variability and a common externally forced signal. Our synthetic ensembles illustrate an important role for internal variability in the temporal evolution of global and regional CO2 flux and produce a wide range of possible trends over 1990-1999 and 2000-2009. We assume a specific externally forced signal and calculate the likelihood of the observed trend given the distribution of synthetic trends during these two periods. Over the decade 1990-1999, three of the four observation-based products exhibit small negative trends in globally integrated sea-air CO2 flux (i.e., enhanced ocean CO2 absorption with time) that are highly probable (44-72% chance of occurrence) in their respective synthetic trend distributions. Over the decade 2000-2009, however, three of the four products show large negative trends in globally integrated sea-air CO2 flux that are somewhat improbable (17-19% chance of occurrence). Our synthetic ensembles suggest that the largest observation-based positive trends in global and Southern Ocean CO2 flux over 1990-1999 and the largest negative trends over 2000-2009 are somewhat improbable (<30% chance of occurrence). Our approach provides a new understanding of the role of internal and external processes in driving sea-air CO2 flux variability.

The COVID-19 global pandemic and associated government lockdowns dramatically altered human activity, providing a window into how changes in individual behavior, enacted en masse, impact atmospheric composition. The resulting reductions in anthropogenic activity represent an unprecedented event that yields a glimpse into a future where emissions to the atmosphere are reduced. While air pollutants and greenhouse gases share many common anthropogenic sources, there is a sharp difference in the response of their atmospheric concentrations to COVID-19 emissions changes due in large part to their different lifetimes. Here, we discuss two key takeaways from modeling and observational studies. First, despite dramatic declines in mobility and associated vehicular emissions, the atmospheric growth rates of greenhouse gases were not slowed. Second, it demonstrated empirically that the response of atmospheric composition to emissions changes is heavily modulated by factors including carbon cycle feedbacks to CH4 and CO2, background pollutant levels, the timing and location of emissions changes, and climate feedbacks on air quality.

The ocean has absorbed the equivalent of 39% of industrial-age fossil carbon emissions, significantly modulating the growth rate of atmospheric CO2 and its associated impacts on climate. Despite the importance of the ocean carbon sink to climate, our understanding of the causes of its interannual-to-decadal variability remains limited. This hinders our ability to attribute its past behavior and project its future. A key period of interest is the 1990s, when the ocean carbon sink did not grow as expected. Previous explanations of this behavior have focused on variability internal to the ocean or associated with coupled atmosphere/ocean modes. Here, we use an idealized upper ocean box model to illustrate that two external forcings are sufficient to explain the pattern and magnitude of sink variability since the mid-1980s. First, the global-scale reduction in the decadal-average ocean carbon sink in the 1990s is attributable to the slowed growth rate of atmospheric pCO2. The acceleration of atmospheric pCO2 growth after 2001 drove recovery of the sink. Second, the global sea surface temperature response to the 1991 eruption of Mt Pinatubo explains the timing of the global sink within the 1990s. These results are consistent with previous experiments using ocean hindcast models with and without forcing from variable atmospheric pCO2 and climate variability. The fact that variability in the growth rate of atmospheric pCO2 directly imprints on the ocean sink implies that there will be an immediate reduction in ocean carbon uptake as atmospheric pCO2 responds to cuts in anthropogenic emissions.

The physical circulation of the Southern Ocean sets the surface concentration and thus air-sea exchange of CO2. However, we have a limited understanding of the three-dimensional circulation that brings deep carbon-rich waters to the surface. Here, we introduce and analyze a novel high-resolution ocean model simulation with active biogeochemistry and online Lagrangian particle tracking. We focus our attention on a subset of particles with high dissolved inorganic carbon (DIC) that originate below 1000 m and eventually upwell into the surface mixed layer. We find that 71% of the DIC-enriched water upwelling across 1000 m is concentrated near topographic features, which occupy just 33% of the Antarctic Circumpolar Current. Once particles upwell to the surface mixed layer, their DIC decorrelates on timescales of ~1.5 months—an order of magnitude longer than their residence time. Our results show that Southern Ocean bathymetry plays a key role in delivering carbon-rich waters to the surface.