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.

Earth’s Northern and Southern Hemispheres reflect essentially equal amounts of sunlight. How—and whether—this hemispheric albedo symmetry is maintained remains a mystery. We decompose Earth’s hemispheric albedo symmetry into components associated with the surface, clear-sky atmosphere, and different cloud types as defined by cloud effective pressure and optical thickness. Climatologically, greater reflection by the surface, aerosols, and high clouds in the Northern Hemisphere is balanced by greater low and mid-level cloudiness in the Southern Hemisphere. Both hemispheres have darkened at similar rates over the past two decades; whether the decrease in clear-sky asymmetry from declining aerosol is breaking all-sky symmetry due to more rapid Northern Hemisphere darkening remains uncertain. Regional trends and natural experiments including sea ice loss and volcanic eruptions provide strong evidence against the hypothesis that extratropical low clouds compensate changing clear-sky asymmetries on decadal timescales but some evidence that tropical high cloud shifts may do so.

Earth’s Northern and Southern Hemispheres reflect identical amounts of sunlight. How — and whether — this hemispheric albedo symmetry is maintained remains a mystery. We decompose Earth’s hemispheric albedo symmetry into components associated with different cloud types as defined by cloud effective pressure and optical thickness. Greater reflection by the surface, clear-sky atmosphere, and high clouds in the Northern Hemisphere is balanced by low and mid clouds (dominated by stratocumulus) in the Southern Hemisphere. Both hemispheres have darkened by ~0.5-0.8 W/m2/decade due to decreasing low and mid cloud and surface reflection, partially offset by increasing high cloud reflection. Cloud reflection trends largely follow cloud fraction, with the exception of decreasing stratocumulus albedo in both hemispheres. Hypotheses that all-sky symmetry is maintained despite clear-sky changes via adjustments in high clouds within the Intertropical Convergence Zone or in low and mid clouds in the Southern Ocean are not supported at interannual or decadal timescales.