Marine cloud brightening (MCB), also known as marine cloud seeding or marine cloud engineering, may be a way to make stratocumulus clouds over the sea brighter, thus reflecting more sunlight back into space in order to limit global warming. It is one of two such methods that might feasibly have a substantial climate impact, but is lower in the atmosphere than stratospheric aerosol injection. It may be able to keep local areas from overheating. If used on a large scale it might increase the Earth's albedo; and so, in combination with greenhouse gas emissions reduction, limit climate change and its risks to people and the environment. If implemented, the cooling effect would be expected to be felt rapidly and to be reversible on fairly short time scales. However, technical barriers remain to large-scale marine cloud brightening, and it could not offset all the current warming. As clouds are complicated and poorly understood, the risks of marine cloud brightening are unclear as of 2025. Very small droplets of sea water are sprayed into the air to increase cloud reflectivity. The fine particles of sea salt enhance cloud condensation nuclei, making more cloud droplets so making the clouds more reflective. MCB could be implemented using fleets of unmanned rotor ships to disperse seawater mist into the air. Small-scale field tests were conducted on the Great Barrier Reef in 2024.
Basic principles Marine cloud brightening is based on phenomena that are currently observed in the climate system. Today, emissions particles, such as soot, mix with clouds in the atmosphere and increase the amount of sunlight they reflect, reducing warming. This cooling effect is estimated at between 0.5 and 1.5 °C (0.9 and 2.7 °F), and is one of the most important unknowns in climate. Marine cloud brightening proposes to generate a similar effect using benign material, such as sea salt. Marine stratocumulus clouds are thought to be the most suitable because of their prevalence, coverage, accessibility, and generally low cloud drop number concentration. MCB also makes the clouds last longer. Although stratospheric aerosol injection would be much higher up, it could diffuse sunlight and so also brighten low-level marine clouds. Most clouds are quite reflective, redirecting incoming solar radiation back into space. Increasing clouds' albedo would increase the portion of incoming solar radiation that is reflected, in turn cooling the planet. Clouds consist of water droplets, and clouds with smaller droplets are more reflective (because of the Twomey effect). Cloud condensation nuclei are necessary for water droplet formation. The central idea underlying marine cloud brightening is to add aerosols to atmospheric locations where clouds form. These would then act as cloud condensation nuclei, increasing the cloud albedo. Marine cloud brightening on a small scale already occurs unintentionally due to the aerosols in ships' exhaust, leaving ship tracks. Changes to shipping regulations enacted by the United Nations' International Maritime Organization to reduce certain aerosols are hypothesized to be leading to reduced cloud cover and increased oceanic warming, providing additional support to the potential effectiveness of marine cloud brightening at modifying ocean temperature. Different cloud regimes are likely to have differing susceptibility to brightening strategies, with marine stratocumulus clouds (low, layered clouds over ocean regions) most sensitive to aerosol changes. These marine stratocumulus clouds are thus typically proposed as the target. They are common over the cooler regions of subtropical and midlatitude oceans, where their coverage can average over 50% over a year. The leading possible source of additional cloud condensation nuclei is salt from seawater, although there are others. Even though the importance of aerosols for the formation of clouds is, in general, well understood, many uncertainties remain. The IPCC Fifth Assessment Report considers aerosol-cloud interactions as one of the current major challenges in climate modeling in general. In particular, the number of droplets does not increase proportionally when more aerosols are present, and can even decrease. Extrapolating the effects of particles on clouds observed on the microphysical scale to the regional, climatically relevant, scale is not straightforward. For example deployment in the South Pacific or South Atlantic could increase rainfall in western and central Africa but reduce it in southern Africa.
Proposed local use It has been suggested that MCB should be used to preserve Arctic sea ice.
Climatic impacts
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