Marine cloud brightening (MCB), also known as marine

cloud seeding Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation, mitigate hail, or disperse fog. The usual objective is to increase rain or snow, either for its own sake or to prevent precipitation from ...

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 Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

. 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 Solar radiation reduction due to volcanic eruptions, considered the best analogue for stratospheric aerosol injection. Stratospheric aerosol injection (SAI) is a proposed method of solar geoengineering (or solar radiation modification) to reduc ...

. It may be able to keep local areas from overheating. If used on a large scale it might reduce the Earth's

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

; 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 Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximate ...

are sprayed into the air to increase cloud reflectivity. The fine particles of sea salt enhance

cloud condensation nuclei Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 μm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...

, 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 The Great Barrier Reef is the world's largest coral reef system, composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...

in 2024.

Basic principles

Ship Tracks off the California Coast - NASA Earth Observatory

Marine cloud brightening is based on phenomena that are currently observed in the climate system. Today, emissions particles, such as

soot Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. Soot is considered a hazardous substance with carcinogenic properties. Most broadly, the term includes all the particulate matter produced b ...

, 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

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 Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 μm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...

are necessary for water droplet formation. The central idea underlying marine cloud brightening is to add

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...

s 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 Exhaust, exhaustive, or exhaustion may refer to: Law * Exhaustion of intellectual property rights, limits to intellectual property rights in patent and copyright law ** Exhaustion doctrine, in patent law ** Exhaustion doctrine under U.S. law, i ...

, leaving

ship tracks Ship tracks are clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles (aerosols) from exhaust to form a cloud seed. More and more water accumulates on the seed until a vi ...

. 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 cloud A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and t ...

s (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 In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

from

seawater Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...

, 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 The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in IPCC#Assessment reports, a series of such reports and was completed in 2014.IPCC (2014The IPCC's Fifth Assessment Report (A ...

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 The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

South Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...

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

Reduction in global warming

The modeling evidence of the global climatic effects of marine cloud brightening remains limited. Current modeling research indicates that marine cloud brightening could substantially cool the planet. A 2020 study found a substantial increase in cloud reflectivity from shipping in the southeast Atlantic basin, suggesting that a regional-scale test of MCB in stratocumulus‐dominated regions could be successful. Studies in the late 2010s estimated that this technique could produce up to 2 W/m² of negative

radiative forcing Radiative forcing (or climate forcing) is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentrations of greenhouse gases ...

, which is less than human-caused radiative forcing of almost 3 W/m². The climatic impacts of marine cloud brightening would be rapidly responsive and reversible. If the brightening activity were to change in intensity, or stop altogether, then the clouds' brightness would respond within a few days to weeks, as the cloud condensation nuclei particles

precipitate In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...

naturally. A 2024 study used a unique reduced complexity modeling framework to evaluate the effectiveness of marine cloud brightening across different cloud types. The study reported that anthropogenic aerosols have likely offset approximately one-third of greenhouse gas–induced warming since the industrial era, and marine cloud brightening leverages this effect by enhancing cloud reflectivity via modified droplet concentrations. Using an open-source parcel model, commercial CFD-based radiation transfer module, and assimilated meteorological data, the authors simulated albedo responses to varying droplet sizes and concentrations. They found optimal droplet radii (20–35 µm) led to a 28–57% increase in cloud albedo, with effectiveness strongly dependent on exceeding ambient aerosol levels by ~30% and on spatial deployment patterns. Although their approach omits aerosol–cloud feedbacks, the model reproduced observed radiative effects with surprising accuracy, highlighting the feasibility of reduced-complexity frameworks for geoengineering assessment. Their results also underscore the importance of considering background droplet size and concentration—not just meteorological variables—when designing cloud brightening interventions. Again unlike stratospheric aerosol injection, marine cloud brightening might be able to be used regionally, albeit in a limited manner. Marine stratocumulus clouds are common in particular regions, specifically the eastern Pacific Ocean and the eastern South Atlantic Ocean. A typical finding among simulation studies was a persistent cooling of the Pacific, similar to the

La Niña LA most frequently refers to Los Angeles, the second most populous city in the United States of America. La, LA, or L.A. may also refer to: Arts and entertainment Music *La (musical note), or A, the sixth note *"L.A.", a song by Elliott Smit ...

phenomenon, and, despite the localized nature of the albedo change, an increase in polar sea ice. Studies aim at making simulation findings derived from different models comparable.

Side effects

There is some potential for changes to precipitation patterns and amplitude, although modeling suggests that the changes are likely less than those for stratospheric aerosol injection and considerably smaller than for unabated anthropogenic global warming. The effects may be like

. Regional implementations of MCB would need care to avoid causing possibly adverse consequences in areas far away from the region they are aiming to help. For example, a potential Marine Cloud Brightening aimed at cooling the Western United States could risk causing increasing heat in Europe, due to climate teleconnections such as unintended perturbation of the

Atlantic meridional overturning circulation The Atlantic meridional overturning circulation (AMOC) is the main ocean current system in the Atlantic Ocean.IPCC, 2021Annex VII: Glossary

Research

Marine cloud brightening was originally suggested by John Latham (physicist)">John Latham in 1990. Because clouds remain a major source of uncertainty in climate change, some research projects into cloud reflectivity in the general climate change context have provided insight into marine cloud brightening specifically. For example, one project released smoke behind ships in the Pacific Ocean and monitored the particulates' impact on clouds. Although this was done in order to better understand clouds and climate change, the research has implications for marine cloud brightening. A research coalition called the Marine Cloud Brightening Project was formed in order to coordinate research activities. Its proposed program includes modeling, field experiments, technology development and policy research to study cloud-aerosol effects and marine cloud brightening. The proposed program currently serves as a model for process-level (environmentally benign) experimental programs in the atmosphere.Formed in 2009 by Kelly Wanser with support from Ken Caldeira, the project is now housed at the University of Washington. The shipping industry may have been carrying out an unintentional experiment in marine cloud brightening due to the emissions of ships and causing a global temperature reduction of as much as 0.25 ˚C lower than they would otherwise have been. A 2020 study found a substantial increase in cloud reflectivity from shipping in the southeast Atlantic, suggesting that a regional-scale test of MCB in stratocumulus‐dominated regions could be successful. Marine cloud brightening is being field tested as a way to shade and cool the

in Australia, as part of the Reef Restoration and Adaptation Program. As of 2024 it is thought that the salt spray can deliver particles into low clouds. Although research is not yet complete, experts on the project say that if deployed it would not effect any other countries. Unlike experiments in some other places, this research is supported locally and by most Australians.

Proposed methods

The leading proposed method for marine cloud brightening is to generate a fine mist of salt from seawater, and to deliver into targeted banks of marine stratocumulus clouds from ships traversing the ocean. This requires technology that can generate optimally-sized (~200 nm) sea-salt particles and deliver them at sufficient force and scale to penetrate low-lying marine clouds. The resulting spray mist must then be delivered continuously into target clouds over the ocean. In the earliest published studies, John Latham and

Stephen Salter Professor Stephen Hugh Salter, (7 December 1938 – 23 February 2024) was a South African-born Scottish academic who was Emeritus Professor of Engineering Design at the University of Edinburgh and inventor of the eponymous Salter's duck wav ...

proposed a fleet of around 1500 unmanned Rotor ships, or Flettner ships, that would spray mist created from seawater into the air. Subsequent researchers determined that transport efficiency was only relevant for use at scale, and that for research requirements, standard ships could be used for transport. (Some researchers considered aircraft as an option, but concluded that it would be too costly.) Droplet generation and delivery technology is critical to progress, and technology research has been focused on solving this challenging problem. As of 2025 how far the plume would travel and how much would reach the cloud layer is not known. Other methods were proposed and discounted, including: * Using small droplets of seawater into the air through ocean foams. When bubbles in the foams burst, they loft small droplets of seawater. * Using

piezoelectric Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied stress (mechanics), mechanical s ...

transducers. This would create Faraday waves at a free surface. If the waves are steep enough, droplets of sea water will be thrown from the crests and the resulting salt particles can enter into the clouds. However, a significant amount of energy is required. * Using engine or smoke emissions as a source for CCN. Paraffin oil particles have also been proposed, though their viability has been discounted.

Costs

The costs of marine cloud brightening remain largely unknown. A report of the US National Academies suggested roughly five billion US dollars annually for a large deployment program.

Governance

Marine cloud brightening would be governed primarily by

international law International law, also known as public international law and the law of nations, is the set of Rule of law, rules, norms, Customary law, legal customs and standards that State (polity), states and other actors feel an obligation to, and generall ...

because it would likely take place outside of countries'

territorial waters Territorial waters are informally an area of water where a sovereign state has jurisdiction, including internal waters, the territorial sea, the contiguous zone, the exclusive economic zone, and potentially the extended continental shelf ( ...

, and because it would affect the environment of other countries and of the oceans. For the most part, the international law governing solar radiation management in general would apply. For example, according to

customary international law Customary international law consists of international legal obligations arising from established or usual international practices, which are less formal customary expectations of behavior often unwritten as opposed to formal written treaties or c ...

, if a country were to conduct or approve a marine cloud brightening activity that would pose significant risk of harm to the environments of other countries or of the oceans, then that country would be obligated to minimize this risk pursuant to a

due diligence Due diligence is the investigation or exercise of care that a reasonable business or person is normally expected to take before entering into an agreement or contract with another party or an act with a certain standard of care. Due diligence ...

standard. In this, the country would need to require authorization for the activity (if it were to be conducted by a private actor), perform a prior

environmental impact assessment Environmental impact assessment (EIA) is the assessment of the environmental impact, environmental consequences of a plan, policy, program, or actual projects prior to the decision to move forward with the proposed action. In this context, the te ...

, notify and cooperate with potentially affected countries, and inform the public. Marine cloud brightening activities would be further governed by the international law of the sea, and particularly by the

United Nations Convention on the Law of the Sea The United Nations Convention on the Law of the Sea (UNCLOS), also called the Law of the Sea Convention or the Law of the Sea Treaty, is an international treaty that establishes a legal framework for all marine and maritime activities. , 169 sov ...

(UNCLOS). Parties to the UNCLOS are obligated to "protect and preserve the marine environment," including by preventing, reducing, and controlling pollution of the marine environment from any source. The "marine environment" is not defined but is widely interpreted as including the ocean's water, lifeforms, and the air above. "Pollution of the marine environment" is defined in a way that includes global warming and greenhouse gases. The UNCLOS could thus be interpreted as obligating the involved parties to use methods such as marine cloud brightening if these were found to be effective and environmentally benign. Whether marine cloud brightening itself could be such pollution of the marine environment is unclear. At the same time, in combating pollution, Parties are "not to transfer, directly or indirectly, damage or hazards from one area to another or transform one type of pollution into another." If marine cloud brightening were found to cause damage or hazards, the UNCLOS could prohibit it. If marine cloud brightening activities were to be "marine scientific research"—also an undefined term—then UNCLOS Parties have a right to conduct the research, subject to some qualifications. Like all other ships, those that would conduct marine cloud brightening must bear the flag of the country that has given them permission to do so and to which the ship has a genuine link, even if the ship is unmanned or automated. The flagged state must exercise its jurisdiction over those ships. The legal implications would depend on, among other things, whether the activity were to occur in

, an

exclusive economic zone An exclusive economic zone (EEZ), as prescribed by the 1982 United Nations Convention on the Law of the Sea, is an area of the sea in which a sovereign state has exclusive rights regarding the exploration and use of marine natural resource, reso ...

(EEZ), or the

high seas The terms international waters or transboundary waters apply where any of the following types of bodies of water (or their drainage basins) transcend international boundaries: oceans, large marine ecosystems, enclosed or semi-enclosed regiona ...

; and whether the activity was scientific research or not. Coastal states would need to approve any marine cloud brightening activities in their territorial waters. In the EEZ, the ship must comply with the coastal state's laws and regulations. It appears that the state conducting marine cloud brightening activities in another state's EEZ would not need the latter's permission, unless the activity were marine scientific research. In that case, the coastal state should grant permission in normal circumstances. States would be generally free to conduct marine cloud brightening activities on the high seas, provided that this is done with "due regard" for other states' interests. There is some legal unclarity regarding unmanned or automated ships. As of 2025 MCB is being considered for addition to the

London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter 1972, commonly called the "London Convention" or "LC '72" and also abbreviated as Marine Dumping, is an agreement to control pollution of the sea by dum ...

, which might mean that parties to the convention would have to assess projects under Annex V of the convention’s London Protocol.

Advantages and disadvantages

Marine cloud brightening appears to have most of the advantages and disadvantages of

solar radiation management Solar radiation modification (SRM) (or solar geoengineering) is a group of large-scale approaches to reduce global warming by increasing the amount of sunlight that is reflected away from Earth and back to space. It is not intended to replace e ...

in general. For example, it presently appears to be inexpensive relative to suffering climate change damages and greenhouse gas emissions abatement, fast acting, and reversible in its direct climatic effects. Some advantages and disadvantages are specific to it, relative to other proposed solar radiation management techniques. Compared with other proposed solar radiation management methods, such as stratospheric aerosols injection, marine cloud brightening may be able to be partially localized in its effects. This could, for example, be used to stabilize the

West Antarctic Ice Sheet The West Antarctic Ice Sheet (WAIS) is the segment of the Antarctic ice sheet, continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. It is cla ...

. Furthermore, marine cloud brightening, as it is currently envisioned, would use only natural substances sea water and wind, instead of introducing human-made substances into the environment. Potential disadvantages include that specific MCB implementations could have a varying effect across time; the same intervention might even become a net contributor to global warming some years after being first launched, though this could be avoided with careful planning.

Notes

References

{{Climate change, state=expanded Climate change policy Planetary engineering Geoengineering