Marine cloud brightening (MCB), also called marine cloud seeding or marine cloud engineering, is a method that may make stratocumulus clouds over the ocean brighter. Brighter clouds reflect more sunlight back into space, which could help reduce global warming. It is one of two methods that might significantly affect the climate, but it works in the lower part of the atmosphere, unlike stratospheric aerosol injection, which operates higher up. MCB may help prevent certain areas from becoming too hot. If used widely, it could increase Earth's albedo, or reflectivity, and when combined with reducing greenhouse gas emissions, might help slow climate change and its effects on people and the environment. If used, the cooling effect would likely happen quickly and could be stopped or reversed in a short time. However, technical challenges remain, and MCB cannot fully counteract current warming. Since clouds are complex and not fully understood, the risks of MCB are still unclear as of 2025.

Tiny seawater droplets are sprayed into the air to make clouds more reflective. Sea salt particles help form more cloud droplets, increasing cloud brightness. MCB could be carried out using unmanned ships that release seawater mist into the air. Small tests were conducted near the Great Barrier Reef in 2024.

Basic principles

Marine cloud brightening is based on natural events that scientists observe in the climate system today. Emissions from human activities, such as soot, mix with clouds in the atmosphere and increase the amount of sunlight they reflect, which helps reduce warming. This cooling effect is estimated to be between 0.5 and 1.5 °C (0.9 and 2.7 °F), and it is one of the most important unknowns in climate science. Marine cloud brightening suggests using harmless materials, like sea salt, to create a similar effect. Marine stratocumulus clouds are considered the best choice because they are common, cover large areas, are easily accessible, and usually have fewer water droplets. Marine cloud brightening may also help clouds last longer. While a method called stratospheric aerosol injection would place particles higher in the atmosphere, it could also brighten low-level marine clouds by diffusing sunlight.

Most clouds are highly reflective, sending sunlight back into space. Increasing the reflectiveness of clouds, known as albedo, would reflect more sunlight and cool the planet. Clouds are made of water droplets, and clouds with smaller droplets are more reflective because of the Twomey effect. Cloud condensation nuclei are needed for water droplets to form. The main idea behind marine cloud brightening is to add aerosols to areas where clouds form. These aerosols would act as cloud condensation nuclei, increasing the cloud's albedo.

Marine cloud brightening already happens unintentionally on a small scale because of aerosols in ship exhaust, which create visible patterns called ship tracks. Changes in shipping rules by the United Nations' International Maritime Organization to reduce certain aerosols are thought to be causing less cloud cover and more ocean warming. This supports the idea that marine cloud brightening could help change ocean temperatures. Different types of clouds may respond differently to brightening strategies, with marine stratocumulus clouds (low, layered clouds over oceans) being most affected by changes in aerosols. These clouds are often found over cooler regions of subtropical and midlatitude oceans, where they can cover more than 50% of the area for a year. The main source of additional cloud condensation nuclei is salt from seawater, though other sources may exist.

Although scientists generally understand how aerosols help form clouds, many questions remain. The IPCC Fifth Assessment Report states that how aerosols affect clouds is one of the biggest challenges in climate modeling. For example, adding more aerosols does not always increase the number of droplets in clouds and may even decrease it. Predicting how small-scale effects of particles on clouds influence larger climate patterns is difficult. For instance, using marine cloud brightening in the South Pacific or South Atlantic could increase rainfall in western and central Africa but decrease it in southern Africa.

Climatic impacts

Research about how marine cloud brightening affects the global climate is still limited. Current studies suggest that this method could significantly lower Earth's temperature. A 2020 study found that shipping activities in the southeast Atlantic increased cloud reflectivity, showing that testing marine cloud brightening in areas with many stratocumulus clouds might work.

Studies from the late 2010s estimated that this technique could create up to 2 W/m² of cooling, which is less than the warming caused by human activities (about 3 W/m²). The effects of marine cloud brightening would happen quickly and could be reversed if the process stopped. If the brightness of clouds changed or ended, the clouds would return to their normal state within days or weeks as particles naturally fall from the air.

A 2024 study used a simplified model to test marine cloud brightening across different cloud types. It found that human-made particles in the air may have reduced warming from greenhouse gases by about one-third since the industrial era. Marine cloud brightening uses this effect by increasing cloud reflectivity through changes in droplet size. The study also showed that designing cloud brightening efforts must consider both droplet size and concentration, not just weather conditions.

Unlike stratospheric aerosol injection, marine cloud brightening could be used in specific regions, though only in limited ways. Stratocumulus clouds are common in the eastern Pacific Ocean and the eastern South Atlantic Ocean. Simulations often showed cooling in the Pacific similar to the La Niña phenomenon, along with increased polar sea ice. Researchers are working to compare results from different models to better understand these effects.

Marine cloud brightening might change rainfall patterns, but models suggest these changes would likely be smaller than those from stratospheric aerosol injection or uncontrolled human-caused warming. These effects could resemble those of La Niña.

Using marine cloud brightening in one region could cause unintended effects in distant areas. For example, efforts to cool the Western United States might lead to increased heat in Europe due to climate connections like changes in the Atlantic meridional overturning circulation.

Research

Marine cloud brightening was first proposed by John Latham in 1990. Since clouds are a major unknown in climate change studies, some research has explored how cloud reflectivity affects the climate. For example, one study released smoke behind ships in the Pacific Ocean and observed how tiny particles in the smoke influenced clouds. This research aimed to better understand clouds and climate change, but it also provided information about marine cloud brightening.

A group of researchers called the Marine Cloud Brightening Project was created in 2009 by Kelly Wanser with help from Ken Caldeira. The project is now based at the University of Washington. It plans to study cloud-aerosol effects through modeling, field experiments, technology development, and policy research. The project serves as an example of how to conduct safe, step-by-step experiments in the atmosphere.

The shipping industry may have accidentally tested marine cloud brightening. Ship emissions have been linked to a global temperature drop of up to 0.25 ˚C. A 2020 study found that shipping increased cloud reflectivity in the southeast Atlantic, suggesting that testing marine cloud brightening in areas with certain types of clouds could work.

Marine cloud brightening is being tested as a way to cool the Great Barrier Reef in Australia as part of the Reef Restoration and Adaptation Program. As of 2024, scientists believe that salt spray can deliver tiny particles into low clouds. While research is still ongoing, experts say that if used, the method would not affect other countries. This project is supported by local communities and most Australians.

Proposed methods

The main idea for making clouds brighter over the ocean is to create a fine salt mist from seawater and send it into specific areas of low-lying marine clouds using ships. This process needs special technology to make salt particles about 200 nm in size and to release them with enough power and in large amounts to reach the clouds. The mist must be sent continuously into the clouds over the ocean.

In the earliest studies, scientists John Latham and Stephen Salter suggested using around 1,500 unmanned Rotor ships, also called Flettner ships, to spray seawater mist into the air. Later research found that using standard ships was enough for testing, and that using large numbers of ships was only important for large-scale use. Some scientists considered using airplanes, but decided it would be too expensive. Creating and delivering tiny droplets is a key challenge, and research has focused on solving this problem.

As of 2025, it is still unknown how far the mist would spread or how much of it would reach the cloud layer.

Other methods were suggested but not used, including:

• Spraying small seawater droplets into the air using ocean foams. When bubbles in the foams burst, they release tiny droplets.
• Using piezoelectric transducers to create waves on water surfaces. If the waves are large enough, seawater droplets can be thrown into the air and enter clouds. However, this method requires a lot of energy.
• Using engine or smoke emissions to create cloud condensation nuclei (CCN). Paraffin oil particles were also considered, but their usefulness was not supported.

Costs

The costs of marine cloud brightening are not fully understood. A report from the US National Academies estimated that a large-scale program would require about five billion US dollars each year.

Governance

Marine cloud brightening (MCB) would mostly be controlled by international laws because it would likely happen outside of countries' territorial waters and could affect the environment of other nations and the oceans. The same international rules that apply to solar radiation management would mostly govern MCB. For example, under customary international law, if a country planned or approved an MCB activity that could harm the environment of other countries or the oceans, that country would need to take steps to reduce the risk. This includes requiring permission for the activity if a private group is involved, doing an environmental impact assessment before starting, informing and working with other countries that might be affected, and sharing information with the public.

The international law of the sea, especially the United Nations Convention on the Law of the Sea (UNCLOS), would also apply to MCB. Countries that signed UNCLOS must protect the marine environment and reduce pollution from any source. The term "marine environment" is not clearly defined but usually includes ocean water, living things, and the air above the ocean. "Pollution of the marine environment" includes global warming and greenhouse gases. This means UNCLOS might require countries to use methods like MCB if they are found to be effective and safe for the environment. However, it is unclear if MCB itself could be considered pollution. At the same time, UNCLOS says countries should not move harm or pollution from one area to another. If MCB caused harm, UNCLOS might stop it. If MCB is considered "marine scientific research" (a term not clearly defined), then countries have the right to do it, but with some rules. Any ships used for MCB must display the flag of the country that approved their use, even if the ship is unmanned or automated. The country that gave permission must manage the ship's actions. Legal rules depend on where the MCB happens—inside a country's territorial waters, in an exclusive economic zone (EEZ), or on the high seas—and whether the activity is scientific research. If MCB happens in a country's territorial waters, that country must agree to it. In an EEZ, the ship must follow the coastal country's laws. A country might not need permission to do MCB in another country's EEZ unless it is scientific research, in which case the coastal country should usually allow it. Countries can generally do MCB on the high seas as long as they consider the interests of other countries. There are still unclear rules about unmanned or automated ships.

As of 2025, MCB is being considered for inclusion in the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter. This might require countries to evaluate MCB projects under the London Protocol's Annex V.

Advantages and disadvantages

Marine cloud brightening seems to share many of the same benefits and risks as other solar radiation management methods. It is currently less costly than dealing with the harm caused by climate change or reducing greenhouse gas emissions. It works quickly and its direct effects on the climate can be reversed. Some benefits and risks are unique to marine cloud brightening compared to other solar radiation management ideas.

Compared to methods like injecting aerosols into the stratosphere, marine cloud brightening may have effects that can be limited to certain areas. This could help protect the West Antarctic Ice Sheet, for example. Marine cloud brightening, as planned, would use only natural materials—seawater and wind—rather than adding human-made substances to the environment.

Possible risks include that specific ways of using marine cloud brightening might have different results over time. In some cases, the same action could increase global warming years after it is first used. However, careful planning might prevent this outcome.