Marine cloud brightening

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The exhaust from ships already causes more and brighter clouds above the oceans.

Marine cloud brightening is a proposed solar radiation management climate engineering technique that would make clouds brighter, reflecting a small fraction of incoming sunlight back into space in order to offset anthropogenic global warming. Along with stratospheric aerosol injection, it is one of the two solar radiation management methods that may most feasibly have a substantial climate impact.[1]The intention is that increasing the Earth's albedo, in combination with greenhouse gas emissions reduction, carbon dioxide removal, and adaptation, would reduce climate change and its risks to people and the environment. If implemented, the cooling effect is expected to be felt rapidly and to be reversible on fairly short time scales. However, technical barriers remain to large-scale marine cloud brightening. There are risks with such modification of complex climate systems.

Basic principles[edit]

Most clouds are quite reflective, bouncing 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. The marine environment has a deficit of cloud condensation nuclei due to lower levels of dust and pollution at sea,[citation needed] so marine cloud brightening would be more effective over the ocean than over land. In fact, marine cloud brightening on a small scale already occurs unintentionally due to the aerosols in ships' exhaust, leaving ship tracks.[2] 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.[3][4] These marine stratocumulus clouds are thus typically proposed as the suited target. They are common over the cooler regions of subtropical and midlatitude oceans, where their coverage can exceed 50% in the annual mean.[5] The leading possible source of additional cloud condensation nuclei is salt from seawater, although there are others.[6] Even though the importance of aerosols for the formation of clouds is, in general, well understood, many uncertainties remain. In fact, the latest IPCC report considers aerosol-cloud interactions as one of the current major challenges in climate modeling in general.[7] In particular, the number of droplets does not increase proportionally when more aerosols are present and can even decrease.[8][9] Extrapolating the effects of particles on clouds observed on the microphysical scale to the regional, climatically relevant scale, is not straightforward.[10]

Climatic impacts[edit]

The modeling evidence of the climatic effects of marine cloud brightening remains scant.[1] Current modeling research indicates that marine cloud brightening could substantially cool the planet. Unlike stratospheric aerosol injection, this capacity is limited due to the existing amount of marine stratocumulus clouds. One study estimated that it could could produce 3.7 W/m2 of globally averaged negative forcing. This would counteract the warming caused a doubling of the preindustrial atmospheric carbon dioxide concentration, or an estimated 3 degrees Celsius,[3] although models have indicated less capacity.[11]

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 days to weeks, as the cloud condensation nuclei particles precipitate naturally.[1]

Again unlike stratospheric aerosol injection, marine cloud brightening might be able to be used regionally, albeit in a limited manner.[12] 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” phenomenon, and, despite the localized nature of the albedo change, an increase in polar sea ice.[13][11][14][15][16] Recent studies aim at making simulation findings derived from different models comparable.[17][18]

There is some potential for changes to precipitation patterns and amplitude,[19][20][14] although modeling suggests that the changes are likely less than those for stratospheric aerosol injection and considerably smaller than for unabated anthropogenic global warming.[1]


Marine cloud brightening was originally suggested by John Latham in 1990.[21]

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.[22] Although this was done in order to better understand clouds and climate change, the research has implications for marine cloud brightening.

Bower, Jones and Choularton created a model to analyze the effectiveness of albedo modification on clouds.[23] A 2006 study simplified the model.[24] Their model illustrates the practicality of the technique. It demonstrates droplet size is not of that much importance, that location of clouds is of limited importance, and that significant cooling can be achieved with a level of .03.

A research coalition call the Marine Cloud Brightening Project formed in order to coordinate research activities.

A handful of outdoor tests of marine cloud brightening have been tentatively sketched out.[25]

Proposed methods[edit]

The leading proposed method for marine cloud brightening would be to spray seawater upward. To do that, John Latham and Stephen Salter proposed a fleet of around 1500 unmanned Rotor ships, or Flettner ships, that would spray mist created from seawater into the air.[3][26] The power for the rotors and the ship could be generated from underwater turbines. The vessels would spray sea water droplets at a rate of approximately 50 cubic meter per second over a large portion of Earth's ocean surface.[27] This method would require that the spray nozzles not become clogged. Salter and colleagues proposed using active hydrofoils with controlled pitch for power.[28]

Another method to get small droplets of seawater into the air is through ocean foams. When bubbles in the foams burst, they loft small droplets of seawater.[29]

Still a third possibility is to use a a piezoelectric transducer. 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.[30]

A fourth possibility is the electrostatic atomization of seawater drops. This technique would utilize mobile spray platforms that move to adjust to changing weather conditions. These too could be on unmanned ships.

Some researchers considered aircraft as an option, but concluded that it would be too costly.[31]

Another possibility is to use engine or smoke emissions as a source for CCN.[1] Paraffin oil particles could also be used effectively.[22]

A final, indirect method is to enhance the natural sulfur cycle in the Southern Ocean by fertilizing a small portion with iron. This would, in turn, enhance dimethyl sulfide production.[32] This may be able to cool the Antarctic region in particular, slowing Antarctic ice melting.[33] Such techniques would also sequester carbon.[34] However, this technique can give only 0.016 W/m2 of globally averaged negative forcing,[35] which is practically insignificant as a contribution to reducing global warming. On the other hand, its effects would be concentrated on the climate of Antarctica.


The costs of marine cloud brightening remain largely unknown. One academic paper implied annual costs of approximately 50 to 100 million UK pounds (roughly 75 to 150 million US dollars).[3] A report of the US National Academies suggested roughly five billion US dollars annually for a large deployment program (reducing radiative forcing by 5 W/m2).[1]


Marine cloud brightening would be governed primarily by international law because it would likely take place outside of countries' territorial waters, and because it would effect 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, 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 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, notify and cooperate with potentially affected countries, inform the public, and develop plans for a possible emergency.

Marine cloud brightening activities would be furthered governed by the international law of sea, and particularly by the United Nations Convention on the Law of the Sea (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.[36] The "marine environment" is not defined but is widely interpreted as including the ocean's water, lifeforms, and the air above.[37] "Pollution of the marine environment" is defined in a way that includes global warming and greenhouse gases.[38][39] The UNCLOS could thus be interpreted as obligating it 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."[40] 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.[41] 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.[42] The flagged state must exercise its jurisdiction over those ships.[43] The legal implications would depend on, among other things, whether the activity were to occur in territorial waters, an exclusive economic zone (EEZ), or the high seas; 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.[44] 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.[45] 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.[46]

Advantages and disadvantages[edit]

Marine cloud brightening appears to have most of the advantages and disadvantages of solar radiation management 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.[12] This could, for example, be used to stabilize the West Antarctic Ice Sheet. Furthermore, marine cloud brightening, as it is currently envisioned, would use only natural substances sea water and wind, instead os introducing human-made substances into the environment.

Marine cloud brightening has limited effectiveness to cool the planet. The effectiveness of marine cloud brightening would depend upon local conditions, such as wind patterns.

See also[edit]


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