Arctic Marine Cloud Brightening

That aerosols contribute to cloud formation has long since been observed in the formation of ship tracks of normal ocean vessels (Hobbs et al. 2000), and studies on such “inadvertent” MCB provide valuable analogues for MCB (Patel and Shand 2022). In 1990 Latham already came up with the idea to use specially designed boats for this. This idea has since then developed in several research programs, including lab experiments (Cooper et al. 2014). In 2014, Neukermans et al. designed a spray device that would use the salt from seawater. As an alternative distribution method, Claudel et al. (2023) have recently suggested using unmanned aerial vehicles (UAVs) to seed clouds. 

Currently, Australian scientists are conducting research to determine whether it might be possible to use MCB to reduce coral bleaching (Latham et al. 2013) and cool the waters around the Great Barrier Reef (Tollefson 2021; for a research project, see https://gbrrestoration.org/program/cooling-by-cloud-brightening/). A novel research project by the Cambridge Centre for Climate Repair and Delft University will also specifically examine the relevance of MCB for attempts to refreeze the Arctic. 

The advocacy group Silver Lining’s 2023 report Near-term Climate Risk and Intervention suggested that 4- to 5-year research programs working on modeling, technical, and outdoor experiments could significantly advance MCB, though the feasibility of such a timeline is questionable. The IPCC AR6 WG1 notes that many uncertainties remain around MCB, especially since climate models do not represent the relevant cloud processes well (IPCC 2021, chapter 4). Hoffmann and Feingold (2021), for example, found that small differences in particle seeding size could have significant impacts on the effect of MCB.

Advocates of Arctic MCB envision large unmanned fleets of vessels operating throughout the season, but for now, these ships only exist on the drawing board, and it is uncertain whether the measure would be scalable (National Research Council 2015). Wood (2021) estimates that 10,000 to 100,000 vessels would need to operate ‘over the majority of the 54 % of the Earth's surface that is over ocean and remote from land’ to compensate for the forcing caused by a doubling of atmospheric CO2. Moreover, the IPCC AR6 Working Group 1 report cautions that MCB requires the presence of a specific type of cloud (IPCC 2021, chapter 4). It might be that MCB could also be effective in areas regardless of clouds, due to the radiative effect of the aerosols themselves (Ahlm et al. 2017). The Mahfouz et al. (2023) coupled model study recently highlighted the need to better understand these interactions between aerosols and solar radiation to determine the effectiveness of MCB. Manshausen et al. (2022) note that invisible ship tracks create significant radiative forcing. This implies that MCB could potentially exhibit stronger and earlier detectable impacts on climate than previously expected.

The previously mentioned Silver Lining Report (2023) is very optimistic about the potential of MCB. However, the absence of modeling results and the current lack of means for large-scale distribution raise doubts.

Many studies show that MCB could cause significant cooling over the Arctic. The Parkes et al. (2012) model study shows that polar MCB would allow significantly more sea ice to remain in a double CO2 world. Likewise, Latham et al. (2012) also found MCB ‘significantly reduces sea-ice fraction loss during the summer months’. Furthermore, Latham et al. (2014) found that MCB might help stabilize the West Antarctic ice sheet, and could also be a tangible way to cool surrounding permafrost areas and limit methane release from its thaw (see methane measures). The Kim et al. (2020) model study found that MCB could produce significant cooling over East Asia, and could restore sea ice in the Sea of Okhotsk. Mahfouz et al. (2023) also show that MCB could be effective in cooling Arctic temperatures.In comparison to other measures, Zhao et al. (2020) posit that MCB has several climate benefits over surface albedo modification (see Marine Surface Albedo Modification).

Multiple model studies have shown that MCB could cool the oceans and lead to regional cooling, but it is unsure how effective MCB could be in lowering global temperatures (National Research Council 2015). In their large model ensemble study, Stjern et al. (2018) found that MCB would lower temperatures by −0.96K relative to RCP4.5. A review by Lawrence et al. (2018) found large differences in potential cooling of MCB that ranged from 0.8 to 5.4 W/m2. The IPCC AR6 Working Group 1 report gives 1–5 W/m2 as the global mean radiative forcing potential (IPCC 2021, chapter 4) but expresses low to medium confidence in the reported forcing numbers due to a lack of understanding of the relevant warming processes (IPCC 2021, chapter 6). Hirasawa et al. (2023) suggest that an AI Model like the one they used could optimize MCB forcing patterns.

The costs of operationalising MCB at scale are still largely unknown but will likely be low in comparison to the projected benefits and costs associated with inaction. The National Research Council estimated logistical costs of around USD 5 million per week to produce 0.01W/m2 (NASEM 2015). In an interview with the Guardian, David Kind gives a rough estimate that an effective Arctic MCB fleet of 500 to 1000 vessels would cost up to £40bn, with subsequent annual continuation costs of around £10bn (Anthony 2022).

Like other solar radiation management (SRM) measures, MCB could impact precipitation patterns and the hydrological cycle (IPCC 2021, chapter 4). In their large model ensemble study, Stjern et al. (2018) found that MCB would reduce global precipitation by −2.35%. Bala et al. (2012) found equally important effects, although these would be less impactful than those of land-based albedo changes.

The National Research Council report states that MCB might reduce light availability thereby affecting weather patterns and local ecosystems through reduced photosynthesis (NASEM 2015) and having an impact on ocean circulation and carbon sequestration (Partanen et al. 2016; Lauvset et al. 2017). Horowitz et al. (2020) found that sea salt aerosol distribution by MCB would decrease tropospheric ozone and extend methane lifetime, and although the resulting radiative forcing is minimal, it could also influence air quality.

The inherent patchiness of MCB (i.e., it is not done over land) means that there will be large gradients in radiative forcing. In simulations of global ocean brightening, these radiative changes induce large impacts on downstream precipitation and clouds (Kravitz et al. 2018). Few or no global analyses of teleconnection impacts from localized MCB have been done (Ricke et al. 2021).

MCB could have certain beneficial effects. Although claims from proponents like Kevin Lister and Sev Clarke of technology firm Winwick Business Solutions, are probably too optimistic when they hint at the possibility of MCB to 'influencing where, when, and how much precipitation occurs downwind'. Parkes et al. (2015) found MCB and other geoengineering methods could reduce crop failure rates, and Latham et al. (2012b) showed it could weaken hurricanes. Moreover, Hirasawa et al. (2023) suggest AI models could potentially be used to reduce the negative side effects of MCB.

Diamond et al. (2022) recommend 'collaboration between physical scientists, ecologists, social scientists, and ethicists' to explore potential risks to local communities. It could be, for example, that reduced bioproductivity (see Environmental Risks above) might lead to local fish stocks decline, thereby detrimentally affecting local and indigenous communities and the international fishing industry.

The effect of MCB would only last a couple of days and would, therefore, be relatively straightforward to reverse.

The C2G2 risk analysis report on solar radiation management (SRM) (Felgenhauer et al. 2022) states clearly that, 'A sudden and sustained termination of large amount of Stratospheric Aerosol Injection (SAI) or MCB under a high greenhouse gas emission background would cause a rapid increase in temperature and precipitation at a rate that far exceeds that predicted for future climate change without SRM.' Parker and Irvine (2018) point out that the short time before the effect of MCB wears off would not leave much time to restore the measure in comparison to SAI, whose stratospheric aerosols have a far longer lifetime.

Cloud seeding under domestic regulation is already widely practices, and as the MCB experiments in Australia show, it can be done without international consultation.The Carnegie Climate Governance Initiative brief on SRM (Felgenhauer et al. 2022), states that 'there are no measures, other than soft power, that would stop either researchers or states from taking forward field trials or climate-scale deployments'. They therefore suggest 'early discussions about how these technologies might be governed' since policy makers do not know enough about the technology to make adequate decisions.