Introduction

Two months ago, a team of scientists helmed by researchers from the University of Washington initiated a field test on the deck of the U.S.S. Hornet, a decommissioned aircraft carrier docked off Alameda, California. The experiment entailed spraying extremely tiny sea salt particles into the air using a device that resembles a snow-making device. The researchers wanted to determine if they could consistently spray the right size of particles to ultimately facilitate a process called “marine cloud brightening (MCB).” However, after a public meeting in June to review the experiment, the Alameda City Council voted unanimously to stop the operation.

Marine Cloud Brightening is one approach in a suite of options to combat climate change denominated as “solar radiation modification (SRM),” often called solar geoengineering. SRM approaches seek to reduce the amount of solar (shortwave) energy that reaches the Earth’s surface. Reducing radiative forcing can offset some of the warming effects associated with greenhouse gas emissions. Other SRM approaches include injecting sulfur dioxide or other materials into the stratosphere, or placing highly reflective sunshades in space.

MCB aims to increase the albedo, or reflectivity, of low-level maritime clouds. Increasing cloud albedo could result in deflection of more incoming solar radiation back to space, thus exerting a cooling effect. The approach seeks to accomplish this by injecting seawater spray into the lower marine atmosphere. Ideally, the spray would be converted to fine particles (approximately 50 nanometers) by evaporation and conveyed to clouds by turbulent and convective air motions. The higher aerosol load associated with such operations could lead to a greater number of cloud droplets, smaller in diameter. This phenomenon, known as the “Twomey effect,” could substantially increase the reflectivity of the clouds. Smaller cloud droplets could also extend the lifetime of such clouds. 

One leading proponent of this approach has proposed the deployment of a fleet of unmanned wind-powered ships. These vessels would be equipped with underwater turbines to produce the necessary seawater particles, and devices to inject the particles into the atmosphere.

Given the failure of the world community in arresting climate change, it is likely that the drum beat for deployment of SRM approaches will grow ever louder in the next decade. This piece will focus on marine cloud brightening in terms of potential benefits and risks. 

Potential effectiveness of marine cloud brightening

To date, virtually all assessments of the potential effectiveness of MCB to combat climate change are derived from modeling, and the results reflect high levels of uncertainty and heterogeneous results. For example, one study concluded that delivering a 50-100% increase in droplet concentrations in all marine stratiform clouds could offset the warming associated with a doubling of the concentrations of greenhouse gases in the atmosphere. 

However, another study concluded that increases in droplet concentrations would have to be much higher to achieve that objective, and ultimately might fail to increase albedo sufficiently. Moreover, other studies have found no substantial changes in temperature over wide ranges of the globe, or have concluded that positive temperature impacts might diminish over time. Some models have also suggested that MCB could largely restore sea ice coverage in both the Northern and Southern Hemispheres. 

Potential risks associated with marine cloud brightening

MCB research to date has raised serious concerns that deployment might ultimately create regional “winners” and “losers,” which could undermine principles of equity and justice and exacerbate international tensions. In one study, modeling of MCB deployment in the South Atlantic substantially decreased temperatures in North America, but it also yielded a far more substantial temperature increase over Amazonia, a region already facing serious negative impacts from rising temperatures associated with climate change. In a more recent study, modelling of MCB in the North Pacific revealed that it could result in exacerbated heat stress and hotter summers in several regions of the world by 2050, including northeast Asia, Europe and central North America. 

Another concern is that MCB might alter regional precipitation patterns in ways that could adversely impact food production and ecosystems. For example, one study concluded that MCB deployment could result in a substantial decline in precipitation over northeastern South America, while another, modeling deployment of MCB in the South Atlantic yielded substantial declines in precipitation in the Amazon, perhaps up to 50%. Notably, another study concluded that MCB deployment would actually increase precipitation in the region.  A study modeling MCB in the North Pacific concluded that deployment could result in decreases in precipitation over the Sahel and the Western United States. This highlights high levels of uncertainty in assessing regional impacts, with results potentially dependent on the magnitude of sea salt injections and regions of deployment. 

A final risk associated with MCB deployment, and one common to all SRM approaches, is termed the “termination” or “rebound” effect, meaning that sudden cessation of the use of the approach could result in extremely rapid and large temperature increases. Temperatures could rise four to six times faster than under a business as usual scenario. “As a result, the world would be hit with one massive heatwave – which could cause unprecedented and unparalleled damage to society.” 

Society could substantially ameliorate the threat of “termination shock” by committing to rapid decarbonization in conjunction with MCB deployment. However, there’s a very real threat that MCB deployment might actually pull society in the opposite direction. If the approach proved to be highly effective in masking the manifestations of climate change, society might be lulled into a false sense of complacency, which could denude its commitment to climate mitigation policies. This phenomena is often termed the “moral hazard.” Absent a full-throated commitment to rapidly reducing emissions, MCB deployment would need to be “sustained for centuries,” posing an unprecedented governance challenge for future generations.

The future of MCB?

Given the highly speculative benefits of MCB, as well as the serious potential risks that deployment might pose in large swathes of the world, it’s far from clear that it should be considered a credible mechanism to address climate change. Given the recent experience in Alameda, it’s also far from clear if communities are willing to even permit field research on this approach. 

However, if interest in MCB as a climate response mechanism continues, a carefully designed research program will be needed to adequate characterize its potential impacts. This would include a far more granular understanding of aerosol and cloud physics, including quantification of the microphysical-dynamical boundary layer feedbacks, assessment of the potential negative impacts of MCB deployment on ocean circulation patterns related to changes in sea surface temperatures, and a more robust understanding of potential regional deployment of MCB on global systems.

As is true with all solar geoengineering approaches, the future of MCB remains “cloudy.”