Geoengineering clouds: a hazy proposition? (Part 2)

Marine cloud brightening (MCB)

By increasing the reflectivity, or ‘brightness’, of marine stratocumulus clouds, the aim is to reduce incoming solar energy and prevent further warming. By enhancing the cloud condensation nuclei (CCN), the surface area of droplets is increased which results in bigger clouds to deflect solar energy (Caldeira et al, 2013).

Figure 1: Solar radiative potential of different cloud types (Source: Earth Gauge).

Essentially, MCB is similar to the sulphate aerosol technique in that an aerosol is sprayed into the atmosphere. CCN is enhanced by spraying a salty seawater mist into the marine atmosphere. Marine stratocumulus clouds are particularly proposed for MCB techniques as they are low-lying, thicker and more sensitive to aerosol additions – thus their albedo effect is more efficient than higher, thinner clouds (Latham et al, 2014). The video below explains the technique in more depth and scientific research on MCB.

Cloud albedo feedbacks strongly control atmospheric circulation and climate, as well as mediating radiation, water and heat transfer. For example, by mediating water and heat transfer, clouds in the tropics play a necessary role in tropical circulations. The primary tropical circulation is also known as the Inter Tropical Convergence Zone: a rain belt that shifts from the northern to the southern tropics (Bony et al, 2015). Furthermore, the liquid water content of clouds strongly regulates rainfall intensity during the Indian Monsoon (Maitra et al, 2014).

Climatic impacts

Many climatic modelling studies assume that enhancing cloud CCN will automatically increase albedo and thus cause cooling – however changes in cloud microphysical properties and how this will implicate albedo and cooling is difficult to to predict (Caldeira et al, 2013).

Some studies suggest radiative forcing, from a doubling of CCN, has the potential to offset the warming of a doubling of atmospheric CO₂ concentrations (approx. 440 ppm) as well as retain polar sea-ice extent (Latham et al, 2012). Enhancing cloud CCN would further influence the Earth’s hydrological system. Assuming a CCN enhancement of all marine clouds, global mean precipitation precipitation would decrease by 1.3%, however terrestrial runoff would increase by 7.5% (Bala et al, 2011). However, there is no certainty in regards to the spatial distribution of this runoff and its local implications.

With the frequency of droughts expected to increase in a warming world, extra rainfall may be required in arid regions. A marine brightening simulation has proposed a reduction in terrestrial dry spell frequency (Aswathy et al, 2015). Moreover, MCB can potentially reduce crop failure frequency in water stressed regions (Parkes et al, 2015).

In polar regions experiencing amplified global warming due to sea-ice albedo feedbacks, MCB techniques have the potential to reduce sea-ice loss and dampen feedback effects. This would be achieved by increasing sea-ice thickness and reducing sea-ice loss during summer months (Latham et al, 2012), by causing localised ocean surface cooling (Latham et al, 2014). Subsequently, this would help to reduce methane release from subsea permafrost in the East Siberian Arctic Shelf - a potent positive feedback for climate warming (Shakhova et al, 2015). Relative to other geoengineering methods, therefore, MCB techniques can be more efficiently targeted in polar regions (Parkes et al, 2012).

Ecological impacts

Although MCB techniques can slightly decrease net primary productivity in some light-limited oceans e.g. off the Peruvian coast, the regional effect of this is more pronounced (Partanen et al, 2016). This would result in a decrease in phytoplankton carbon uptake. Though this may seem minor relative to the radiative offsetting of MCB, phytoplankton play a large role in regulating the global carbon cycle (Litchman et al, 2015). 

Figure 2: Phytoplankton carbon capture via photosynthesis, using sunlight, across the sea-air interface (Source: Najjar, 2009).

Parallel to sulphate aerosols, there is a potential for salt damage to terrestrial ecosystems due to long-distance transport of marine salt aerosols. A recent study suggests that MCB would result in a decrease in gross primary productivity in tropical rainforests, with some models proposing a dieback of the Amazon rainforest (Muri et al, 2015). Thus, changing the balance of limited nutrients in ecosystems makes it difficult to predict future ecological impacts.

On the other hand, MCB techniques could potentially reduce coral bleaching episodes in the Great Barrier Reef, Caribbean and Polynesia. This would be due to a reduction in sea surface temperature (Latham et al, 2013). Currently, coral bleaching is a pressing concern as the worst destruction of coral was recorded this year (BBC News, 2016).

Figure 3: The use of MCB to prevent coral bleaching (Source: Anderson, 2013).

In conclusion, the MCB technique shares similarities with sulphate aerosol methods: in both climatic impacts and ecological concerns. The abundance of sea salt, coupled with potential global impacts would make it a relatively cheap, efficient and straightforward technique to implement. However, current models are contradictory and it is difficult to predict the spatial heterogeneity of impacts. Although MCB techniques could create localised ocean cooling, the ocean is essentially a 'global conveyor belt' that needs to be thought of in a holistic sense: as localised changes can have unpredictable global consequences. I feel more research needs to be undertaken to understand the implications of MCB techniques on all spatial scales, coupled with potential localised experiments, due to uncertainty with modelling.