Introduction

Geoengineering is the large-scale intervention in the Earth’s climate system by limiting incoming solar radiation or removing carbon dioxide from the atmosphere. To limit incoming solar radiation, the reflectivity or albedo of the Earth’s surface or atmosphere can be increased. A variety of geoengineering techniques have been proposed and analyzed (see Figure 1).

Figure 1: A diagram illustrating multiple albedo-enhancing solar radiation management techniques

Overview of Techniques

Stratospheric Aerosol Injection

Stratospheric aerosol involves injecting aerosols, usually sulfur dioxide, into the stratosphere to reflect incoming solar radiation before it reaches the Earth’s surface (Crutzen, 2006). This technique can be deployed relatively fast but is expensive, usually costing about 18 billion USD annually (Barrett, 2008).

Marine Cloud Brightening

Since oceans have a very low albedo, increasing the size and brightness of clouds that cover them would be very beneficial to increase the amount of reflected solar radiation (Seitz, 2011). Clouds undergo this change when aerosols, like sea water mist, interact with them, increasing their number of water droplets (Wood and Ackerman, 2013; Latham et al., 2012).

Seawater Microbubbles

Air bubbles in the micrometre range can be added into oceans to increase their reflectivity, which can reduce global temperature by 3℃ (Seitz, 2011). Microbubbles have a short lifetime, which can be extended using insoluble gases (Johnson and Cooke, 1981; Schutt et al., 2003). This technique can be applied on a regional scale with limited energy supply (Seitz, 2011).

Crop Bio-Geoengineering

Some plant characteristics offer naturally higher reflectivity, such as leaves angled away from direct sunlight, decreased vegetative clumping and chlorophyll deficiency (Genesio et al., 2020; Ni-Meister, Yang and Kiang, 2010; Otterman and Brakke, 1991). The presence of was and hairs on the leaves also enhance albedo (Gates et al., 1965; Grant et al., 2003; Febrero et al., 1998). Each of these features has potential to be augmented through selective breeding, genetic modification and artificial enhancement (Singarayer and Davies-Barnard, 2012).

Discussion of Technqiues

The use of aerosols requires the development of new technology to reach higher altitudes (Smith and Wagner, 2018). Marine cloud brightening has developed efficient nozzles (Taylor, 1964; Foster et al., 2020). Microbubbles can be produced using existing ships but further research is needed to extend microbubble lifetime (Crook, Jackson and Forster, 2016; Seitz, 2011). Crop bioengineering uses well-established methods but further research on plant genomes is needed (Ridgwell et al., 2009).

Each geoengineering technique has general environmental concerns associated with it (ie. altered precipitation patterns, heat distribution, atmospheric composition), but before they can be addressed scientists must confirm the exact effects through future research (Aziz et al., 2014; Ridgwell et al., 2009). Studies show that public awareness of geoengineering is very low but people who show concern for climate change often support it (Pidgeon et al., 2012).

Stratospheric aerosol injection is the most costly technique (18 billion USD/year) (Teller, Wood and Hyde, 1997). Cloud brightening cost is highly variable, but annual estimation is only 75-150 million USD (Salter, Sortino and Latham, 2008). Microbubble injection into oceans seems to only be economically worthwhile at the regional scale (Seitz, 2011). Existing large-scale cultivation puts crop bio-geoengineering in an ideal position for economic feasibility (Ridgwell et al., 2009).

Conclusion

The utilization of geoengineering techniques present a potential short-term solution to climate change. Each technique has its own associated benefits and risks, however determining which is optimal is premature at this point and more research must be done to isolate an optimal combination of techniques.

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