A Review of Methods:
A range of studies have resorted to SRM as a 'last resort' or an 'emergency geoengineering measure' due to the large amount of uncertainty surrounding the method. However, the option of SRM is fairly attractive when looking at the rapid cooling capabilities provided. The table below by Caldeira et al, 2013 highlights space-based schemes and stratospheric aerosols as the most effective proposed methods. When assessing all schemes, stratospheric aerosol pumping highlights the most potential due to its quick deployment, high potential, low cost and medium risk. Therefore, this post will concentrate predominantly on this method.
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| A summary of SRM strategies assessed by potential, deployment speed, cost and risk (Source: Caldeira et al, 2013) |
It must also be noted that all SRM approaches have no effect on the carbon dioxide levels already present in the atmosphere. Their sole purpose is to reduce global temperatures. Therefore, if implementation of such management were to occur, it would have to be done with CDR schemes working to also reduce atmospheric CO2 levels.
Is SRM a viable management strategy?
A range of issues have been highlighted surrounding aerosol pumping but with these, scientists have produced counter arguments. I will attempt to review a few of these here.
It has been suggested that the sulphur particles ejected into the atmosphere can contribute to depletion of the stratospheric ozone layer, leading to further solar radiation. However, Calderia et al. (2013) highlight that this is only a short term issue. He highlighted how this effect will diminish over time as fewer chloroflourocarbons become capable of reaching the stratosphere. Other authors have suggested that SRM will have a negative effect on solar energy efficiency, due to decreased solar radiation (Robock, 2008). However, Calderia believes that SRM will in fact scatter incoming sunlight providing a greater fraction of diffuse light. This is expected to expose more parts of individual plants to light, leading to more global photosynthesis and greater CO2 absorbance. General arguments against SRM also raise concerns over the side effect of increased acid rain from sulphur additions. However, through simulation models Kravitz et al (2009) found that on a global scale, the additional acid rain caused is likely to be relatively small with modest consequences. In addition, managing incoming solar radiation has been linked to a global increase in biomass due to the CO2 fertilisation effect where plant photosynthesis increases with CO2 concentrations. If SRM successfully maintained the global temperatures we have today well into the future, Pongratz et al (2012) indicated that CO2 levels will continue rising and once reaching 800ppm (compared to 400ppm today), crop yields will be 8-21% greater than current yields, dependent on the crop. The reason for this is a reduced temperature stress on plants associated with higher CO2 concentrations with the continued effect of CO2 fertilisation. Thus allowing plants to photosynthesis more, without being affected by increased temperatures. Despite this, 800ppm CO2 concentrations will lead to extensive ocean acidification and a range of health issues, making this scenario not only unlikely, but dangerous too.
Importantly, it must also be noted that SRM schemes have no positive effect on ocean acidification (Matthews et al, 2009). Furthermore, this method of management is taken unilaterally across the globe, introducing issues surrounding governance and matters of who decides when and if such a scheme should take place (Barrett, 2014). Rather than acting as a means to prevent climate conflict, it could act a source to stimulate international conflict. However, the potential for SRM in reducing global temperatures to prolong the time required to reduce carbon dioxide levels through emission reductions looks attractive.
Thinking into the future:
Imagine yourself in 2100, living in rural India where climate change has led to an array of monsoons, flooding, vastly reduced crop yields and death and famine to a large number of friends and family. The country is beginning to slip into a dire situation and food insecurity is rapidly increasing. Agreed emissions reductions across the globe have been unsuccessful and future targets seem extremely unrealistic. However, in the midst of this crisis, there remains talk of one, potentially risky management strategy - Solar Radiation Management. You, as well as the general public see no other option and democracy begins to speak. Governments are pressured into implementing this emergency solution. Nationwide aerosol spraying begins despite countries across the globe strongly opposing the idea. Neighbouring countries begin to protest and before long, this will be an issue of global concern as the atmosphere is shared by all. Either a peaceful resolution is sought or conflict arises.
What should we do?
Further research on SRM is required through small, controlled experiments. Once better understood, any use of it should be restricted to fine-tuned injections to manage global issues such as melting of the Greenland ice sheets. Spraying should be at low concentrations and done following natural seasons. Over time, the side effects will be managed as all use is small scale. Furthermore, with time the strategy is very likely to improve in efficiency and become more controllable, a belief shared by (Morenzo-Cruz & Keith, 2013). In this scenario, countries that are hit by climate crisis in the future will be more aware of side effects and a global SRM organisation would have already been set up, providing them with the best course of action. By no means should SRM provide a justification for further CO2 emissions, it can simply be used to prevent overcoming tipping points, providing time for CRD technologies to become affordable and efficient and allow reduction targets to be implemented and met. Only when SRM is used in this context, will it be a justifiable management strategy.
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