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| Fig. 1: Schematic of stratospheric aerosol injection deployed by tethered balloon (Source: Wikimedia Commons ) |
It is becoming increasingly likely that preventing anthropogenic global warming of 2°C by the year 2100 will require interventions beyond emissions mitigation. Solar radiation management (SRM) strategies have been proposed which aim to cool the earth by reducing the amount of solar energy that reaches the earth surface. The SRM technology that has received the most attention is stratospheric aerosol injection (SAI) which involves injecting reflective aerosols into the stratosphere to induce cooling via increased atmospheric albedo. In this way, SAI would mimic the cooling effect observed after major volcanic eruptions whereby large amounts of sulphates are emitted into the atmosphere. Most SAI studies are based on sulfate aerosols, though several other materials have been proposed including alumina, calcium carbonate, dust and diamond. Typical estimates for the mass of aerosols required are in the range of 1-10 million tonnes (Mt) to be deployed annually at an altitude of roughly 20 km. [1] The most common proposed methods of deployment of the aerosols include aircraft, airships, or tethered balloons as depicted in Fig 1.
Proponents of SAI argue that it is an inexpensive technology relative to emissions mitigation efforts that would produce the same amount of cooling. Depending on the scenario considered, reported cost estimates vary widely. One early study suggested that SAI could be conducted at an annual cost of $0.2 billion per year per W/m2 of cooling. [2] With current anthropogenic CO2 radiative forcing estimated to be ~2W/m2, this equates to ∼ $0.4 billion annually to offset current anthropogenic warming by SAI. [2] On the other hand, more recent studies have estimated annual costs ranging from $7 billion to $90 billion depending on the cooling target, deployment method, and future emissions mitigation efforts, [1,3] For comparison, meeting global climate goals through emission mitigation efforts alone are estimated to cost 0.2-2.5% of 2030 global GDP, equivalent to about $200 billion to $2 trillion per year. [4]
Although the direct cost of SAI may seem relatively inexpensive, widespread support for SAI is lagging due to a host of climatological and ecological uncertainties associated with deploying ∼10 Mt of sulfate aerosols into the atmosphere every year. Much of the research towards understanding these risks relies on climate data following major volcanic eruptions. Following the 1991 Mount Pinatubo eruption, significant reductions in rainfall over land were observed with a corresponding decrease in river discharge. As a result, studies suggest that SAI could adversely impact the global hydrological cycle thus potentially altering the world's food supply. [2] In this way, SAI could potentially exacerbate some of the worst effects of climate change. In addition, SAI is expected to lead to ozone depletion (at least temporarily) and it would not resolve the problem of ocean acidification caused by rising atmospheric CO2 concentrations. [1] Another uncertainty is the reversibility of SAI once deployed if adverse climate affects occur. These drawbacks along with the ethical dilemma of deliberately altering the climate for over 7 billion people will likely prevent serious considerations of SAI deployment in the near future.
© Eli Mueller. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. Moriyama et al., "The Cost of Stratospheric Climate Engineering Revisited." Mitig. Adapt. Strateg. 22, 1207 (2017).
[2] "Geoengineering the Climate: Science, Governance and Uncertainty," Royal Society, September 2009.
[3] W. Smith, "The Cost of Stratospheric Aerosol Injection Rhrough 2100," Environ. Res. Lett. 15, 114004 (2020).
[4] J. McClellan, D.W. Keith and J. Apt, "Cost Analysis of Stratospheric Albedo Modification Delivery Systems." Environ. Res. Lett. 7, 034019 (2012).
