The Problem
Human emissions of gasses such as carbon dioxide absorb radiation emitted by the earth’s surface essentially acting like a blanket that keeps in heat. The difference between the amount of radiation coming into and out of the atmosphere is known as radiative forcing (RF). For example, if the energy that is going out is greater than that which is coming in this is known as negative RF and as such the planet will cool(1),(2). Ideally, a decrease in the greenhouse gasses in the atmosphere would cause the temperatures to return to the pre-industrial era. As this may not be possible it will be important to try to mitigate the risks that come with rising temperatures such as solar radiation management (SRM).
What is MCB and why might it help?
Seeding clouds refers to artificially adding nuclei to the atmosphere so that vapour condenses around them. Marine Cloud Brightening (MCB) seeds certain clouds using seawater as the condensation nuclei, which may increase the concentration of cloud droplets. The reflectivity and possibly longevity of the clouds would then increase, essentially brightening the clouds. The idea is to use this reflection to attempt to produce a negative RF and thus a cooling effect on the planet by reflecting sunlight out of the atmosphere(3). The nuclei that aren’t used as condensation nuclei are also seen in some experiments to substantially add to this effect as they also scatter radiation outside of the cloud. This would be done to attempt to prevent the negative effects of warming such as the polar ice melting.
Some key advantages to MCB are that if there were negative consequences, the process can be terminated quickly and there is a degree of flexibility in which areas to seed with clouds. This localisation might be able to be used to weaken hurricanes or protect coral reefs by cooling certain areas of water. It is also possible that if the seeding could produce fewer, bigger drops, creating a warming effect could be useful in increasing rainfall in areas where it is required and become even more flexible with heat and air flows(8).
Results of recent experimentation and simulations
The southeast Atlantic shipping corridor can help determine how MCB would affect the earth energy balance as the winds there effectively contain the pollution from the ships which essentially uses the same principle of seeding and causes an increase in cloud brightness(4). A recent study calculated that within this corridor there’s been a significant reduction in RF.
There is a lot of uncertainty involved in the increase of condensation nuclei within clouds as there are many processes that can take place and thus simulations have achieved varying results. In one simulation, a warming effect was predicted and in other simulations, the results are largely dependent on meteorological conditions or the structure of the clouds.
However, in many studies, a cooling effect is observed, but some with unintended effects such as a significant drying or increases in the rain in certain areas. A study using multiple models to obtain a comparison found an average reduction in forcing that would account for most of the human increase from 1750(5).
The simulations from a recent study(6) have predicted that from 2030 to 2059, implementation of MCB is predicted to decrease the mean annual hottest days and coolest nights by 0.4–1.7°C and 0.3–2.1°C, respectively, for most of the Sahara-Sahel-Arabian Peninsula zone(6). The study claims that it appears increasingly likely that human society will face a future where the alternatives are between catastrophic climate change and the MCB scheme. (2)
Challenges & current problems
There are several problems for which there are currently no full solutions; such as there are no fully established designs for some of the tools necessary for condensation nuclei production. Related to this issue is the fact that there is a level of uncertainty as to whether the nuclei will be small enough to scatter at the right wavelengths and the possibility of the nuclei causing a decrease in ozone.(7) It couldn’t be deployed unless there was a comprehensive examination to assess its risks, an international body would have to be set up and investigate themselves to determine it being safe enough. Another problem is that if a large amount of the working MCB devices stopped working there would be a large rise in temperature that could have disastrous effects. Thus, the designs of the system need to be developed to prevent clogging and allow for a consistent flow.
Other Solar Radiation Management approaches
Stratospheric sulphur injection is a similar method to MCB as it utilises releasing sulphur species into the atmosphere. It is currently considered to be one of the most feasible schemes because previous volcanic eruptions such as Pinatubo in 1991 have provided significant data against which model predictions. However, they are more difficult to test as they typically last a lot longer than MCB so MCB can be tested much easier in a limited area and again there is a risk of ozone depletion.
Another SRM is called Ocean albedo modification (OAM) and basically aims to increase the reflectivity of oceans using microbubbles to cause a negative forcing and cool the planet. Comparatively, OAM has a larger effect on precipitation and evaporation than MCB. MCB appears to cause more of a negative RF and as such would likely be cooler. (8)
[4] Diamond, M. S., Director, H. M., Eastman, R., Possner, A., & Wood, R.(2020). Substantial Cloud Brighteningfrom Shipping in Subtropical LowClouds. AGU Advances, 1,e2019AV000111. [cited 5 August 2021]. Available from: https://doi.org/10.1029/2019AV000111
[5] IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. [cited 5 August 2021]. Available from: http://www.climatechange2013.org/images/report/WG1AR5_SPM_FINAL.pdf,
[6] Stjern, C. W., Muri, H., Ahlm, L., Boucher, O., Cole, J. N. S., Ji, D., Jones, A., Haywood, J., Kravitz, B., Lenton, A., Moore, J. C., Niemeier, U., Phipps, S. J., Schmidt, H., Watanabe, S., and Kristjánsson, J. E.: Response to marine cloud brightening in a multi-model ensemble, Atmos. Chem. Phys., 18, 621–634, [cited 5 August 2021]. Available from: https://doi.org/10.5194/acp-18-621-2018.
[7] Zhu, Y., Zhang, Z. and Crabbe, M.J.C. (2021), “Extreme climate response to marine cloud brightening in the arid Sahara-Sahel-Arabian Peninsula zone”, International Journal of Climate Change Strategies and Management, Vol. ahead-of-print No. ahead-of-print. [cited 5 August 2021]. Available from: https://doi.org/10.1108/IJCCSM-06-2020-0051
[8] Horowitz, H. M., Holmes, C., Wright,A., Sherwen, T., Wang, X., Evans, M.,et al. (2020). Effects of sea salt aerosol emissions for marine cloud brightening on atmospheric chemistry: Implications for radiative forcing. GeophysicalResearch Letters, 47, e2019GL085838. [cited 5 August 2021]. Available from: https://doi.org/10.1029/2019GL085838
[9] Zhao, M., Cao, L., Duan, L., Bala, G., and Caldeira, K.: Comparison of climate response to marine cloud brightening and ocean albedo modification: A model study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1877, https://doi.org/10.5194/egusphere-egu21-1877, 2021. [cited 5 August 2021]. Available from: https://ui.adsabs.harvard.edu/abs/2021EGUGA..23.1877Z/abstract