Article
Cirrus Cloud Thinning
Cirrus cloud thinning (CCT) would cool the Earth by thinning heat-trapping cirrus clouds. Currently, scientists are not certain that this strategy would work at all, and there are no practical ideas to implement it. This article explores the potential advantages of this idea and the challenges it faces.
Key takeaways
- Cirrus clouds are thin, wispy clouds made of ice that trap a lot of heat.
- Adding the right particles in the right places might thin cirrus clouds allowing more heat to escape to space.
- The understanding of cirrus cloud thinning is limited, and there are no concrete ideas for how to do it.
CCT is an idea to decrease the lifetime of some cirrus clouds to allow more heat to escape to space. CCT would work by seeding regions where cirrus clouds would have formed without seed particles. In principle, seeding would produce thinner, shorter-lived cirrus clouds, reducing their warming effect and cooling the planet.1 There are uncertainties in the feasibility and risks of this cooling strategy, and there are currently no practical ideas for its implementation.
The climate is changing due to the build-up of greenhouse gases, especially carbon dioxide, in the atmosphere, primarily from the burning of fossil fuels. These greenhouse gases warm the planet by preventing heat from escaping to space.
Most sunlight reflection methods, e.g., stratospheric aerosol injection and marine cloud brightening, would counteract the warming effects of greenhouse gases by increasing the amount of sunlight reflected to space.
In contrast, CCT would lower temperatures by increasing the amount of heat escaping to space. This cooling mechanism would more directly offset the heat-trapping effect of elevated greenhouse gas concentrations.1
Researchers have hypothesised that this approach to cooling might be preferable as it might counteract some of the changes in rainfall associated with the heat-trapping effect of greenhouse gases.2
Seeding cirrus clouds to shorten their lifespan
All clouds reflect some sunlight back to space and prevent some heat from leaving the atmosphere. Cirrus clouds are wispy, high clouds that form from ice at very cold temperatures (below −35°C), and trap heat more effectively than they reflect sunlight, leading to a net warming effect.3
Since cirrus clouds trap heat and reflect some sunlight, their heating effect is greatest in darker places. This means CCT would be most effective at darker times of the year and at high latitudes. Limiting CCT to high-latitude clouds during winter would maximise its cooling effect.5
CCT would only work on certain cirrus clouds. Some cirrus clouds form when ice crystals grow around “seed” particles, such as dust or other solid particles suspended in the air. These cirrus clouds form at higher temperatures and lower altitudes, and they are relatively thin and short-lived.6 CCT would not work on these cirrus clouds.7
Other cirrus clouds form without seed particles. Without these seed particles, the air must get much colder to start forming ice crystals. However, when this process does occur, smaller ice crystals form, leading to thicker, longer-lived cirrus clouds. As a result, unseeded cirrus clouds are thicker and more effective at trapping heat.8
CCT would target these unseeded cirrus clouds, adding tiny particles of silver iodide or bismuth tri-iodide which would stimulate the formation of ice crystals.7 Just as in the naturally seeded clouds, these modified cirrus clouds would form fewer, larger ice crystals, leading to thinner, shorter-lived cirrus clouds, allowing more heat to escape to space.
Substantial uncertainties around CCT’s effectiveness and consequences
Climate model studies suggest that, at best, cooling from CCT might offset most of the warming seen to date, but not more,9 though the cooling would be patchy, stronger in some places and weaker in others.5 However, there are substantial uncertainties, and if done wrong, it may even cause warming.10
Researchers are unsure what fraction of cirrus clouds form with and without seed particles in today’s climate.11 This fundamental uncertainty makes it difficult to know how effective CCT could be, which regions should be seeded, and how much material is needed.
Adding particles to the wrong places or adding too many particles – “overseeding” – could cause cirrus clouds to form in places they would not have or could make them thicker. This would be counterproductive, producing warming, rather than cooling.12
Even if it works, CCT would produce an uneven cooling effect. This would lead to shifts in rainfall patterns and could cause increases in extreme rainfall in some regions and increases in droughts in others.13,14
It is unclear if CCT would work
Cirrus clouds have a net warming effect, and CCT may offer a way to reduce that warming effect. It is unique among sunlight reflection methods15 in that it would decrease the amount of heat the Earth’s atmosphere traps, more closely offsetting the warming effect of greenhouse gases.
All studies of CCT to date have been idealised climate modelling studies, providing insights into cirrus cloud formation and processes.1 There are currently no detailed engineering ideas for how to achieve CCT in practice, nor have there been any field tests.
Whether and how CCT could be implemented has yet to be studied in detail, so it is not clear if this idea would work in practice.
Open questions
- What fraction of cirrus clouds currently form without seed particles?
- Would seeding cirrus clouds cool the planet as suggested by some climate models?
- Can a feasible approach for seeding cirrus clouds, using aircraft or drones, be developed?
Ask us a question!
Endnotes
- Lohman U, Gasparini B. (2017). A cirrus cloud climate dial. Science. American Association for the Advancement of Science. https://doi.org/10.1126/science.aan3325
- Gasparini B, McGraw Z, Storelvmo T, et al. (2020). To what extent can cirrus cloud seeding counteract global warming? Environmental Research Letters, 15(5). https://doi.org/10.1088/1748-9326/ab71a3
- Kärcher B. (2017). Cirrus Clouds and Their Response to Anthropogenic Activities. Current Climate Change Reports. Springer. https://doi.org/10.1007/s40641-017-0060-3
- Contains public sector information licensed under the Open Government Licence v3.0
- Storelvmo T, Herger N. (2014). Cirrus cloud susceptibility to the injection of ice nuclei in the upper troposphere. Journal of Geophysical Research, 119(5), 2375–2389. https://doi.org/10.1002/2013JD020816
- Heymsfield AJ, Krämer M, Luebke A, et al. (2017). Cirrus Clouds. Meteorological Monographs, 58, 2.1-2.26. https://doi.org/10.1175/amsmonographs-d-16-0010.1
- Mitchell DL, Finnegan W. (2009). Modification of cirrus clouds to reduce global warming. Environmental Research Letters, 4(4). https://doi.org/10.1088/1748-9326/4/4/045102
- Gruber S, Blahak U, Haenel F, et al. (2019). A Process Study on Thinning of Arctic Winter Cirrus Clouds With High-Resolution ICON-ART Simulations. Journal of Geophysical Research: Atmospheres, 124(11), 5860–5888. https://doi.org/10.1029/2018JD029815
- Lohman & Gasparini (2017)1 noted that CCT could offset about half the warming induced by a doubling of carbon dioxide since pre-industrial times or a radiative forcing of about 2 to 3 W m−2. According to the Intergovernmental Panel on Climate Change, greenhouse gases have exerted a radiative forcing of about 2.72 W m−2 over pre-industrial values.16 Radiative forcing is a measure of how a factor influences Earth’s energy balance, indicating its warming or cooling effect on the planet.
- Tully C, Neubauer D, Villanueva D, et al. (2023). Does prognostic seeding along flight tracks produce the desired effects of cirrus cloud thinning? Atmospheric Chemistry and Physics, 23(13), 7673–7698. https://doi.org/10.5194/acp-23-7673-2023
- Gasparini B, Sullivan SC, Sokol AB, et al. (2023). Opinion: Tropical cirrus – from micro-scale processes to climate-scale impacts. Atmospheric Chemistry and Physics, 23(24), 15413–15444. https://doi.org/10.5194/acp-23-15413-2023
- Tully C, Neubauer D, Omanovic N, et al. (2022). Cirrus cloud thinning using a more physically based ice microphysics scheme in the ECHAM-HAM general circulation model. Atmospheric Chemistry and Physics, 22(17), 11455–11484. https://doi.org/10.5194/acp-22-11455-2022
- Kristjánsson JE, Muri H, Schmidt H. (2015). The hydrological cycle response to cirrus cloud thinning. Geophysical Research Letters, 42(24), 10807–10815. https://doi.org/10.1002/2015GL066795
- Ricke K, Wan JS, Saenger M, et al. (2023). Hydrological Consequences of Solar Geoengineering. Rev. Earth Planet. Sci. 2023, 51, 447–70. https://doi.org/10.1146/annurev-earth-031920-083456
- CCT does not produce its cooling effect by increasing the reflection of light and so it is not strictly a sunlight reflection method. However, as it raises similar issues to other sunlight reflection methods, it is typically grouped with them.
- Intergovernmental Panel on Climate Change (IPCC). (2023). Technical Summary. In Climate Change 2021 – The Physical Science Basis (pp. 35–144). Cambridge University Press. https://doi.org/10.1017/9781009157896.002
Citation
Reuse this work freely
All visualisations, data, and code produced by SRM360 are open access under the Creative Commons BY license. You are free to use, distribute, and reproduce these in any medium, provided that SRM360 and the authors are credited.
Data produced by third parties and made available by SRM360 is subject to the licence terms of the original third-party authors. We will always indicate the original source of such data in our documentation, so please review the licence of any third-party data before use and redistribution.