Cirrus Cloud Thinning
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Description and purpose of the technology
Cirrus Cloud Thinning (CCT) is a solar geoengineering proposal which aims to eliminate or thin cirrus clouds to allow heat to escape into space. Wispy, elongated “cirrus” clouds are found at high altitudes, and often absorb more incoming solar energy than they reflect, because they form in cold temperatures and consist of ice crystals. If the ice crystals are numerous and small, cirrus clouds prevent long-wave terrestrial radiation from escaping back into space, and have a climate impact similar to greenhouse gases. In the presence of natural ice-forming nuclei such as dust particles, the ice crystals that form are fewer and larger, have a shorter lifespan and trap less outgoing heat.
Proponents of CCT propose injecting ice-forming nuclei—such as bismuth triiodide or aerosols such as sulfuric or nitric acid—into the atmosphere at altitudes where cirrus clouds form. This, they say, would produce cirrus clouds with larger ice crystals and shorter life spans, while also reducing their optical depth, which would mean more long-wave terrestrial radiation would escape into space. [1]
Researchers admit that the injection of “too many” ice-nucleating particles into cirrus clouds may produce the opposite effect—more and thicker clouds that trap even more heat and which could lead to increased warming. [2] Other researchers underscore the risks of the unpredictable side effects of CCT, including large regional and seasonal changes to precipitation, and the differing consequences of seeding in the Southern or Northern Hemispheres. [3]
Actors involved
Studies on CCT have so far been limited to modelling exercises and analysis of cirrus cloud dynamics by research institutions. In 2006, the US-based Desert Research Institute started a five-year study to examine the concentration of small ice crystals in cirrus clouds, as well as the concentration of natural and anthropogenic aerosol concentrations in them, in order to provide more accurate data for global climate modelling studies.
The Geoengineering Model Intercomparison Project (GeoMIP) is an international collaboration between climate modelling centres, including research institutions in Canada, China, Denmark, Germany, Japan, Norway, UK and the USA, and is modelling various types of solar geoengineering, including CCT. A research group at the ETH Zurich Institute for Atmospheric and Climate Science has also simulated CCT with a global climate model, and is participating in joint research programs with other institutions, among them Arctic Winter Cirrus Thinning (AWiCiT), a German research project. AWiCiT has been modelling CCT over the Arctic in winter by seeding with ice nucleating particles, with the aim of reducing Arctic warming and slowing down Arctic ice melting.
Other studies on CCT are being conducted by researchers at the University of Leeds, UK, as well as at Zhejiang University, China, as part of China’s Geoengineering Programme. [4]
Impacts of the technology
As with all solar geoengineering techniques, CCT could have considerable impacts on regional climates. Its deployment could cause unintended changes in the hydrological cycle and atmospheric circulation, due to the fact that the climate system is complex and highly nonlinear in its behaviour. Perturbing one element of it in this way could lead to unforeseen changes with wide-ranging consequences. [6]
In simulations modelling CCT and continued carbon dioxide concentration increases, researchers have found that CCT may enhance the hydrological cycle and therefore increase precipitation overall. Although seeding is predicted to decrease the frequency of the most extreme precipitation globally, extreme precipitation events could occur more frequently in certain areas, including the Sahel and Central America, and the Indian monsoon could be strengthened. [5] This could have devastating impacts on millions of vulnerable people and the livelihoods of many communities.
Another potential problem with CCT is over-seeding, if too many nuclei were to be released, causing the formation of numerous additional ice crystals. As a consequence, the cirrus clouds would become optically thicker and less permeable to terrestrial radiation, leading to additional warming of the atmosphere. Models simulating “Cirrus Cloud Thickening” have shown that a weaker hydrological cycle would result, with effects comparable to doubling carbon dioxide concentrations in the atmosphere. [7] This would obviously cause serious harm to ecosystems and communities. Furthermore, the point at which over-seeding would occur is not known, adding considerable uncertainty to current models.
Similarly, seeding would need to be avoided in cloud-free regions with high relative humidity where no cirrus clouds form. In these areas, seeding could lead to cirrus cloud formation rather than thinning, resulting in a warming effect on the climate. These interconnected factors mean that CCT could either increase or decrease global temperatures. The influence of CCT on lower-lying clouds is also poorly understood, whereby CCT could enhance or dampen cloud reflectivity, allowing more or less heat to escape, which would likely result in additional climate impacts. [8]
Another significant concern is that CCT could be operated at a local and unilateral scale with the intention of creating climate responses in certain areas. This might be attractive to governments as it could theoretically be used to suppress some extreme weather events, such as heat waves. [9] Another example of small-scale deployment that has been proposed would be to avoid further melting of Arctic sea ice. [10] This kind of localised deployment could cause negative impacts on non-target neighbouring regions and communities, and potentially precipitate serious conflicts. Climatic events are unlikely to be contained—one country avoiding a heatwave could cause flooding in another. Or, rather than stopping Arctic ice melting, the technology could also be used to melt it completely in order to open up lucrative shipping routes.
Reality check
CCT is still a theoretical concept, and research into its effects is currently limited to climate modelling and based on assumptions that could be wrong. Researchers do not even know which substances would effectively seed cirrus clouds and which technological challenges may emerge. Recent studies found that none of the current cloud seeding strategies could achieve significant cooling through CCT, due to complex microphysical mechanisms that limit the climatic response, and due to large uncertainties in both cloud and surface climate responses. These more recent studies directly contradict previous findings that CCT could be an effective geoengineering option. [11]
Further reading
ETC Group and Heinrich Böll Foundation, “Geoengineering Map”, https://map.geoengineeringmonitor.org/
End notes
[1] Storelvmo, et al. (2013) Cirrus cloud seeding has potential to cool climate, in: Geophys. Res. Lett., Vol. 40(1): 178 – 182, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL054201; Reynolds (2019) Solar geoengineering to reduce climate change: a review of governance proposals, in: Proc. R. Soc. A, Vol. 475, https://royalsocietypublishing.org/doi/10.1098/rspa.2019.0255; ETC Group and Heinrich Böll Foundation (2020) Geoengineering Map, https://map.geoengineeringmonitor.org/
[2] Lohmann and Gasparini (2017) A cirrus cloud climate dial?, in: Science, Vol. 357(6347): 248 – 249, https://science.sciencemag.org/content/357/6348/248/tab-pdf
[3] Muri, et al. (2014) The climatic effect of modifying cirrus clouds in a climate engineering framework, in: Journal of Geophysical Research: Atmospheres, Vol. 119(7): 4174 – 4191, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JD021063
[4] ETC Group and Heinrich Böll Foundation (2020) Geoengineering Map, https://map.geoengineeringmonitor.org/
[5] Kristjánsson, et al. (2015) The hydrological cycle response to cirrus cloud thinning, in: Geophys. Res. Lett., Vol. 42(24):10,807 – 10,815, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL066795; Gasparini, et al. (2020) To what extent can cirrus cloud seeding counteract global warming?, in Environ. Res. Lett., accepted manuscript published online: January 30, 2020, https://iopscience.iop.org/article/10.1088/1748-9326/ab71a3
[6] Muri, et al. (2014)
[7] Kristjánsson, et al. (2015)
[8] Lohmann and Gasparini (2017)
[9] Quaas, et al. (2016) Regional climate engineering by radiation management: Prerequisites and prospects,” in: Earth’s Future, Vol. 4(12), 618 – 625, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016EF000440
[10] Lohmann and Gasparini (2017)
[11] Gasparini and Lohmann (2016), Why cirrus cloud seeding cannot substantially cool the planet, in: Journal of Geophysical Research: Atmospheres, Vol. 121(9), 4877 – 4893, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JD024666; ibid (Gasparini, et al. (2020))
Cirrus Cloud Thinning
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Descripción y propósito de la tecnología
Cirrus Cloud Thinning (CCT) is a solar geoengineering proposal which aims to eliminate or thin cirrus clouds to allow heat to escape into space. Wispy, elongated “cirrus” clouds are found at high altitudes, and often absorb more incoming solar energy than they reflect, because they form in cold temperatures and consist of ice crystals. If the ice crystals are numerous and small, cirrus clouds prevent long-wave terrestrial radiation from escaping back into space, and have a climate impact similar to greenhouse gases. In the presence of natural ice-forming nuclei such as dust particles, the ice crystals that form are fewer and larger, have a shorter lifespan and trap less outgoing heat.
Proponents of CCT propose injecting ice-forming nuclei—such as bismuth triiodide or aerosols such as sulfuric or nitric acid—into the atmosphere at altitudes where cirrus clouds form. This, they say, would produce cirrus clouds with larger ice crystals and shorter life spans, while also reducing their optical depth, which would mean more long-wave terrestrial radiation would escape into space. [1]
Researchers admit that the injection of “too many” ice-nucleating particles into cirrus clouds may produce the opposite effect—more and thicker clouds that trap even more heat and which could lead to increased warming. [2] Other researchers underscore the risks of the unpredictable side effects of CCT, including large regional and seasonal changes to precipitation, and the differing consequences of seeding in the Southern or Northern Hemispheres. [3]
Actores involucrados
Studies on CCT have so far been limited to modelling exercises and analysis of cirrus cloud dynamics by research institutions. In 2006, the US-based Desert Research Institute started a five-year study to examine the concentration of small ice crystals in cirrus clouds, as well as the concentration of natural and anthropogenic aerosol concentrations in them, in order to provide more accurate data for global climate modelling studies.
The Geoengineering Model Intercomparison Project (GeoMIP) is an international collaboration between climate modelling centres, including research institutions in Canada, China, Denmark, Germany, Japan, Norway, UK and the USA, and is modelling various types of solar geoengineering, including CCT. A research group at the ETH Zurich Institute for Atmospheric and Climate Science has also simulated CCT with a global climate model, and is participating in joint research programs with other institutions, among them Arctic Winter Cirrus Thinning (AWiCiT), a German research project. AWiCiT has been modelling CCT over the Arctic in winter by seeding with ice nucleating particles, with the aim of reducing Arctic warming and slowing down Arctic ice melting.
Other studies on CCT are being conducted by researchers at the University of Leeds, UK, as well as at Zhejiang University, China, as part of China’s Geoengineering Programme. [4]
Impactos de la tecnología
As with all solar geoengineering techniques, CCT could have considerable impacts on regional climates. Its deployment could cause unintended changes in the hydrological cycle and atmospheric circulation, due to the fact that the climate system is complex and highly nonlinear in its behaviour. Perturbing one element of it in this way could lead to unforeseen changes with wide-ranging consequences. [6]
In simulations modelling CCT and continued carbon dioxide concentration increases, researchers have found that CCT may enhance the hydrological cycle and therefore increase precipitation overall. Although seeding is predicted to decrease the frequency of the most extreme precipitation globally, extreme precipitation events could occur more frequently in certain areas, including the Sahel and Central America, and the Indian monsoon could be strengthened. [5] This could have devastating impacts on millions of vulnerable people and the livelihoods of many communities.
Another potential problem with CCT is over-seeding, if too many nuclei were to be released, causing the formation of numerous additional ice crystals. As a consequence, the cirrus clouds would become optically thicker and less permeable to terrestrial radiation, leading to additional warming of the atmosphere. Models simulating “Cirrus Cloud Thickening” have shown that a weaker hydrological cycle would result, with effects comparable to doubling carbon dioxide concentrations in the atmosphere. [7] This would obviously cause serious harm to ecosystems and communities. Furthermore, the point at which over-seeding would occur is not known, adding considerable uncertainty to current models.
Similarly, seeding would need to be avoided in cloud-free regions with high relative humidity where no cirrus clouds form. In these areas, seeding could lead to cirrus cloud formation rather than thinning, resulting in a warming effect on the climate. These interconnected factors mean that CCT could either increase or decrease global temperatures. The influence of CCT on lower-lying clouds is also poorly understood, whereby CCT could enhance or dampen cloud reflectivity, allowing more or less heat to escape, which would likely result in additional climate impacts. [8]
Another significant concern is that CCT could be operated at a local and unilateral scale with the intention of creating climate responses in certain areas. This might be attractive to governments as it could theoretically be used to suppress some extreme weather events, such as heat waves. [9] Another example of small-scale deployment that has been proposed would be to avoid further melting of Arctic sea ice. [10] This kind of localised deployment could cause negative impacts on non-target neighbouring regions and communities, and potentially precipitate serious conflicts. Climatic events are unlikely to be contained—one country avoiding a heatwave could cause flooding in another. Or, rather than stopping Arctic ice melting, the technology could also be used to melt it completely in order to open up lucrative shipping routes.
Visión realista
CCT is still a theoretical concept, and research into its effects is currently limited to climate modelling and based on assumptions that could be wrong. Researchers do not even know which substances would effectively seed cirrus clouds and which technological challenges may emerge. Recent studies found that none of the current cloud seeding strategies could achieve significant cooling through CCT, due to complex microphysical mechanisms that limit the climatic response, and due to large uncertainties in both cloud and surface climate responses. These more recent studies directly contradict previous findings that CCT could be an effective geoengineering option. [11]
Lectura complementaria
ETC Group and Heinrich Böll Foundation, “Geoengineering Map”, https://map.geoengineeringmonitor.org/
Notas finales
[1] Storelvmo, et al. (2013) Cirrus cloud seeding has potential to cool climate, in: Geophys. Res. Lett., Vol. 40(1): 178 – 182, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL054201; Reynolds (2019) Solar geoengineering to reduce climate change: a review of governance proposals, in: Proc. R. Soc. A, Vol. 475, https://royalsocietypublishing.org/doi/10.1098/rspa.2019.0255; ETC Group and Heinrich Böll Foundation (2020) Geoengineering Map, https://map.geoengineeringmonitor.org/
[2] Lohmann and Gasparini (2017) A cirrus cloud climate dial?, in: Science, Vol. 357(6347): 248 – 249, https://science.sciencemag.org/content/357/6348/248/tab-pdf
[3] Muri, et al. (2014) The climatic effect of modifying cirrus clouds in a climate engineering framework, in: Journal of Geophysical Research: Atmospheres, Vol. 119(7): 4174 – 4191, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JD021063
[4] ETC Group and Heinrich Böll Foundation (2020) Geoengineering Map, https://map.geoengineeringmonitor.org/
[5] Kristjánsson, et al. (2015) The hydrological cycle response to cirrus cloud thinning, in: Geophys. Res. Lett., Vol. 42(24):10,807 – 10,815, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL066795; Gasparini, et al. (2020) To what extent can cirrus cloud seeding counteract global warming?, in Environ. Res. Lett., accepted manuscript published online: January 30, 2020, https://iopscience.iop.org/article/10.1088/1748-9326/ab71a3
[6] Muri, et al. (2014)
[7] Kristjánsson, et al. (2015)
[8] Lohmann and Gasparini (2017)
[9] Quaas, et al. (2016) Regional climate engineering by radiation management: Prerequisites and prospects,” in: Earth’s Future, Vol. 4(12), 618 – 625, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016EF000440
[10] Lohmann and Gasparini (2017)
[11] Gasparini and Lohmann (2016), Why cirrus cloud seeding cannot substantially cool the planet, in: Journal of Geophysical Research: Atmospheres, Vol. 121(9), 4877 – 4893, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JD024666; ibid (Gasparini, et al. (2020))