Cirrus cloud thinning (CCT) is a proposed form of climate engineering. Cirrus clouds are high cold ice that, like other clouds, both reflect sunlight and absorb warming infrared radiation. However, they differ from other types of clouds in that, on average, infrared absorption outweighs sunlight reflection, resulting in a net warming effect on the climate. [1] Therefore, thinning or removing these clouds would reduce their heat trapping capacity, resulting in a cooling effect on Earth's climate. This could be a potential tool to reduce anthropogenic global warming. [2] Cirrus cloud thinning is an alternative category of climate engineering, in addition to solar radiation management and greenhouse gas removal.
In 2021 the IPCC described CCT as a proposal "to reduce the amount of cirrus clouds by injecting ice nucleating substances in the upper troposphere." However it reported low confidence in the cooling effect of CCT, due to limited understanding of cirrus microphysics, its interaction with aerosols, and the complexity of seeding strategy. CCT may also increase global precipitation. [3]
Typical cirrus clouds may be susceptible to modification to reduce their lifetime and optical thickness, and hence their net positive radiative forcing (in contrast to the typical low, warm liquid clouds). Material to seed such modification could be delivered via drones or by aircraft. Scientists believe that cirrus clouds in the high latitude upper troposphere are formed by homogeneous freezing, resulting in large numbers of small ice crystals. If effective ice nuclei were introduced into this environment, the cirrus may instead form by heterogeneous freezing. If the concentration of ice nuclei is seeded such that the resulting cloud particle density is less than that for the natural case, the cloud particles should grow larger due to less water vapor competition and attain higher settling velocities. By seeding with aerosols, ice crystals could grow rapidly and deplete water vapor, suppress nucleation and any growth of ice crystals by homogeneous nucleation. The net effect would be a reduced optical thickness and a reduced cloud lifetime, allowing more infrared radiation to be emitted at the top of the atmosphere, as the ice particles sediment out. [4] Less upper tropospheric water vapor and infrared radiation in the atmosphere would consequently cool the climate.
Bismuth tri-iodide (BiI3) has been proposed as the seeding material, as it is effective as ice nuclei for temperatures colder than -10 °C, [2] non-toxic and relatively inexpensive compared to e.g. silver iodide. [5] The seeding aerosols would need to be added regularly, as it would sediment out along with the large ice crystals.
In contrast to solar radiation management techniques, which would be most effective during the day time at lower latitudes, cirrus cloud thinning would be most effective at high latitudes and high solar zenith angles, where the background concentrations of aerosol are low. [6]
The cloud-aerosol-climate interactions important for cirrus cloud thinning are not well understood. Factors that related to the heterogeneous freezing process are uncertain, as ice growth kinetics are not well documented. Vertical velocities are essential for the activation of ice nuclei, but remain uncertain due to lack of observations. Heterogeneous freezing may already be common in cirrus, [7] which could limit the cooling potential of the technique. There are significant uncertainties associated with not only ice nucleation processes in cirrus clouds and the fraction of nucleation that occurs via heterogeneous and homogeneous freezing, but also its representation in climate models. “Over-seeding” might lead to warming, as opposed to the desired cooling. [8] Several studies assess the potential and viability of cirrus cloud thinning and the effectiveness of the technique remains a subject of debate. [9] [10] [11]
Due to the lack of realistic representation of ice crystal nucleation in Earth system models, some studies have used a simplified representation of cirrus cloud thinning by increasing the terminal velocity of ice crystals below the homogeneous freezing threshold of about -38 °C. [12] [13] [14] [15] [16]
Cirrus cloud formation may be effected by secondary organic aerosols, i.e. particles produced by natural plant life. [17] [18]
Some modelling of cirrus cloud seeding indicates significant reductions in climate damage due to CO2 increase. [19]
Cirrus is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto rocky or metallic dust particles at high altitudes. Globally, they form anywhere between 4,000 and 20,000 meters above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions.
Cloud albedo is a measure of the albedo or reflectivity of a cloud. Clouds regulate the amount of solar radiation absorbed by a planet and its solar surface irradiance. Generally, increased cloud cover correlates to a higher albedo and a lower absorption of solar energy. Cloud albedo strongly influences the Earth's energy budget, accounting for approximately half of Earth's albedo. Cloud albedo depends on the total mass of water, the size and shape of the droplets or particles and their distribution in space. Thick clouds reflect a large amount of incoming solar radiation, translating to a high albedo. Thin clouds tend to transmit more solar radiation and, therefore, have a low albedo. Changes in cloud albedo caused by variations in cloud properties have a significant effect on global climate.
Cloud feedback is a type of climate change feedback that has been difficult to quantify in contemporary climate models. It can affect the magnitude of internally generated climate variability or they can affect the magnitude of climate change resulting from external radiative forcings. Cloud representations vary among global climate models, and small changes in cloud cover have a large impact on the climate.
Contrails or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. They are composed primarily of water, in the form of ice crystals. The combination of water vapor in aircraft engine exhaust and the low ambient temperatures at high altitudes causes the trails' formation. Impurities in the engine exhaust from the fuel, including sulfur compounds provide some of the particles that serve as nucleation sites for water droplet growth in the exhaust. If water droplets form, they can freeze to form ice particles that compose a contrail. Their formation can also be triggered by changes in air pressure in wingtip vortices, or in the air over the entire wing surface. Contrails, and other clouds caused directly by human activity, are called homogenitus.
Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.
Ice crystals are solid ice in symmetrical shapes including hexagonal columns, hexagonal plates, and dendritic crystals. Ice crystals are responsible for various atmospheric optic displays and cloud formations.
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid below its freezing point without it becoming a solid. It is achieved in the absence of a seed crystal or nucleus around which a crystal structure can form. The supercooling of water can be achieved without any special techniques other than chemical demineralization, down to −48.3 °C (−54.9 °F). Supercooled water can occur naturally, for example in the atmosphere, animals or plants.
Global dimming was the name given to a decline in the amount of sunlight reaching the Earth's surface, a measure also known as global direct solar irradiance. It was observed soon after the first systematic measurements of solar irradiance began in the 1950s, and continued until 1980s, with an observed reduction of 4–5% per decade, even though solar activity did not vary more than the usual at the time. Instead, global dimming had been attributed to an increase in atmospheric particulate matter, predominantly sulfate aerosols, as the result of rapidly growing air pollution due to post-war industrialization. After 1980s, global dimming started to reverse, alongside reductions in particulate emissions, in what has been described as global brightening, although this reversal is only considered "partial" for now. This reversal has also been globally uneven, as some of the brightening over the developed countries in the 1980s and 1990s had been counteracted by the increased dimming from the industrialization of the developing countries and the expansion of the global shipping industry, although they have also been making rapid progress in cleaning up air pollution in the recent years.
Nephology is the study of clouds and cloud formation. British meteorologist Luke Howard was a major researcher within this field, establishing a cloud classification system. While this branch of meteorology still exists today, the term nephology, or nephologist is rarely used. The term came into use at the end of the nineteenth century, and fell out of common use by the middle of the twentieth. Recently, interest in nephology has surged as many meteorologists have begun to focus on the relationship between clouds and global warming which is a major source of uncertainty regarding "...estimates and interpretations of the Earth’s changing energy budget."
Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This can affect the radiative properties of clouds and the overall atmosphere. Water requires a non-gaseous surface to make the transition from a vapour to a liquid; this process is called condensation.
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both, along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.
Planetary engineering is the development and application of technology for the purpose of influencing the environment of a planet. Planetary engineering encompasses a variety of methods such as terraforming, seeding, and geoengineering.
In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears. For example, if a volume of water is cooled below 0 °C, it will tend to freeze into ice, but volumes of water cooled only a few degrees below 0 °C often stay completely free of ice for long periods (supercooling). At these conditions, nucleation of ice is either slow or does not occur at all. However, at lower temperatures nucleation is fast, and ice crystals appear after little or no delay.
Cosmics Leaving Outdoor Droplets (CLOUD) is an experiment being run at CERN by a group of researchers led by Jasper Kirkby to investigate the microphysics between galactic cosmic rays (GCRs) and aerosols under controlled conditions. This is a fixed-target experiment that began operation in November 2009, though it was originally proposed in 2000.
Solar geoengineering, or solar radiation modification (SRM), is a type of climate engineering in which sunlight would be reflected back to outer space to limit or offset human-caused climate change. There are multiple potential approaches, with stratospheric aerosol injection (SAI) being the most-studied method, followed by marine cloud brightening (MCB). Other methods have been proposed, including a variety of space-based approaches, but they are generally considered less viable, and are not taken seriously by the Intergovernmental Panel on Climate Change. SRM methods could have a rapid cooling effect on atmospheric temperature, but if the intervention were to suddenly stop for any reason, the cooling would soon stop as well. It is estimated that the cooling impact from SAI would cease 1–3 years after the last aerosol injection, while the impact from marine cloud brightening would disappear in just 10 days. Contrastingly, once any carbon dioxide is added to the atmosphere and not removed, its warming impact does not decrease for a century, and some of it will persist for hundreds to thousands of years. As such, solar geoengineering is not a substitute for reducing greenhouse gas emissions but would act as a temporary measure to limit warming while emissions of greenhouse gases are reduced and carbon dioxide is removed.
Marine cloud brightening also known as marine cloud seeding and marine cloud engineering is a proposed solar radiation management climate engineering technique that would make clouds brighter, reflecting a small fraction of incoming sunlight back into space in order to offset anthropogenic global warming. Along with stratospheric aerosol injection, it is one of the two solar radiation management methods that may most feasibly have a substantial climate impact. The intention is that increasing the Earth's albedo, in combination with greenhouse gas emissions reduction, carbon dioxide removal, and adaptation, would reduce climate change and its risks to people and the environment. If implemented, the cooling effect is expected to be felt rapidly and to be reversible on fairly short time scales. However, technical barriers remain to large-scale marine cloud brightening. There are also risks with such modification of complex climate systems.
Stratospheric aerosol injection is a proposed method of solar geoengineering to reduce global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming and increased albedo, which occurs naturally from volcanic winter. It appears that stratospheric aerosol injection, at a moderate intensity, could counter most changes to temperature and precipitation, take effect rapidly, have low direct implementation costs, and be reversible in its direct climatic effects. The Intergovernmental Panel on Climate Change concludes that it "is the most-researched [solar geoengineering] method, with high agreement that it could limit warming to below 1.5 °C (2.7 °F)." However, like other solar geoengineering approaches, stratospheric aerosol injection would do so imperfectly and other effects are possible, particularly if used in a suboptimal manner.
An ice nucleus, also known as an ice nucleating particle (INP), is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere.
Joyce Penner is an atmospheric scientist known for her research on climate change, especially on the impact of aerosols and clouds.
Ulrike Lohmann is a climate researcher and professor for atmospheric physics at the ETH Zurich. She is known for her research on aerosol particles in clouds.