Cloud Brightening and Climate Change

Transcription

1 Cloud Brightening and Climate Change 89 Hannele Korhonen and Antti-Ilari Partanen Contents Definitions Aerosols and Cloud Albedo Cloud Brightening with Sea-Salt Aerosol Climate Effects of Cloud Brightening Conclusions References Abstract Atmospheric greenhouse gas emissions show no declining trend, which has motivated ideas of deliberate climate engineering by modifying the reflectivity of the Earth. One suggested method is cloud brightening, in which artificial emissions of aerosol particles are used to enhance the reflectivity of clouds. While climate models suggest that cloud brightening might be able to offset at least some of the predicted global warming, large uncertainties remain related to the efficiency as well as to the environmental and regional climate impacts of the method. Keywords Climate engineering Geoengineering Solar radiation management Cloud whitening Aerosol Sea salt H. Korhonen (*) A.-I. Partanen Finnish Meteorological Institute, Kuopio, Finland Bill Freedman (ed.), Global Environmental Change, DOI / _50, # Springer Science+Business Media Dordrecht

2 778 H. Korhonen and A.-I. Partanen Definitions Cloud brightening is a climate engineering method which aims to decrease the global temperature by deliberately injecting sea spray aerosol particles into the atmosphere in regions with persistent marine stratocumulus clouds. Cloud condensation nucleus (CCN) is an atmospheric aerosol particle about which supersaturated water vapor can condense to form a cloud droplet. Cloud albedo effect (i.e., first indirect effect) of aerosol particles refers to the mechanism by which aerosols modify the cloud droplet concentration and hence the reflectivity of the clouds. Aerosols and Cloud Albedo Clouds are an important regulator of the Earth s energy balance. On a global scale, they block about one fifth of the solar radiation from reaching the planet s surface and at the same time also trap a large fraction of the thermal radiation emitted by the Earth s surface and prevent it from escaping into space. These two effects of the clouds have opposite effects on the surface temperature below them: the former prevents the absorption of some of the sun s energy at the surface and thus causes cooling, whereas the latter tends to warm the surface. (See chapter Global Climate Change, Introduction and Radiative Forcing and the Greenhouse Gases for more detailed discussion on clouds and radiation.) The amount of solar radiation that the clouds are capable of reflecting back into space depends strongly on the atmospheric conditions, for example, on humidity, temperature profile, and convection. However, for many types of clouds, it also depends on the number concentration of atmospheric aerosol particles, which can act as nuclei for cloud droplet formation. The increase in aerosol particles available as cloud condensation nuclei (CCN) typically increases the optical thickness (i.e., reflectivity) of the clouds. This is because when the same amount of cloud water is divided among more numerous droplets, the total surface area of the droplets increases causing higher reflectivity. This effect is known as the cloud albedo effect (or first indirect effect) of aerosol particles. As a rule of thumb, the change in cloud albedo is proportional to the relative change in cloud droplet concentration, i.e., the reflective properties of clouds which have a low initial droplet concentration can be changed with less additional droplets than the properties of clouds that have a high initial droplet concentration. Because of this, the strength of the cloud albedo effect varies in different types of environments. Over the continents, there is typically a large abundance of aerosol particles that are suitable CCN, and thus, clouds have high cloud droplet number concentrations (several hundred or even couple of thousand per cubic centimeter). However, in remote marine regions the aerosol concentrations are often fairly low, around 100 droplets per cubic centimeter. Such low droplet concentrations make the albedo of marine stratocumulus clouds susceptible to brightening from changes in atmospheric aerosol concentrations.

3 89 Cloud Brightening and Climate Change 779 One example of the susceptibility of the marine clouds to a perturbation from aerosol injection is ship tracks, which can be detected from satellite images as long, thin and optically thick clouds following the ship plumes. Ship tracks form when the ships exhaust releases new sulfur-containing particles into the atmosphere, and these particles act as CCN upon cloud formation. As a clearly visible demonstration of the effect of aerosol particles on cloud albedo, the ship tracks have inspired ideas of modifying marine clouds by deliberately injecting aerosol particles to regions with persistent cloud cover in order to brighten the marine clouds and thus to cool the climate. Cloud Brightening with Sea-Salt Aerosol The use of deliberate aerosol injections in order to increase the reflectivity of marine stratocumulus clouds was first suggested several decades ago (Latham 1990). Current research on the topic concentrates on artificial emissions of sea spray which could be produced with a fleet of unmanned, wind-powered vessels that can be remotely steered beneath marine clouds and emit seawater droplets into the air at a very high rate (Fig. 89.1) (Salter et al. 2008). Once in the atmosphere, the water from the injected droplets will evaporate leaving behind particles consisting mainly of sea salt. It is currently uncertain how large a fraction of the released particles would be transported to altitudes where they could act as CCN and thus increase the cloud droplet concentration compared to an unperturbed situation. The efficiency of artificial cloud brightening is expected to be highly dependent on several factors, such as the effect of droplet evaporation on the atmospheric temperature profile, vertical velocity of atmospheric flows, and background aerosol concentration (Korhonen et al. 2010). For marine regions to be suitable for cloud brightening, they must be frequently occupied by persistent low-lying stratocumulus decks, so that particles injected close to the sea surface can easily reach the cloud base. In order to have a significant effect on the global radiation balance, the injection regions should also receive a fair amount of solar radiation. Based on these requirements, several climate model studies have identified the stratocumulus regions off the west coasts of North and South America and of Southern Africa as the most suitable all-year-round injection sites. It should be noted, however, that also other regions might prove favorable if the additional aerosol particles were released into the atmosphere directly at the cloud altitude (e.g., from planes) instead of at the ocean surface. It has also been previously suggested that marine clouds could be brightened by fertilizing the oceans with iron in order to enhance dimethyl sulfide (DMS) emissions from phytoplankton. The gaseous DMS is known to get oxidized in the atmosphere after which it can form new aerosol particles to act as CCN. However, a wealth of recent field observations and modeling studies has shown that even large increases in DMS emissions will probably not lead to significant cloud brightening (Quinn and Bates 2011).

4 780 H. Korhonen and A.-I. Partanen Fig Schematic illustration of the suggested cloud brightening and cloud albedo effect. The vessel injects seawater droplets that evaporate and the remaining sea-salt particles are transported to the cloud level. They act as cloud condensation nuclei and increase the cloud droplet concentration and thus cloud reflectivity. Particles that end up outside clouds also scatter solar radiation, but the effect is minor compared to the cloud albedo effect Climate Effects of Cloud Brightening To date, no large-scale experiments of cloud brightening have been conducted, and therefore, atmospheric models are currently the only source of estimates on the potential cooling efficiency of cloud brightening. Several climate model studies have indicated that cloud brightening could significantly cool the planet provided that one could at least quadruple the natural cloud droplet concentration (from about 100 cm 3 to about 400 cm 3 ) over large fraction of the oceans. It has been estimated that under the unrealistic assumption that all marine low-level clouds were modified, the resulting cooling could more than offset the warming from doubling of preindustrial atmospheric CO 2 concentration (Latham et al. 2008). On the other hand, if only the most suited stratocumulus regions off the coasts of the Americas and Southern Africa (comprising approximately 3 % of the Earth s surface area) were modified, cloud brightening could counteract about 35 % of the warming of doubled CO 2 concentration and postpone global warming by 25 years (Jones et al. 2009). It is important to notice that even though large-scale cloud brightening might be able to help restore the global mean temperature to preindustrial values as well as preserve the polar ice sheets, climate models suggest that cloud geoengineering would result in large regional differences in temperature change (Rasch et al. 2009). While this is likely to be true also for other climate engineering methods that reflect sunlight back into space, the effect could be even stronger for cloud brightening due to the strong localized modification of the energy balance. The regions experiencing most cooling would likely be found under or close to the modified clouds, although also the Arctic could experience significant cooling (Jones et al. 2009).

5 89 Cloud Brightening and Climate Change 781 Many potential environmental effects of strong localized cooling remain uncertain, but concerns have been raised, e.g., the effects on ocean currents, marine ecosystems, as well as large-scale weather patterns such as El Niño. Large-scale cloud brightening is predicted to decrease global mean precipitation because less sunlight reaching the Earth s surface would reduce evaporation from the surface and thus also reduce the atmospheric water available for rain formation (Rasch et al. 2009). However, different climate models and cloud brightening scenarios give very different results with respect to the geographical distribution of the precipitation change. For example, one model study has predicted a very strong decrease of rainfall over the Amazonia with potentially detrimental consequences on vast areas of rain forest (Jones et al. 2009). This effect has not, however, been reproduced by other models which highlights the uncertainty of predicting future precipitation changes. Overall, while cloud brightening may be able to counteract at least some of the precipitation increase resulting from increased CO 2 concentrations on a global scale, it is very unlikely that even the global means of temperature and precipitation could be simultaneously kept at present-day values. While most climate model studies to date have assumed that the cloud droplet concentration could be increased fairly homogeneously in large regions over the oceans, this is unlikely to be the case in the real atmosphere. More detailed studies of the fate of the injected particles have revealed that spatial heterogeneity is highly likely, in part due to varying background concentrations and transport in the atmosphere (Korhonen et al. 2010; Partanen et al. in press). Furthermore, according to simulations made with models that are able to resolve the structure of individual clouds, sea-salt injections would probably increase the cloud reflectivity only under certain favorable meteorological conditions and have negligible effect under others (Wang et al. 2011). A further concern with the brightening efficiency is that poor mixing of the injected particles with the surrounding air could lead to a requirement of a very large number of spraying vessels in order to spread particles into a sufficiently large area. On the other hand, it has been recently found that even particles that are transported outside clouded areas could contribute to climate cooling by scattering solar radiation (so-called aerosol direct effect). This effect is, however, likely to be relatively small compared to the cloud albedo effect (Partanen et al. in press). One advantage of cloud brightening as a climate engineering option is that the lifetime of sea-salt particles in the atmosphere is short. Thus, should the need arise, the system could be stopped immediately after which the direct cooling effects would last only for a few days. It is, however, possible that some of the environmental effects caused by the method could be irreversible or take a long time to reverse even upon termination. Furthermore, it is crucial to understand that a shutdown of cloud brightening or any other climate engineering method in a world with high greenhouse gas concentrations could lead to a very rapid warming with potentially large detrimental ecosystem and societal impacts.

6 782 H. Korhonen and A.-I. Partanen Conclusions Many climate model studies suggest that cloud brightening with sea-salt injections could have a significant effect on the Earth s radiation balance and at least partly offset changes in global mean temperature and precipitation caused by increased greenhouse gas emissions. It is, however, clear that regional climate conditions could not be restored simultaneously everywhere on the globe. In addition, recent studies suggest that the first climate simulations may have been overly optimistic concerning the efficiency of cloud brightening. It is therefore too early to estimate the costs of deploying the method on a large scale. Further improvement of the scientific understanding on the feasibility or efficacy of cloud brightening would require controlled small-scale field studies of cloud seeding with sea-salt aerosol. Such experiments would be considerably easier to conduct with minimal effects on the surrounding environment than those testing climate engineering with stratospheric sulfate particles. However, proposals of any field studies on these methods currently face opposition from the general public based on the moral and ethical issues of climate engineering. References Jones A, Haywood J, Boucher O (2009) Climate impacts of geoengineering marine stratocumulus clouds. J Geophys Res 114:D doi: /2008jd Korhonen H, Carslaw KS, Romakkaniemi S (2010) Enhancement of marine cloud albedo via controlled sea spray injections: a global model study of the influence of emission rates, microphysics and transport. Atmos Chem Phys 10: Latham J (1990) Control of global warming? Nature 347: Latham J, Rasch P, Chen C-C, Kettles L, Gadian A, Gettelman A, Morrison H, Bower K, Chourlaton T (2008) Global temperature stabilization via controlled albedo enhancement of low-level maritime clouds. Philos Trans R Soc A 366: Partanen A-I, Kokkola H, Romakkaniemi S, Kerminen V-M, Lehtinen KEJ, Bergman T, Arola A, Korhonen H (2012) Direct and indirect effects of sea spray geoengineering and the role of injected particle size. J Geophys Res 117, D02203, doi: /2011jd Quinn PK, Bates TS (2011) The case against climate regulation via oceanic phytoplankton sulphur emissions. Nature 480:51 56 Rasch P, Latham J, Chen C-C (2009) Geoengineering by cloud seeding: influence on sea ice and climate system. Environ Res Lett 4. doi: / /4/4/ Salter S, Sortino G, Latham J (2008) Sea-going hardware for the cloud albedo method of reversing global warming. Philos Trans R Soc A 366: Wang H, Rasch PJ, Feingold G (2011) Manipulating marine stratocumulus cloud amount and albedo: a process-modelling study of aerosol-cloud-precipitation interactions in response to injection of cloud condensation nuclei. Atmos Chem Phys 11: