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Cloud reflectivity modification is a climate engineering technique (or more specifically, a solar radiation management technique) that would seek to reflect a very small fraction of sunlight back into space in order to offset many of the negative impacts of climate change. In particular, it would seek to alter clouds to make them more or less reflective, in order to change their effect on climate. There are two hypothesised forms: cirrus thinning and marine cloud brightening. High, cold cirrus clouds tend to allow sunlight through, yet trap infra-red (or longwave) radiation from Earth. Therefore, thinning or removing these clouds reduces their longwave trapping capacity and results in a cooling effect on Earth's climate.〔Muri, H., J. E. Kristjánsson, T. Storelvmo, and M. A. Pfeffer (2014), The climatic effects of modifying cirrus clouds in aa climate engineering framework, J. Geophys. Res. Atmos., 119, 4174–4191, (doi:10.1002/2013JD021063 ).〕 Conversely, low, warm clouds tend to be highly reflective to sunlight, especially when the cloud is bright due to the presence of smaller droplets (because of the Twomey effect). Therefore, modifying these clouds to make them more reflective cools the climate. This proposed technique is known as 'marine cloud brightening', 'marine sky brightening'〔Schäfer et al., (2015). The European Transdisciplinary Assessment of Climate Engineering (EuTRACE): Removing Greenhouse Gases from the Atmosphere and Reflecting Sunlight away from Earth. Funded by the European Union’s Seventh Framework Programme under Grant Agreement 306993.〕 or 'cloud whitening' of low cloud. By modifying the reflectivity of clouds, the albedo of Earth would be altered. The intention is that this technique, in combination with greenhouse gas emissions reduction (and possibly other climate engineering techniques) will be sufficient to control global warming. The effect is expected to be fully reversible, as the cloud condensation nuclei particles precipitate naturally. However, like any planetary-scale project dealing with the complex climate system, there is a non-trivial risk of unintended consequences. ==Basic principles== Low level stratocumulus clouds cover approximately one-third of the oceans surface. These clouds possess Albedo levels of 0.3-0.7.〔Schwartz and Slingo, 1996 S.E. Schwartz and A. Slingo In: P. Crutzen, Editor, Clouds, Chemistry and Climate: Proceedings of NATO Advanced Research Workshop (1996).〕 The levels of albedo present in the low-level marine stratocumulus clouds create a significant global cooling effect. Theoretically, by increasing the albedo of the clouds, via spraying sea water(NaCl) droplets approximately 1 μm in size, the cooling effects of the clouds are increased and potentially their longevity is enhanced.〔Computational Assessment Of A Proposed Technique For Global Warming Mitigation Via Albedo-enhancement Of Marine Stratocumulus Clouds K Bower - T Choularton - J Latham - J Sahraei - S Salter - Atmospheric Research - 2006.〕 By creating cloud condensation nuclei, it might be possible to change the albedo of clouds to make them appear whiter. This can be done using a variety of chemicals and techniques, with seawater sprays from ships being a simple example. Broadly speaking, the marine environment has a deficit of cloud condensation nuclei due to lower levels of dust and pollution at sea, so this technique is more effective over the ocean than over land. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cloud reflectivity modification」の詳細全文を読む スポンサード リンク
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