Clouds not only contribute to the greenhouse warming; they are also highly reflective, thus contributing to the planetary albedo. The planetary albedo and the albedo under clear skies are measured by satellites. Together with the estimated cloud cover, the albedo of clouds can be determined indirectly (Figure 15). The albedo of clouds strongly varies with latitude because the albedo of liquid water strongly increases with a decreasing solar elevation. This effect is related to the cloud optical depth, i.e. the path of a sunray in a cloud. In this respect also the amount of liquid water and the amount of ice-crystals in a cloud are important. Ice-crystals are formed at very low temperature and, hence, depend on the altitude of the cloud. Also the downward backradiation varies with the cloud-level because long-wave radiation emitted by clouds depends on temperature. The net effect under annual mean conditions is that the cooling through reflection dominates the greenhouse contribution. Hence, on net, climate is cooler with clouds than under clear-sky conditions. More important is the role of clouds when temperature changes. In other words, can clouds reduce the global warming (i.e., produce a negative feedback) due to the enhanced greenhouse effect? Feedback mechanisms related to clouds are extremely complex. Predictions of the enhanced greenhouse effect with large atmosphere models differ mostly because the strength of the cloud feedback and even its sign is highly uncertain. Processes that determine the strength of the cloud feedback involve cloud cover, cloud-level, cloud droplet size distribution and the interplay between temperature and the presence of ice crystals or liquid droplets. In this respect also the amount of condensation nuclei is important (see hereafter).
Looking from the top of a mountain into a valley, one will notice a hazy atmosphere that reflects some sunlight. The effect is caused by the scattering of sunlight on solid and liquid particles in the atmosphere. All these floating particles are called aerosols. The total effect of aerosols on the planetary albedo is estimated at 0.05. Like clouds, aerosols have a dualistic character in determining the radiation budget. The outgoing long-wave radiation is hampered by aerosols, such that the effective transmittance of the atmosphere (te) decreases. The net effect of aerosols on climate is very difficult to estimate because of the general lack of observational data, together with the extraordinary diversity of tropospheric aerosols (in size, optical properties, atmospheric lifetimes and chemical composition). However, stratospheric aerosols are an exception. For these particles it is estimated that changes in the short-wave radiation budget dominate those in the long-wave radiation budget. Hence, aerosols in the stratosphere tend to cool Earth's climate.
Again aerosol concentrations in the lower troposphere as well as in the stratosphere are not constant. Concentration of aerosols fluctuates strongly by infrequent occurrence of dust storms, volcano eruptions, biomass combustion and anthropogenic input in industrial, urban and agricultural areas. The latter is the main cause of the haze that reduces visibility over industrialised areas. Once particles are levitated into moving air currents, they can be transported over long distances. Convection may also transport aerosols higher into the troposphere. However, the relative high concentration of aerosols in the stratosphere is not directly related to the above sources because the air exchange between troposphere and stratosphere is very slow. The perturbations of stratospheric aerosol concentration are strongly related to high energetic eruptions of volcanoes. There are two types of particles volcanoes inject into the stratosphere. Volcanic ash is typical silicate dust created from the fragmentation of solid rock. Absorption of solar radiation by volcanic dust prevents the short-wave radiation from reaching the Earth's surface. More important, however, is the back scattering on sulphate aerosols that increases the planetary albedo. Most of the volcanic ash falls out of stratosphere within a few months, but sulphate aerosols may remain there for several years. On average, major volcanic eruptions are thought to reduce the global mean temperature by 0.5°C for up to two years (e.g., the eruption of Mt. Pinatubo in June 1991).
Stratospheric sulphate aerosols are produced out of carbonyl sulphide (COS) and out of one of the volcanic gases released during an eruption: sulphur dioxide (SO2). The responsible process, called gas-to-particle-conversion, is another important source of very small particles in the troposphere. These very small particles occur by direct nucleation from the gas phase but are mostly created by accretion onto pre-existing particles. Sulphur-containing gases are major participants in gas-to-particle-conversion. Sulphuric acid is formed out of reactive gases, like dimethylsulphide (CH3SCH3), hydrogen sulphide (H2S), and sulphur dioxide. Sulphuric acid quickly condenses out onto cloud droplets and aerosol surfaces. Under natural conditions the main source is dimethylsulphide emitted into the atmosphere as a biological waste product from the ocean surface. However, today, the anthropogenic source of sulphur dioxide induces a severe increase of sulphate aerosols in the troposphere. Coal and oil both contain significant quantities of sulphur. Hence, anthropogenic input of sulphur dioxide is related to that of carbon dioxide. Therefore, this anthropogenic climate effect can be estimated reasonable well. Recently, calculations with climate models suggest that this increase of sulphate aerosols in the troposphere may temporarily and partly offset the enhanced greenhouse effect.
This direct cooling effect of tropospheric sulphate aerosols may be enhanced by an indirect effect. Most aerosols can act as so-called cloud condensation nuclei (CCN), i.e., by condensation of water vapour on aerosols cloud droplets are formed. More aerosols can cause an increase of the number of small droplets whenever the amount of cloud water stays the same. Smaller droplets increase the reflectivity of clouds. Hence, the cloud albedo over continents, especially over more polluted areas, is somewhat higher than over oceans. Because of the lack of observational data and due to the complexity of processes involved, the possible influence of anthropogenic SO2-emission on cloud albedo and subsequent on climate is difficult to quantify. Moreover, the direct and indirect cooling effect by tropospheric sulphate aerosols is not a solution to offset the enhanced greenhouse effect for two reasons. First, the anthropogenic SO2-emission is the main source of acid rain. Second, the residence time of tropospheric sulphate aerosols is very short (a few weeks at most) compared to that of most greenhouse gases (decades to centuries). Moreover, radiative forcing by tropospheric sulphate aerosols is highly regional, while the atmospheric CO2-concentration increases globally.