Cloud albedo

Another family of feedbacks arises because the radical differences in the albedo (reflectivity) of ice, snow, and clouds compared to the rest of the planetary surface, which causes a loss of the absorption of solar radiation and thereby cools the planet. Indeed, the high albedo of snow and ice cover may be a factor that hastens the transition into ice ages once they have been initiated. Of course, the opposite holds due to decreasing albedo at the end of an ice age. As simple as this concept may appear to be, the cloud-albedo feedback is not easy to quantify because clouds reflect solar radiation (albedo effect) but absorb... 

Cloud albedo is calculated to be sensitive to the droplet population. Twomey (1977) showed theoretically that albedo is enhanced by the addition of particles to the atmosphere. 

The putative feedback involves the influence of emissions of this aerosolgenic gas, (CH3)2S, that influences cloud albedo and hence either the temperature of the seawater in which the phytoplankton live or the amount of light available for their photosynthesis. Figure 17-9 represents the hypothetical feedback loop, and emphasizes that even the sign of the feedback is not known. Contradictory evidence has been developed... 

If we let Ti be the predictive temperature that we have now, 255 K, we can quickly see what happens if A changes from Ai, to a new value A2. The current albedo of Earth is ca. 0.3, and current fractional cloud cover is ca. 0.5. Ice and snow cover are minimal and most of the Earth is oceans with Aocean 0.1. Forests have forests 0.1 so most of the noncloudy Earth has. A % 0.1. This gives an expression for average cloud albedo ... 

Charleson RJ, JE Lovelock, MO Andreae, SG Warren (1987) Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature (London) 326 655-661. 

Challenger F, Simpson M (1948) Studies on biological methylation. Part 12. A precursor of the dimethyl sulphide evolved by Polysiphonia fastigiata. Dimethyl-2-carboxyethylsulphonium hydroxide and its salts. J Chem Soc 3 1591-1597 Charlson RJ, Lovelock JE, Andreae MO, Warren SG (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326 655-661 Coley PD, Aide T (1991) Comparison of herbivory and plant defenses in temperate and tropical broad-leaved forests. In Price P, Lewinsohn T, Fernandes G, Benson W (eds) Plant-animal interactions evolutionary ecology in tropical and temperate regions. Wileys, New York, pp 25 19... 

Typical cloud albedos for thick clouds in the boundary layer are 0.5 over the ocean in midlatitudes i.e., half of the incoming solar radiation is scattered back out to space (Baker, 1997). This approximation, Eq. (JJ), illustrates why a change in the number of cloud droplets and their size affects the cloud albedo and hence the radiative forcing (see Problem 9). 

Not only do CCN affect the number of cloud droplets formed, but they also affect the size distribution of these droplets. This also affects cloud albedo and its sensitivity to changes in the number concentration (see Eqs. (JJ) and (KK)). Figure 14.47, for example, shows the size distribution for cloud droplets measured in urban and nonurban air around Denver, Colorado (Al-kezweeny et al., 1993). The median volume diameter was 14 jj,m for the urban air cloud, and this was only 50% of that of the much larger droplets in the... 

Falkowski, P. G., Y. Kim, Z. Kolber, C. Wilson, C. Wirick, and R. Cess, Natural versus Anthropogenic Factors Affecting Low-Level Cloud Albedo over the North Atlantic, Science, 256, 1311-1313 (1992). 

Hegg, D. A., Cloud Condensation Nucleus-Sulfate Mass Relationship and Cloud Albedo, J. Geophys. Res., 99, 25903-25907 (1994). 

Kim, Y., and R. D. Cess, Effect of Anthropogenic Sulfate Aerosols on Low-Level Cloud Albedo over Oceans, J. Geophys. Res., 98, 14883-14885 (1993). 

Penner, J. E Cloud Albedo, Greenhouse Effects, Atmospheric Chemistry, and Climate Change, J. Air Waste Manage. Assoc., 40, 456-461 (1990). 

Platnick, S., and S. Twomey, Determining the Susceptibility of Cloud Albedo to Changes in Droplet Concentration with the Advanced Very High Resolution Radiometer, J. Appl. Meteorol., 33, 334-347 (1994). 

Schwartz, S. E., Are Global Cloud Albedo and Climate Controlled by Marine Phytoplankton Nature, 336, 441-445 (1988). 

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