Albedo

Planetary albedo is the ratio between incoming and reflected radiation at the top of the atmosphere. This includes effects of reflection from the atmosphere, mainly clouds, and surface albedo. On average 24% of incoming radiation is reflected by low altitude clouds and water vapor, and ozone in the stratosphere. Low clouds reflect most sunlight and have little effect on the energy reflected by the earth, helping to cool the current climate. In other words, low clouds do not trap energy that is headed for space. While higher temperatures are the result of high clouds that reflect less radiation and trap more emitted energy. Changes in the atmosphere can alter climate by changing the amount of solar radiation that reaches the Earth’s surface.

Surface albedo is the ratio of incoming radiation to reflected radiation where the atmosphere comes in contact with Earth’s surfaces (the boundary between earth surface and the atmosphere). Reflectivity is the capacity of an object to reflect solar radiation. It depends on radiation wavelength and the physical composition of the object. Soil reflectivity varies because of variations of moisture content, particle size, organic matter content, surface roughness, and mineral composition. Vegetation reflectivity varies with how much of the ground it covers, leaf size and area and plant growth stage. Snow reflectivity varies with crystal size, compaction, age, and liquid water content. Water reflectivity is affected by turbidity, depth, and concentrations of small aquatic plants called phytoplankton. Water albedo is lowest when the sun is near zenith and increases to near 100% when the sun is near the horizon. Here are samples of some of Earth’s surface albedos in percentages.

Albedo varies with geographic region and time of year since snow and ice are generally highly reflective. The temperature difference between the Tropics and the Poles is the driving force behind the circulation of the Earth’s atmosphere and oceans, thus creating winds and ocean currents that carry excess heat and moisture. When the moisture encounters cooler temperatures as it moves to the poles, clouds form and reduce the emission of energy to space. Any change in surface albedo will alter climate by drastically changing the amount of solar energy absorbed by the planet.

When the angle of the sun to the surface is low (closer to the horizon), solar energy is less intense since it is spread out over a larger area. Changes in this angle are one of the controlling factors that that make latitude one of the strongest influences on climate. The other controlling factor is the length of day. For latitudes of 66.5* and above, the length ranges from zero during winter solstice to 24 hours during summer solstice. The Equator has a constant 12-hour day all year long. The seasonal range of temperature therefore decreases from high latitudes to the tropics.

Human Effects on Earth’s Albedo

Suggestions have been made that human modifications of the Earth’s surface may be altering the planet’s albedo. It has been said that overgrazing in desert regions can increase surface albedo as much as 20%. It has also been estimated that such changes may suppress rainfall, which can enhance the process of desertification. And again it’s possible for extensive deforestation in tropical rain forests to increase surface albedo and result in a major climatic change.