The energy that warms Earth’s lower atmosphere comes from the Sun, but sunlight does not warm the lower atmosphere directly. This region of the atmosphere warms from below. Most of the short wavelength, electromagnetic energy from the Sun passes through the atmosphere and is absorbed by the Earth, which warms up as a result. As the Earth warms, it emits some long-wave radiation back out, heating up the lower atmosphere above it. Eventually, this energy radiates from the atmosphere back into space.
The Earth does not absorb all of the electromagnetic energy that hits it. Some reflects back out into space. This is important for the Earth’s energy balance, because only absorbed energy contributes to the temperature of the Earth/atmosphere system. The proportion of the total energy reflected by the Earth (or any object) is called the Albedo.
Albedo values range from 0 to 1. A perfectly absorptive surface has an albedo of 0, an object with an albedo of 0.5 absorbs as much energy as it reflects back, and a perfectly reflective surface has an albedo of 1. If the Earth had an albedo of 1, it would absorb no energy from the Sun and the planet would be much colder. Earth has an average albedo of around 0.39*, which means that it absorbs a little more energy than it reflects.
The Earth's average albedo depends on the composition and physical state of its surface. Land and liquid water have relatively low albedos (10 to 40 %), while ice and snow have higher albedos, typically between 70 and 90%. Multiple factors and processes can change the albedo of a surface over time. For example, as water turns to ice the albedo increases, and as soot and dirt settle on fresh ice or snow, the albedo decreases. Similarly, vegetation cover influences the albedo of the landscape: deserts have albedos of 20-35%, savannas are around 15%, and rainforests are around 5%.
There are natural cycles in albedo. Seasonally, winter snow and ice cover will increase the albedo of Earth’s temperate and polar regions. The same happens on geological timescales, with the coming and going of ice ages. Particles suspended in the atmosphere influence the Earth’s albedo. Soot particles from combustion have low albedos, and absorb energy directly, contributing to warming. Aerosols such as airborne sulfates strongly reflect shortwave radiation, so their presence in the atmosphere increases the Earth’s overall albedo. Clouds, which are made of condensed water vapor, also increase the Earth's albedo.
Human activity influences land use, vegetation cover, and the concentration of aerosols, soot particles and water vapor in the atmosphere. This makes the effect of human activity on the Earth’s albedo difficult assess.Video Demonstration
*The precise value of an object's albedo varies over different wavelengths and is influenced by shape and angle of incidence.