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Surface albedo is defined as the ratio of radiosity J e to the irradiance E e (flux per unit area) received by a surface. [2] The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. [3]
In planetary geology, an albedo feature is a large area on the surface of a planet (or other Solar System body) which shows a contrast in brightness or darkness with adjacent areas. Historically, albedo features were the first (and usually only) features to be seen and named on Mars and Mercury .
Cloud albedo is a measure of the albedo or reflectivity of a cloud. Clouds regulate the amount of solar radiation absorbed by a planet and its solar surface irradiance . Generally, increased cloud cover correlates to a higher albedo and a lower absorption of solar energy .
Earth's albedo varies by a factor of 6, from 0.12 in the cloud-free case to 0.76 in the case of altostratus cloud. The absolute magnitude in the table corresponds to an albedo of 0.434. Due to the variability of the weather, Earth's apparent magnitude cannot be predicted as accurately as that of most other planets. [20]
Earth has an albedo of about 0.306 and a solar irradiance (L / 4 π D 2) of 1361 W m −2 at its mean orbital radius of 1.5×10 8 km. The calculation with ε=1 and remaining physical constants then gives an Earth effective temperature of 254 K (−19 °C). [11] The actual temperature of Earth's surface is an average 288 K (15 °C) as of 2020. [12]
However, it also increases the global albedo from 15% to 30%, and this reduces the amount of solar radiation absorbed by the Earth by about 44 W/m 2. Thus, there is a net cooling of about 13 W/m 2. [22] If the clouds were removed with all else remaining the same, the Earth would lose this much cooling and the global temperatures would increase.
Diffuse reflection on sphere and flat disk, each for the case of a geometric albedo of 1. For the hypothetical case of a plane surface, the geometric albedo is the albedo of the surface when the illumination is provided by a beam of radiation that comes in perpendicular to the surface.
Radiative forcing is defined in the IPCC Sixth Assessment Report as follows: "The change in the net, downward minus upward, radiative flux (expressed in W/m 2) due to a change in an external driver of climate change, such as a change in the concentration of carbon dioxide (CO 2), the concentration of volcanic aerosols or the output of the Sun." [3]: 2245