average albedo of Earth is about 0.3. This is far higher than for the ocean primarily because of the contribution of clouds. Earth's surface albedo is regularly estimated via Earth observation satellite sensors such as NASA's MODIS instruments on board the Terra and Aqua satellites, and the CERES instrument on the Suomi NPP and JPSS. As the amount of reflected radiation is only measured for a single direction by satellite, not all directions, a mathematical model is used to translate a sample set of satellite reflectance measurements into estimates of directional-hemispherical reflectance and bi-hemispherical reflectance (e.g.,). These calculations are based on the bidirectional reflectance distribution function (BRDF), which describes how the reflectance of a given surface depends on the view angle of the observer and the solar angle. BDRF can facilitate translations of observations of reflectance into albedo. Earth's average surface temperature due to its albedo and the greenhouse effect is currently about . If Earth were frozen entirely (and hence be more reflective), the average temperature of the planet would drop below . If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about . In contrast, if the entire Earth was covered by water – a so-called ocean planet – the average temperature on the planet would rise to almost . In 2021, scientists reported that Earth dimmed by ~0.5% over two decades (1998–2017) as measured by earthshine using modern photometric techniques. This may have both been co-caused by climate change as well as a substantial increase in global warming. However, the link to climate change has not been explored to date and it is unclear whether or not this represents an ongoing trend. White-sky, black-sky, and blue-sky albedo For land surfaces, it has been shown that the albedo at a particular solar zenith angle θi can be approximated by the proportionate sum of two terms: the directional-hemispherical reflectance at that solar zenith angle, , sometimes referred to as black-sky albedo, and the bi-hemispherical reflectance, , sometimes referred to as white-sky albedo. with being the proportion of direct radiation from a given solar angle, and being the proportion of diffuse illumination, the actual albedo (also called blue-sky albedo) can then be given as: This formula is important because it allows the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the