18.14: Radiation: Applications
The average temperature of Earth is the subject of much current discussion. Earth is in radiative contact with both the Sun and dark space; it receives almost all its energy from the radiation of the Sun and reflects some of it into outer space. Dark space is very cold, about 3 K, so Earth radiates energy into it. For instance, heat transfer occurs from soil and grasses, the rate of which can be so rapid that frost can occur on clear summer evenings, even in warm latitudes.
The average temperature of Earth is determined by its energy balance. As a first approximation, it is the temperature at which Earth radiates heat to space as fast as it receives energy from the Sun. An important parameter in calculating the temperature of Earth is its emissivity. On average, Earth's emissivity is about 0.65, but the calculation of this value is complicated by the considerable day-to-day variation in highly reflective cloud coverage.
The often-mentioned greenhouse effect is directly related to the variation of Earth's emissivity with wavelength. The greenhouse effect is a natural phenomenon responsible for providing temperatures suitable for life on Earth; it also makes Venus unsuitable for human life. The greenhouse effect is central to the discussion of global warming due to carbon dioxide and methane emissions into Earth's atmosphere due to industry, transportation, and farming. Changes in the global climate could lead to more intense storms, precipitation changes affecting agriculture, reduction in rainforest biodiversity, and rising sea levels.