Earth's radiation balance

From Wikipedia, the free encyclopedia
Jump to: navigation, search
These maps show monthly net radiation in watts per square meter. Places where the amounts of incoming and outgoing energy were in balance are yellow. Places where more energy was coming in than going out (positive net radiation) are red. Places where more energy was going out than coming in (negative net radiation) are blue-green. The measurements were made by the Clouds and the Earth's Radiant Energy System (CERES) sensors on NASA’s Terra and Aqua satellites. Over the course of a year, the most obvious pattern is seasonal changes in net radiation. Incoming sunlight increases in the hemisphere experiencing summer, which makes the energy imbalance strongly positive (more watts of energy coming in than going out). As the September equinox approaches, a zone of positive net radiation is nearly centered over the equator, and energy deficits lie over the poles. As the season changes into winter, the net radiation becomes negative across much of the Northern Hemisphere and positive in the Southern Hemisphere. The pattern reverses on the March equinox. Averaged over the year, there is a net energy surplus at the equator and a net energy deficit at the poles. This equator-versus-pole energy imbalance is the fundamental driver of atmospheric and oceanic circulation.[1] (click for more detail)

Earth's radiation balance or Earth's energy balance describes the incoming and outgoing thermal radiation. The Earth equilibrium sensitivity describes a steady state, energy balance. Anthropogenic perturbations are responsible for a positive radiative forcing which reduces the net longwave radiation loss out to Space, hence the radiation balance is disturbed, Earth's energy budget changes. This doesn't occur instantaneously due to the slow response/inertia of the cryosphere to react to the new energy budget. The net heat flux is buffered primarily in the Ocean, until a new energy balance, the equilibrium state is established between in-and outgoing radiative forcing and climate response.[2]

Equation[edit]

The incoming solar radiation is short wave, therefore the equation below is called the short wave radiation balance Qs:[citation needed]

Qs = G - R = D + H - R or depending on the albedo (back-reflection to space): = (D+H)(1 - a)
  • G = global radiation
  • D = direct shortwave radiation
  • H = diffuse shortwave radiation
  • R = reflected portion of global radiation (ca. 4%)
  • a = albedo

The Earth's surface and atmosphere emits heat radiation in the infrared spectrum, called long wave radiation. There is little overlap between the long wave radiation spectrum and the solar radiation spectrum. The equation below expresses the long wave radiation balance Ql:

Ql = AE = AO - AG
  • AE = effective radiation
  • AO = radiation of the Earth's surface
  • AG = trapped radiation (radiation forcing, also known as the so called greenhouse effect)

The two equations on incoming and outgoing radiation can be combined to show the net total amount of radiation energy, total radiation balance Qt:

Qt = Qs - Ql = G - R - AE

Measurement[edit]

An instrument for measuring the net radiation balance and albedo. Model shown CNR 1. Courtesy of Kipp & Zonen

The difficulty is to precisely quantify the various internal and external factors influencing the radiation balance. Internal factors include all mechanisms affecting atmospheric composition (volcanism, biological activity, land use change, human activities etc.). The main external factor is solar radiation. The sun's average luminosity changes little over time.

External and internal factors are also closely interconnected. Increased solar radiation for example results in higher average temperatures and higher water vapour content of the atmosphere. Water vapour, a heat trapping gas absorbing infrared radiation emitted by the Earth's surface, can lead to either higher temperatures through radiation forces or lower temperatures as a result of increased cloud formation and hence increased albedo.

See also[edit]

References[edit]

  1. ^ http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=CERES_NETFLUX_M
  2. ^ M, Previdi et al. (2013). "Climate sensitivity in the Anthropocene". Royal Meteorological Society. doi:10.1002/qj.2165.