The Earth's Energy Budget
The state of the global climate depends upon the balance of energy fluxes (flows) into and out of the climate system (made up of the atmosphere, oceans, ice masses, biosphere and geosphere). The most important component in this respect is the atmosphere, and the major energy fluxes of incoming sunlight and outgoing terrestrial radiation from the Earth, which must balance. The nature of the Earth's atmosphere, with its greenhouse gases, also affects the state of the global climate. This fact sheet reviews the energy budget of the Earth-Atmosphere system.
Radiation from the Sun and Earth
The Earth's atmosphere has an important influence on the energy budget of the global climate system. This is determined by the processes involved in solar (Sun) and terrestrial (Earth) energy transfers. Radiation emitted from the Sun has a temperature of approximately 6000°C. The radiation is emitted over a spectrum of wavelengths, with a specific quantity of energy for each wavelength. Most solar energy is emitted with a wavelength of approximately 0.5 microns. This represents radiation in the visible part of the spectrum. The total energy output of the Sun is approximately 64 million Wm-2.
The solar radiation disperses uniformly in all directions. After travelling 93 million miles, only a tiny fraction of the energy emitted by the Sun is intercepted by the Earth. Therefore, the energy arriving at the top of the Earth's atmosphere is many orders of magnitude smaller than that leaving the Sun. The latest satellite measurements indicate a value of 1368Wm-2 for the energy received at the top of the atmosphere. This is known as the solar constant.
The Earth also emits radiation, but since it is much cooler than the Sun, its radiating energy is in the longer wavelength, invisible infrared or heat part of the spectrum. Sometimes, we can indirectly see heat radiation, for example the heat shimmers rising from a tarmac road on a hot sunny day. The energy received by the Earth from the Sun balances the energy lost by the Earth back into space.
The Energy Budget of the Atmosphere
The Earth, however, has an atmosphere, consisting mostly of nitrogen (78%), oxygen (21%) and a number of greenhouse gases, which affect the Sun-Earth energy balance. The average global temperature is in fact 33°C higher than it should be. Certain atmospheric gases absorb radiation at some wavelengths but allow radiation at other wavelengths to pass through unimpeded. The atmosphere is mostly transparent (little absorption) in the visible part of the spectrum, but significant absorption of solar ultra-violet radiation by ozone, and terrestrial infra-red radiation by water vapour, carbon dioxide and other trace gases occurs.
The absorption of terrestrial infra-red radiation is particularly important to the energy budget of the Earth's atmosphere. Such absorption by the trace gases heats the atmosphere, and so the Earth stores more energy near its surface than it would if there was no atmosphere. Consequently the temperature is higher by about 33°C.
This process is popularly known as the greenhouse effect. Glass in a greenhouse is transparent to solar radiation, but opaque to terrestrial infra-red radiation. The glass acts like some of the atmospheric gases and absorbs the outgoing energy. Much of this energy is then re-emitted back into the greenhouse causing the temperature inside to rise. Consequently, the gases in the atmosphere which absorb the outgoing infra-red radiation are known as greenhouse gases and include carbon dioxide, water vapour, nitrous oxide, methane and ozone.
The Greenhouse Effect
As well as absorbing solar and terrestrial radiation, gases in the atmosphere, along with aerosols, also scatter radiation. The scattering of the incoming solar radiation is important because this, too, can alter the overall energy budget of the atmosphere.
The fluxes or flows of energy within the Earth's atmosphere determine the state of our climate. Factors which influence these on a global scale may be regarded as causes of global climate change. Basic principles determine the state of the Earth-Atmosphere energy budget, and consequently the global climate. The atmosphere's greenhouse gases influence this energy balance by absorbing outgoing energy from the Earth, thereby increasing the global average surface temperature. This process is called the natural greenhouse effect. Through man-made emissions of greenhouse gases since 1765, this natural phenomenon is being enhanced with possible consequences of global warming. In the real world it is also necessary to consider the energy flows within and between the other parts of the climate system, including the oceans, ice masses, biosphere and geosphere.