Radiation balance of the Earth-atmosphere system

The Earth-atmosphere system consists of the ensemble of the atmosphere, ocean, continents and ice cover. The climate of this system is controlled by the orbit and rotation of the Earth, the physical state and chemical composition of the surface (including liquid water and ice), and by the density and composition of the atmosphere. This last parameter participates mainly in the control of the radiation balance. For this reason our knowledge of the radiation balance of the Earth-atmosphere system will be summarized briefly in this section. The interested reader is referred to Paltridge and Platt (1976) for further details. 

The Sun can be considered as a black body, the temperature of which is around 5800 K. In accordance with this temperature, 90 % of the energy radiated is composed of radiation with wavelength ranging from 0.4 to 4.0 /im. The intensity maximum is in the visible range around 0.5 /un. 

The infrared radiation is absorbed by atmospheric gases like water vapour, carbon dioxide and, to a lesser extent, ozone. Furthermore, in this wavelength band, 

Atmospheric circulation transports heat from areas of positive radiation balance to areas where there is an energy deficit. A significant characteristic of this circulation is that a portion of the heat is transported in latent form, which means that the heat is delivered by the condensation of water vapour in the moving air. It is estimated that about one-third of the energy crossing latitude of 30° of both hemispheres is in the form of latent heat. Another one-third of the energy transport takes place in the ocean. Thus only 30 % of the heat is transported directly by the atmosphere. 

The surface temperature of the ocean, controlled by the mixing in the upper layers of the water, plays an important role in regulating the heat exchange between the atmosphere and ocean. Unfortunately, these exchange processes are not sufficiently known to determine quantitatively the role of the ocean in the global heat transport. Thus, further work remains to be done to clarify this point which is of great importance for climate research. 

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We have to emphasize that the correct prediction of the future COz concentrations is one of most important tasks of atmospheric science at present. This is explained by the fact that the C02 content of our atmosphere regulates, among other things, the radiation balance of the Earth-atmosphere system by absorbing infrared radiation emitted by the surface. Thus, we cannot exclude the possibility that the increase of the carbon dioxide concentration may cause inadvertent climatic variations in the future (see Chapter 6). 

The study of the atmospheric sulfur cycle is a rapidly expanding field because human activity provides an important sulfur dioxide source. In the atmosphere S02 is converted to sulfate containing aerosol particles which can modify the radiation balance of the Earth-atmosphere system, the optical properties and the precipitation forming ability24 of the air. 

The Earth s climate depends among other parameters (see later) on the chemical composition of the atmosphere. Thus, any variation in the composition raises the possibility of climatic change. First of all, the chemical composition regulates the radiation balance of the Earth-atmosphere system. However, since differences in radiation balance in various geographical regions control the atmospheric circulation, there is also a relationship between composition and dynamic processes. In this chapter we shall deal mainly with the effects of compositional variations on the radiation balance. Moreover, the significance of so-called feedback mechanisms will also be stressed. 

The aim of this book is, first of all, to present the atmospheric cycle of the trace constituents. We will discuss in more detail the trace substances (Chapter 3) with relatively short residence time (<10 yr). The study of these compounds is particularly interesting since their sources and sinks as well as their concentrations are very variable in space and time. They undergo several physical and chemical transformations in the atmosphere. Among these transformations the processes leading to the formation of aerosol particles have unique importance. The aerosol particles control the optical properties of the air, the formation of clouds and precipitation and, together with some gases, the radiation and heat balance of the Earth-atmosphere system. Because of their importance the physical and chemical characteristics of aerosol particles will be summarized in a separate chapter (see Chapter 4). 

The mean emitted power of the Earth-atmosphere system, 240 W corresponds to a blackbody temperature of 255 K. The surface emission, 390 W m, corresponds to a blackbody temperature of 288 K. The 33 K difference between the blackbody temperatures of the Earth s surface and the Earth-atmosphere system is the so-called greenhouse effect. Increases in concentrations of CO2 and other GHGs since the Industrial Revolution are estimated to have contributed about +2.5 W m to the global and annual average radiation balance (IPCC, 1995). 

It is perhaps worth while to point out that most of the attenuation of infrared radiation in the atmosphere is due to water-vapour absorption bands, the other major contributions coming from carbon dioxide and ozone (Hackforth, i960). The existence of wavelength windows of low absorption is of prime importance in the development of laser communication systems, while the presence of strongly absorbing bands is a major factor in determining the radiation balance of the earth s atmosphere. 

Rashchke, E. 1968, The Radiation Balance of the Earth s Atmosphere System from Radiation Measurements of the Nimbus II Meteorological System, Goddard Space Flight Center, TN-D-4859, Greenbelt, Md. 

The Earth-atmosphere system emits thermal infrared radiation. The upward flux from the Earth s surface is —115 units. The cloud-free atmo.sphere emits —33 units back to the Earth s surface and —34 units out to space. The cloudy atmosphere emits —67 units back to Earth and —36 units out to space. Thus —70 units of infrared radiation leave the top of the atmosphere, balancing the net —70 units of. solar radiation penetrating the top of the atmosphere. The net upward flux of infrared radiation at the surface of the Earth is — 15 units, consisting of — 115 units emitted by the Earth and —100 units radiated back to Earth by the cloud-free and cloudy atmosphere. 

Although the total absorbed and emitted energy of the surface-atmosphere system balance at 240 W m , the fluxes of processes transferring energy within the system, between the surface and atmosphere and within the atmosphere itself, can exceed the net 240 W m solar input. Actually, about 70 of the 240 W m in absorbed insolation is absorbed by trace constituents, aerosols, and clouds in the atmosphere (Ramanathan et al., 1987). In addition, since the Earth s observed average surface temperature is 288 K, about 390 W m is radiated upward from the surface, so that trace atmospheric constituents must be responsible... 

Fig. 2.13 Radiation and heat balance of the system earth-atmosphere. Percentages are given in relation to the incoming solar radiation (100% = 343 W m ). I interception of solar radiation by molecules/particles (p) clouds (c), and earth surface (e), D diffuse radiation by molecules/particles (p) clouds (c), R reflexion (albedo) by molecules/particles (p) clouds (c), and earth surface (e), IR infrared dissipation to space by molecules/particles (p) clouds (c), T terrestrial radiation back to space (without absorption), A absorption of terrestrial radiation by molecules, AB atmospheric back-radiation, SH and LH sensible and latent heat, resp.

Radiation forcing Change in the heat balance of the atmosphere-earth system. 

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