Radiation Balance of Earth

The temperature of Earth is determined by the balance between solar radiation input (see Fig. 2-30) and reflection and reradiation of energy from Earth and its atmosphere back into space. Recall from Section 4.3.1 that the solar constant is approximately 1400 W/m2. The projected area intercepting the solar radiation is 7rr2, where r is Earth s radius. Given that Earth s surface area is 47rr2, the average solar radiation received outside Earth s atmosphere is approximately one-fourth the solar constant, or 350 W/m2. 

What average surface temperature would be predicted for Earth if it had no atmosphere Assume it behaves as a blackbody. What would be the wave- 

Approximately 350 W/m2 of solar radiation would be received on average at Earth s surface if atmospheric effects are ignored. Earth reflects approximately 35% of received radiation. Use the Stefan-Boltzmann law—Eq. [4-42]—to solve for T, equating the average solar radiation that would be absorbed by Earth s surface under these conditions to the rate of reradiation  

This estimated average surface temperature for Earth, if it did not have an atmosphere, is much lower than what actually exists. 

The wavelength corresponding to the maximum energy reradiated by Earth at 251 K can be estimated using Wien s displacement law, Eq. [4-43]  

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Black carbon is operationally defined on the basis of its relative resistance to oxidation in combustion experiments it is typically considered to be the carbon that survives oxidation in air at 375 °C but combusts at higher temperatures. However, because of variations in protocols, different laboratories may measure somewhat different values for /be in the same sediment. In sediments, values of /be are t)q)ically somewhat less than 1%. In the atmosphere, black carbon also has a significant role in atmospheric transparency and thus in the radiation balance of Earth (see Section 4.7). 

In addition to atmospheric gases and vapors, aerosols such as sulfate particles, black carbon, and SOA also influence the radiation balance of Earth. The net effect of particles is complex depending on their size, composition, and reflectivity, particles may both absorb and scatter shortwave and longwave radiation. Some particles may make a negative contribution to radiative forcing. For example, particulate emissions from Mt. Pinatubo in 1991 resulted in months of measurably lower surface temperatures. 

Just as in the case for the hydrosphere, the atmosphere participates in all of the major biogeochemical cycles (except for phosphorus). In turn, the chemical composition of the atmosphere dictates its physical and optical properties, the latter being of great importance for the heat balance of Earth and its climate. Both major constituents (O2, H2O) and minor ones (CO2, sulfur, nitrogen, and other carbon compounds) are involved in mediating the amounts and characteristics of both incoming solar and outgoing infrared radiation. 

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 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. 

Discuss the radiation balance of the Earth s surface. Present quantitative data on the trapping of IR radiation in the troposphere. 

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. 

It has become clear only recently that the atmospheric sierosol plays an important role for the climate on earth. It is common to distinguish between direct and indirect effects of the aerosols on the climate. Aerosols effect directly the radiation balance of the earth due to scattering and absorption of electromagnetic radiation (radiative forcing). On the other hand they influence the physics and chemistry of the atmosphere as condensation nuclei for cloud droplets and their chemical reactions with atmospheric trace gases. Though these indirect aerosol effects are difficult to quantify, they are at least as important as the direct radiative forcing. An especially important and complex example for the indirect influence of aerosols on the chemistry and radiation balance of the earth is the role of stratospheric aerosol particles on the polar ozone depletion, which is discussed in more detail below. 

Figure 14 Diurnal variation in individual terms of radiation balance on earth s surface (example).

Pseudokarst takes place mostly in areas where the radiation dryness index R/lr is greater than 1 but no higher than 3, where R is the annual radiation balance of the Earth s surface (30-50 kcal/cm2), ris the annual amount of atmospheric precipitation (g/cm2), and L is the latent heat of vaporization (kcal/g), which makes it possible to regard loess and its susceptibility to pseudokarst processes as planetary-scale phenomenon and to strictly constrain its boundary conditions [3]. 

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. 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

If more solar energy is absorbed than infrared radiation emitted, the earth would warm and a new equilibrium would appear. But, if the earth had more clouds, it would reflect more solar radiation and absorb less. This would have a cooling effect on the planet, lowering the amount of infrared radiation that is escaping to space to balance the lower amount of absorbed solar energy. The earth s radiant energy balance today is 240 watts per square meter. 

FIGURE 1.9 Global average mean radiation and energy balance per unit of earth s surface [adapted with permission from IPCC (1996) with numbers from Kiehl and Trenberth (1997)]. 

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