Radiation balance of the Earth

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. 

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. 

Methane is the most abundant hydrocarbon in the atmosphere. It plays important roles in atmospheric chemistry and the radiative balance of the Earth. Stratospheric oxidation of CH4 provides a means of introducing water vapor above the tropopause. Methane reacts with atomic chlorine in the stratosphere, forming HCl, a reservoir species for chlorine. Some 90% of the CH4 entering the atmosphere is oxidized through reactions initiated by the OH radical. These reactions are discussed in more detail by Wofsy (1976) and Cicerone and Oremland (1988), and are important in controlling the oxidation state of the atmosphere. Methane absorbs infrared radiation in the troposphere, as do CO2 and H2O, and is an important greenhouse gas (Lacis et al., 1981 Ramanathan et al., 1985). 

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 greenhouse effect takes place in the lower part of the atmosphere, called the troposphere. This phenomenon is an enhancement of natural processes which manages the radiation balance at the Earth s surface owing to a permanent increase of the GHG concentrations. For a better understanding of this problem we should consider the chemical composition of the troposphere and residence times of some species of interest, the temperature profile in the whole atmosphere and the increasing concentrations of greenhouse gases. 

Over a time span of a few years the heat balance of the earth can generally be considered to be in balance, which means that the incoming solar radiation, S, is balanced by the outgoing long wave radiation, F. What happens then when suddenly there is a change in either S or f Let us assume, for example, that there is a sudden increase in CO2 concentration to twice the present value. 

The terrestrial planets are almost in energy balance, that is, thermal emission nearly equals absorbed solar power. On Earth only a small internal heat source exists, which manifests itself by a vertical temperature gradient in the outer layers of the crust. Early measurements, mostly from a few deep mines and bore holes, indicated a temperature increase with depth of 10-40 K km With reasonable assumptions on the thermal conductivity of rocks this corresponds to an internal heat source of approximately 2.6 x 10 W (Bullard, 1954). More recent estimates, including data from deep sea drillings, yield a slightly higher value of 4.3 0.6 x 10 W (Williams von Herzen, 1974). In contrast, solar radiation absorbed by the Earth amounts to approximately 1.2 x 10 W. The internal heat flux is, therefore, only 3.5 X lO"" of the absorbed solar radiation and, consequently, the energy balance of the Earth is approximately 1.00035. 

The role of atmospheric CO2 in the greenhouse effect. Carbon dioxide is transparent to incoming sunlight, but it absorbs and re-emits a significant amount of the infrared radiation emitted by the Earth. This alters Earth s energy balance, raising its average temperature. 

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