a The energy conversion with reflection, absorption and long wavelength emission occurs in the atmosphere and at the ground surface. The solar constant (1370 Wm"2) is the amount of energy that arrives at a surface above the atmosphere vertical to the incident radiation from the sun. In contrast, the solar energy arriving at the earth's surface (342 W m"2) is the average amount of energy falling on the half of the earth facing the sun. The parameters are equivalent to the transport equations given in the text

Generally, the net radiation balance of the earth (Rn; n = net) may be expressed in the following equation:

Radiation balance of the upper atmosphere (r„a)

radiation balance = incoming solar radiation - reflection - long-wave outgoing radiation

RnA is the balance of radiation fluxes at the upper boundary of the atmosphere, IsA the short-wave incoming solar radiation at the upper boundary of the atmosphere, psA the ability of the atmosphere (clouds, gases) and soil to reflect incoming solar radiation, ssA is the ability of the atmosphere to emit long-wave radiation and IiA the long-wave emission of the atmosphere (clouds, gases) and soil.

The radiation balance near the soil may be expressed analogously. The radiation balance at the earth's ground surface (R„g) is the sum of short-wave incoming and outgoing solar radiation.

radiation balance = incoming short-wave radiation - reflection + incoming long-wave radiation - long-wave emission

The incoming short-wave radiation IsG is called solar radiation. Isg-AsaIsg denotes the short-wave radiation balance measured by a radiometer with a glass dome and IiA-siGIiG is the long-wave radiation balance measured by a radiometer with selenium glass. The radiation balance is measured with a radiometer with a polyethylene dome which transmits IR radiation.

In contrast to the radiation balance, the energy balance includes further thermal fluxes and thus the balance is in equilibrium.

^nG = RnG - H - 2E - B - S - M = 0 (2.1.4) energy balance = radiation balance - sensible heat flux -latent heat flux - soil heat flux - storage - metabolism = 0

<PnG is the net energy flux at the soil surface and RnG the radiation balance near the soil. The sensible heat flux, H, is proportional to the specific heat capacity of air (cp= 1012 J kg-1 K-1) and the temperature difference between soil and atmosphere AT. p is the density of air: 1.1884 kg m"3 at 20 °C and 1000 hPa air pressure. H is dependent on the coupling of the exchange from surface to atmosphere. This coupling is expressed by the boundary resistance, rb:

In Eq, (2.1.4) /.E is the latent heat flux, whereby X expresses the energy required for evaporation of water (2.454 MJ kg"1 at 20 °C) and E the evaporation (kg m~2).

Long-wave radiation follows the Stefan-Boltz-man law in all cases [see Eq. (2.1.1)].

The annual incoming short-wave solar radiation (solar radiation, Fig. 2,1.3) is unevenly distributed over the earth's surface. Because there are fewer clouds over dry areas of the earth, net radiation is lowest and decreases in polar regions to about 40% and in the tropics to 70% of the radiation in dry areas. The global distribution of radiation reflects the global temperature and in particular low temperatures and frosts.

Only 12% of the outgoing radiation from the earth's surface reaches the universe directly and without change by interaction with gases or clouds in the atmosphere (see Fig. 2.1.2). The effect of trace gases on climate is based on their effect on the outgoing radiation from the earth. This effect is often called the greenhouse effect. As window glass is more permeable to shortwave radiation than to long-wave radiation, short-wave radiation passes through the window glass without resistance and is absorbed by the surfaces of the room. Long-wave radiation is emitted at the temperature of the room, but cannot pass the glass. It is thus "trapped" in the room - as a consequence, the temperature in the greenhouse increases as energy enters, but does not exit. Trace gases in the earth's atmosphere are analogous to the window pane, with water vapour, C02, methane and other trace gases (comparable to the quality of the glass) absorbing long-wave radiation and this influences the temperature of the atmosphere and thus a very large increase in these gases influences the climate (Stott et al. 2001). This outgoing window through which long-wave energy is emitted is made even smaller by ozone and anthropogenic chlorofluorocarbons (CFCs). These trace gases absorb exactly in the long-wave length and so are in the maximum range of outgoing radiation and are thus significantly more effective than trace gases such as C02, methane and nitrous oxide gas (N20), which only decrease the outgoing radiation at the edge of the absorption spectrum (i.e. make the atmosphere more impermeable for long-wave heat radiation; via the "greenhouse effect").

The course of changes in temperature and C02 concentration during the last ice ages is surprisingly similar (Fig. 2.1.4) pointing to a causal connection. The earth, during its history, has always been subjected to considerable oscillation in temperature and C02, as shown in Fig. 2.1.4 A. However, organisms usually had to cope with lower temperatures than those of the present day. Most plant species developed before the climatic changes in the Tertiary, and yet were able to survive these changes. Figure 2.1.4 B shows the changes in the C02 concentration in the last 1000 years. From the course of the curves over this period it becomes clear that early tree felling by humans in central Europe (first to third period of deforestation: 800-1300, see Chap. 4.1) did not change the C02 concentration of the atmosphere because the release of C02 occurred in a predominantly undisturbed environment which assimilated the C02 again. C02 concentration has only increased since the eighteenth century, initially slowly but exponentially since the middle of the twentieth century. Pre-industrial C02 concentrations are assumed to be 280 ppm. Currently, the increase is 1.5 ppm/year. The C02 concentration and the temperatures have reached values never before reached in the recent history of earth. Global effects are discussed in Chapter 5.

The significance of increased trace gas concentrations for the climate can be seen for an event in the earth's history (Norris and Röhl 1999). The boundary between Palaeocene and Eocene

180 "E

180 "E

Minimum temperature

| Fig. 2.1.3. A Global distribution of solar radiation (short wavelength incident radiation of the sun at the earth's surface); B global distribution of frosts (drawn by J. Kaplan; A, after http://daac.gsfc.nasa/CAMPAIGN_DOCS/FTP_SITE/INST DIS/readmew/sre rad.html; Bishop and Rossow 1991)

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