Earth thermal balance

A second problem presented by the increase in carbon dioxide stems from its effect on the thermal balance of the earth. The sun is at a temperature of about 6000 Kelvin and emits most of its energy in the visible and near infrared portion of the spectrum. The atmosphere and the carbon dioxide are transparent to the wavelengths of this portion of spectrum. These wavelengths pass through the atmosphere and warm the surface of the earth. Because of its transparency for short wave length solar energy, carbon dioxide does not reduce the amount of energy received at the earth s surface. ... 

In addition to screening out harmful short-wavelength radiation, the atmosphere is essential in maintaining a reasonably uniform and moderate temperature on Earth s surface. Earth is in overall thermal balance with its surroundings. This means that the planet radiates energy into space at a rate equal to the rate at which it absorbs energy from the Sun. V FIGURE 18.11 shows the distribution of radiation to and from Earth s... 

A FIGURE 18.11 Earth s thermal balance. The amount of radiation reaching the surface of the planet is approximately equal to the amount radiated back into space. 

Consequently, the net OLR has increased by 0.006 W m-2 since 1880. This means that about one-third of today s thermal pollution (0.02 W m-2) is emitted from Earth. In the long term our use of non-renewable energy will cause a global temperature increase up to a point where the net OLR balances the net heat generation. Meanwhile, nature has some means of delaying global warming. 

Since the earth has temperature, it emits radiant energy called thermal radiation or planetary infrared radiation. Measurements by satellites show an average radiant emission from the earth of about 240 watts per square meter. This is equivalent to the radiation that a black body would emit if its temperature is at -19°C (-3°F). This is also the same energy rate as the solar constant averaged over the earth s surface minus the 30% reflected radiation. This shows that the amount of radiation emitted by the earth is closely balanced by the amount of solar energy absorbed and since the earth is in this state of balance, its temperature will change relatively slowly from year to year. 

The amount of energy the earth absorbs from the sun is the same amount it radiates back to space on average over the 500 trillion square meters of surface area. Satellites above the earth s atmosphere can measure the outgoing thermal radiation and show this balance to a high degree of precision. 

Although the continental crust constitutes only 0.6% by mass of the silicate Earth, it contains a very large proportion of incompatible elements (20-70%, depending on element and model considered Rudnick and Fountain (1995)), which include the heat-producing elements and members of a number of radiogenic-isotope systems (Rb-Sr, U-Pb, Sm-Nd, Lu-Hf). Thus the continental crust factors prominently in any mass-balance calculation for the Earth as a whole and in estimates of the thermal structure of the Earth (Sclater et al., 1980). 

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