Although the increase in greenhouse gases is clear, the effect of these
gases on the energy flow of the Earth is difficult to establish due to the
complexity of the Earth (Crowley 2000; Cook et al. 2005; Oerlemans 2005).
The Earth’s temperature varies widely so the use of temperature requires
averaging over a certain number of locations. Instead of just relying on
recorded temperatures, scientists use global features as markers of tem-
perature. One prominent indicator of the long-term increase in the Earth’s
temperature is the retreat of glacial fronts (Figure 2.11). The position of
the ice shelves in the Antarctic Peninsula shows a pattern of loss that is
outside of normal cyclic behavior.
What is the actual contribution of the atmosphere to the Earth’s tem-
perature? A simple thermodynamic model of energy flow is that Earth is
an airless and rapidly rotating planet so that the temperature is uniform
(Sagan & Chyba 1997). In this case, the equilibrium temperature of the
Earth is determined by the distance between the Earth and the Sun, des, and
the solar radiance of the Sun, S, at the distance des:
(2.33)
According to this equation, the temperature of the Earth would increase
due to an increase in the solar radiation (S), an increase in the area of
the Earth (R^2 ), a decrease in the distance to the Sun (des), or a decrease
in the amount of light reflected back as measured by the reflection
coefficient (A). When the values for these parameters are inserted, the
temperature is calculated to be 255 K, or −18°C. The mean overall global
temperature has been measured to be 288 K, or 15°C. The 33°C differ-
ence in temperature is ascribed to the contribution of the atmosphere. The
temperature of the Earth does vary slowly with time due to factors such
T
SR A
des42
21
()
()
=
−
constantFigure 2.11Glacial retreat shown by comparison of satellite images of
Sheldon Glacier, Adelaide Island in March 1986 (left) and February 2001
(right). From Cook et al. (2005).