Terrestrial Infrared Radiation And The Greenhouse Effect

Radiation from the sun is predominantly short-wave, whereas that leaving the earth is long-wave, or infra-red, radiation (see Figure 3.1). The infra-red emission from the surface is slightly less than that from a black body at the same temperature and, accordingly, Stefan's Law (see p. 33) is modified by an emissivity coefficient (e), which is generally between 0.90 and 0.95, i.e. F = eoT4. Figure 3.1 shows that the atmosphere is highly absorbent to infra-red radiation (due to the effects of water vapour, carbon dioxide and other trace gases), except between about 8.5 and 13.0 |m - the 'atmospheric window'. The opaqueness of the atmosphere to infra-red radiation, relative to its transparency to short-wave radiation, is commonly referred to as the greenhouse effect. However, in the case of an actual greenhouse, the effect of the glass roof is probably as significant in reducing cooling by restricting the turbulent heat loss as it is in retaining the infra-red radiation.

The total 'greenhouse' effect results from the net infra-red absorption capacity of water vapour, carbon dioxide and other trace gases - methane (CH4), nitrous oxide (N2O) and tropospheric ozone (O3). These gases absorb strongly at wavelengths within the atmospheric window region, in addition to their other absorbing bands (see Figure 3.1 and Table 3.3). Moreover, because concentrations of these trace gases are low, their radiative effects increase approximately linearly with concentration, whereas the effect of CO2 is related to the logarithm of the concentration. In addition, because of the long atmospheric residence time of nitrous oxide (132 years) and CFCs (65 to 140 years), the cumulative effects of human activities will be substantial. It is estimated that between 1765 and 2000, the radiative effect of increased CO2 concentration was 1.5 W m-2, and of all trace gases about 2.5 W m-2 (cf. the solar constant value of 1366 W m-2).

The net warming contribution of the natural (non-anthropogenic) greenhouse gases to the mean 'effective' planetary temperature of 255 K (corresponding to the emitted infra-red radiation) is approximately 33 K. Water vapour accounts for 21 K of this amount, carbon dioxide 7 K, ozone 2 K, and other trace gases (nitrous oxide, methane) about 3 K. The present global mean surface temperature is 288 K, but the surface was considerably warmer during the early evolution of the earth, when the atmosphere contained large quantities of methane, water vapour and ammonia. The largely carbon dioxide atmosphere of Venus creates a 500 K greenhouse effect on that planet.

Stratospheric ozone absorbs significant amounts of both incoming ultraviolet radiation, harmful to life, and outgoing terrestrial long-wave re-radiation, so that its overall thermal role is a complex one. Its net effect on earth surface temperatures depends on the elevation at which the absorption occurs, being to some extent a trade-off between short- and long-wave absorption in that:

1 An increase of ozone above about 30 km absorbs relatively more incoming short-wave radiation, causing a net decrease of surface temperatures.

2 An increase of ozone below about 25 km absorbs relatively more outgoing long-wave radiation, causing a net increase of surface temperatures.

Long-wave radiation is not merely terrestrial in the narrow sense. The atmosphere radiates to space, and

Table 3.3 Influence of greenhouse gases on atmospheric temperature.

Gas

Centres of

Temperature increase

Global warming potential

main absorption

(K) for X2 present

on a weight basis

bands (Jm)

concentration

(kg-1 of air)f

Water vapour (H2O)

6.3-8.0, >15 (8.3-12.5)*

Carbon dioxide (CO2)

(5.2), (10), 14.7

3.0 ± 1.5

Methane (CH4)

6.52, 7.66

0.3-0.4

1 1

Ozone (O3)

4.7, 9.6, (14.3)

0.9

Nitrous oxide (N2O)

7.78, 8.56, 17.0

0.3

270

Chlorofluoromethanes

(CFCy

4.66, 9.22, 11.82 6

0.1

3400

(CF2Cl2)

8.68, 9.13, 10.93

7100

Notes: * Important in moist atmospheres.

t Refers to direct annual radiative forcing for the surface-troposphere system.

Sources: After Campbell; Ramanathan; Lashof and Ahuja; Luther and Ellingson; IPCC (1992).

Notes: * Important in moist atmospheres.

t Refers to direct annual radiative forcing for the surface-troposphere system.

Sources: After Campbell; Ramanathan; Lashof and Ahuja; Luther and Ellingson; IPCC (1992).

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