The Greenhouse Effect

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The natural greenhouse effect of the earth's atmosphere is attributable primarily to water vapour. It accounts for 21 K of the 33 K difference between the effective temperature of a dry atmosphere and the real atmosphere through the trapping of infra-red radiation. Water vapour is strongly absorptive around 2.4-3.1 jUm, 4.5-6.5 jUm and above 16 jUm. The concept of greenhouse gas-induced warming is commonly applied to the effects of the increases in atmospheric carbon dioxide concentrations resulting from anthropogenic activities, principally the burning of fossil fuels. Sverre Arrhenius in Sweden drew attention to this possibility in 1896, but observational evidence was forthcoming only some forty years later (Callendar, 1938, 1959). However, a careful record of of atmospheric concentrations of carbon dioxide was lacking until Charles Keeling installed calibrated instruments at the Mauna Loa Observatory, Hawaii, in 1957. Within a decade, these observations became the global benchmark . They showed an annual cycle of about 5 ppm at the Observatory, caused by the biospheric uptake and release, and the c. 0.5 per cent annual increase in CO2 from 315 ppm in 1957 to 370 ppm in 2001, due to fossil fuel burning. The annual increase is about half of the total emission due to CO2 uptake by the oceans and the land biosphere. The principal absorption band for radiation by carbon dioxide is around 14-16 Um, but there are others at 2.6 and 4.2 jUm. Most of the effect of increasing CO2 concentration is by enhanced absorption in the latter, as the main band is almost saturated. The sensitivity of mean global air temperature to a doubling of CO2 in the range 2 to 5°C, while a removal of all atmospheric CO2 might lower the mean surface temperature by more than I0°C.

The important role of other trace greenhouse gases (methane and fluorocarbons) recognized in the 1980s and many additional trace gases began to be monitored and their past histories reconstructed from ice core records. These show that the pre-industrial level of CO2 was 280 ppm and methane 750 ppb; these values decreased to about 180 ppm and 350 ppb, respectively, during the maximum phases of continental glaciation in the Ice Age.

The positive feedback effect of CO2 which involves greenhouse gas-induced warming leading to an enhanced hydrological cycle with a larger atmospheric vapour content and therefore further warming, is still not well resolved quantitatively.

clouds are particularly effective since they act as black bodies. For this reason, cloudiness and cloud-top temperature can be mapped from satellites by day and by night using infra-red sensors (see Plates 2, 3 and 15, where high clouds appear cold). Radiative cooling of cloud layers averages about 1.5°C per day.

For the globe as a whole, satellite measurements show that in cloud-free conditions the mean absorbed solar radiation is approximately 285W m-2, whereas the emitted terrestrial radiation is 265W m-2. Including cloud-covered areas, the corresponding global values are 235 W m-2 for both terms. Clouds reduce the absorbed solar radiation by 50 W m-2, but reduce the emitted radiation by only 30 W m-2. Hence global cloud cover causes a net radiative loss of about 20 W m-2, due to the dominance of cloud albedo reducing short-wave radiation absorption. In lower latitudes this effect is much larger (up to -50 to -100 W m-2), whereas in high latitudes the two factors are close to balance, or the increased infra-red absorption by clouds may lead to a small positive value. These results are important in terms of changing concentrations of greenhouse gases, since the net radiative forcing by cloud cover is four times that expected from CO2 doubling (see Chapter 13).

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