As mentioned in Section 2.3, some terrestrial radiation escapes through the atmosphere by means of a window for radiation with wavelengths around 11 pm, though this is much less effective than the window around 0.5 pm for incoming solar radiation. In other words, the atmosphere is much less transparent to LW than it is to SW. This causes what is called the 'natural greenhouse effect' on global temperatures. Heat is trapped, so that the Earth's surface is warmed to around +15°C on average, instead of the -18°C that it would be otherwise (Note 2.L). Nowadays the additional carbon dioxide emitted by the burning of fossil fuels in recent decades (Section 1.3), together with increases in other gases such as methane, has created an enhanced greenhouse effect (EGE), an additional trapping of terrestrial radiation, leading to gradual global warming (Chapter 15).
The name of the effect arose from a mistake in explaining the warmth of a horticultural glasshouse. It used to be thought that the warmth arose from the properties of glass, which readily transmits solar radiation inwards but will not allow the escape of longwave radiation outwards from the surfaces within the greenhouse, as in the atmosphere. However, an experimental greenhouse of a material which does transmit longwave radiation proves to be equally warm. In fact, a glasshouse's temperature is mainly due to shelter from the wind. Nevertheless, the explanation which is incorrect for glasshouses does apply to the Earth's atmosphere, and the name has stuck.
The carbon dioxide atmospheres of Mars and Venus (Table 1.2) lead to appreciable greenhouse warming. The surface on Mars is at -47°C, instead of -57°C without the effect, and on Venus +525°C instead of +50°C [sic]. The fact that we can calculate the actually observed temperatures is some reassurance that we understand the greenhouse effect.
Greenhouse gases other than carbon dioxide are relatively small in amount but inherently much more potent. The global warming potential (GWP) of methane is about 50 (Section 1.3), i.e. its warming effect is 50 times that of a similar amount of carbon dioxide over a period of twenty years. For CFC-12 the GWP is 8,000, making elimination of this gas highly desirable.
The opposite effect results from haze in the sky, notably volcanic dust in the lower stratosphere (Note 2.G). It transmits terrestrial radiation, but absorbs and reflects solar radiation so that less reaches the ground. As a result, haze tends to cool the surface.
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