E. Linacre and B. Geerts
It is unsatisfactory to use the term 'greenhouse' to explain the warming effect of the atmosphere, or certain gases within (such as water vapour and carbon dioxide). In a real greenhouse (a glasshouse in which crops are grown) the temperature is higher than outside in part because the glass prevents the convective transfer of heat. Outside, the daytime convective heating from the surface is rapidly dispersed. The so-called 'greenhouse gases' in the atmosphere operate in an environment of unrestricted convection.
The Earth’s surface is affected by two distinct kinds of radiation - visible radiation from the small area of the Sun, and infrared radiation from the sky all above us, even in the absence of clouds. The latter ('sky radiation') depends on the temperature of the lowest part of the troposphere, which in turn is governed by the absorption of upward radiation from the surface (the ‘terrestrial radiation’). Greenhouse gases increase the efficiency of that absorption, so the sky becomes warmer and radiates more heat back to the ground, which therefore cools less. To maintain an energy balance, the Earth surface must be warmer, hence emitting more radiation (Stefan Boltzmann's equation, Note 2.C). In short, the so-called greenhouse gases are more transparent in the visible than in the infrared spectrum. This effect should be called the 'atmosphere effect' because it typifies the atmosphere of the Earth and other planets, not the greenhouse effect, according to Fleagle and Businger (1).
The same fallacy applies when referring to the atmosphere as a 'blanket' (2). Blankets on a bed warm by preventing convection from the sleeper, whereas greenhouse gases warm the Earth's surface by selective transfer of radiation. Also, there can be no ‘trapping’ of heat by the gases; they simply absorb radiant energy, raising their temperature and hence their emission of radiation. There is no retention of heat, implied by the word ‘trapping’.
(1) Fleagle, R.G. and J.A. Businger, 1963. An introduction to atmospheric physics. Academic Press, 346 pp.
(2) for more info, see Alistair Fraser's web site