The distinction between the greenhouse effect and real greenhouses
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While there are some similarities between the atmospheric "greenhouse effect" and the heating mechanism of the structure from which the name is derived there are important distinctions as well. A greenhouse heats its enclosed space primarily by the prevention of convection cooling. Because a greenhouse is transparent to sunlight, solar energy passes through the enclosure unimpeded and warms the ground inside. The warm ground in turn warms the enclosed air which continues to heat because unlike the warm air near the surface outside, the trapped air is prevented from rising and pulling in cooler air behind it.
The climatic greenhouse effect is similar in that like the greenhouse, the atmosphere passes sunlight nearly unimpeded. Both also limit the rate of thermal energy flowing out of the system but the heat trapping mechanisms are very different. The atmosphere, which like a glass enclosure, is transparent to sunlight but it is not transparent to the energy being radiated back up from the earth's surface. Some of this radiant energy is reabsorbed by greenhouse gases and is prevented from escaping into space. This reabsorbed energy keeps the planet warmer, much the way a blanket warms us by preventing our body heat from escaping.