Basic science at work here, the science of the earths temperature is some what different and is not a true greenhouse effect as just described. There is a warming effect its just based on different principles.
Greenhouses receive solar radiation through the glass. Essentially all of that energy has to escape back out of the greenhouse. The only place for it to escape is back out through the glass.
Heat flow between two points is proportional to the temperature difference, and also proportional to the thermal conductivity. The glass has low thermal conductivity, so the temperature inside the greenhouse has to rise to keep the heat flowing out at the same rate it is coming in – creating a large temperature drop across the quarter inch of glass.
Without the glass, the heat would escape much more efficiently, and the temperature would be lower. The energy flow through the system is the same with or without the glass, but the temperature is higher with the glass in place.
Centinel2012 