The blanket analogy is not a bad one but students need to see where the analogy breaks down.
Some suggest that this phenomena should be called the atmospheric effect instead of the greenhouse effect
Global warming is the name given to the phenomena whereby the surface of the earth gets hotter.
Our planet is warmer with an atmosphere than it would be without.
The atmosphere is different than a greenhouse in that it radiates energy back to Earth rather than simply trapping energy inside.