The greenhouse effect is caused by the warming of?

The planet(s) through solar radiation and the retention of a small portion of this heat through atmospheric gases.

The greenhouse effect is caused by the warming of the greenhouse gases in the atmosphere, mostly water vapor and carbon dioxide. This is a natural effect that has gone on for millions of years.

The enhanced greenhouse effect, what we are experiencing now, is caused by the warming of the extra greenhouse gases that man is adding to the atmosphere, largely by burning fossil fuels, and by deforestation.