The greenhouse effect is not a bad thing. In fact, without greenhouse gases like carbon dioxide, life on earth would not be sustainable. Without greenhouse gases, solar radiation would simply bounce off the planet and escape into space. The earth would be a frozen ball of ice. Greenhouse gases serve as a global blanket, keeping the earth warm enough to sustain life. Thank goodness for the greenhouse effect, and thank goodness for greenhouse gases.
But what if human activity dramatically altered the natural levels of greenhouse gases in the atmosphere? If greenhouse gases dropped significantly, the global blanket would be threadbare and the planet would cool. If greenhouse gases rose substantially, the insulating effect would increase and the earth would warm. That's the theory, and we are all living through the experiment.