How can greenhouse effect cause global warming?

We'll get an atmosphere like Venus that traps too much heat.

think about it this way.  First you have to understand how a greenhouse works.  A very quick explanation for a greenhouse: the heat gets trapped inside and keeps it the plants warm or even hot.  This is how greenhouses are able to grow plants so well.  Think of the earth in the same manner.  There are plants and water inside the greenhouse (earth).  The CO2 and other gasses keep "trap" the heat inside the greenhouse (earth ) making heating up the greenhouse.  This greenhouse effect is heating up the whole earth making areas where they should normally be cold much warmer on "global scale."

The science behind Greenhouse Gases;

Greenhouse gases are rather transparent to visible light, but they absorb and scatter IR Light, which is the frequency range of Earths blackbody heat radiation. The Sun shines down through the atmosphere and heats the Earth. The Earth at the same time is always radiating IR blackbody radiation back into space creating a heat balance. Some of the IR light, however, is absorbed and scattered by the organic greenhouse gases. Therefore, some of the heat that would have escaped into space doesn't. This continuous action over a long period of time very gradually adds up until a different new and hotter heat balance is achieved.

it causes global warming because heat rises we have a layer of Co2 that reflects the heat back to earth instead of going out to space this causes heat to just reflect back and forth in the earth so it is warmer