what causes the greenhouse effect in earth atmosphere

It's pretty simple actually, it all comes down to density. When I'm out at sea, you notice these massive ice chunks bobbing around, and it's because ice is just less dense than the salty water surrounding it. Since the molecules in ice are spread out a bit more than in liquid water, the whole thing ends up being lighter for its volume.

Think of it like this: if you had a bucket of seawater and dropped a chunk of ice in, the ice displaces a weight of water equal to its own weight. Because seawater is denser due to all that salt, it pushes the ice up. That's why only about 1/10 of the iceberg is actually visible above the surface, while the rest of that beast stays hidden deep underwater.