@pschwede Sure.

Basically the way the greenhouse effect works is that CO2 and other greenhouse gases are opaque to infrared light. they may or may not be opaque to visible light, for simplicity sake lets just talk about visible light and infrared light as they are really where most of the heat energy is from anyway.

so for infrared light (what we call radiative heat) the atmosphere is partially opaque and looks like a partially blackened cloud. As such when IR reaches the earth part of that IR is absorbed by the GH gas and heats up the atmosphere directly. To be specific approximately half of the IR light that reaches earth is absorbed by the earths surface specifically, heating it up. In turn some of that IR is also reflected by the surface, and on its way back out to space goes throught he black atmosphere a second time and even more is absorbed.

this means everything heats up. As we know normally things like planets in space, if they are hot, will slowly cool down. They do this by releasing heat energy as IR, radiative heat.

The surface is constantly giving off IR in an attempt to cool itself. If there were no greenhouse gases the atmosphere would be transparent and the IR would just escape into space, cooling everything. But since the atmopshere is opaque to heat the heat just gets reabsorbed by the atmosphere and cant escape.

Even the heat int eh atmosphere itself cant escape very well. When hot air near the surface of the earth gives off its IR in an attempt to cool itself off it cant because it just gets reabsorbed higher int he atmosphere again.

So while a planet with no GH gas could easily allow its heat to escape through a transparent atmosphere when GH is present the heat can no longer escape.

This effect is even worse on visible light. Since the atmosphere is transparent (mostly) to visible light nearly 100% makes it way through the atmosphere to the earth. It is then absorbed and converted to heat and thus when it escapes the same energy that had an easy time getting to earth as visible light is now IR and cant escape. So things heat up even more.