A scientific view of the greenhouse effect

Why do some molecules absorb infrared radiation and not others?  What is the mechanism for absorbing infrared radiation?

Some molecules are very stable in their electron configuration, so they don't absorb photons because it would take too much energy to knock an electron out of its normal orbit.  This is why glass is transparent when other materials are not.

 
The CO2 molecule can bend and twist making its electrons more exposed to photons.  When an infrared photon hits it correctly, the energy of the photon is absorbed which knocks one of the electrons to a higher orbit.  This is not the preferred state of the electron, so three nanoseconds later the electron falls back to its ground state.  However, it has to give up the energy it absorbed, so it emits an infrared photon.  Because of the random nature of quantum physics, the infrared photon is emitted in a random direction.  So the primary effect of CO2 in the atmosphere is to take infrared photons that were traveling up and away from the Earth and send some of them back down.  CO2 is very good at scattering infrared radiation.

The direct effect of CO2 in warming the atmosphere is not huge.  Climate Alarmism depends upon as-of-yet unproven positive feedback models.  There are many feedbacks positive and negative, and these are not fully understood.  Climate scientists admit that they do not yet fully know how to factor in the feedback from clouds, and there is widespread disagreement over clouds.  The alarmists are claiming positive feedback while the skeptics are claiming negative feedback.