In a greenhouse, movement of air is diminished, so the radiative portion of the energy drain must intensify. Physics can predict with certainty that in order to increase outbound radiation from a greenhouse, temperature inside the greenhouse must increase. A greenhouse must warm up.
But the story in the atmosphere is different. We are adding small amounts of infrared-absorbing gases (like carbon dioxide) to the atmosphere. This does not “trap heat” as the saying goes, instead it diminishes the efficiency of the radiation drain. So the fluid dynamics must adjust to maintain energy flow balance. In order to predict what will happen at the Earth’s surface, it is necessary to solve the equations governing fluid dynamics.
The problem is that no one knows how to do it.
McKitrick, like me, is someone with an economics background who is skeptical on global warming. He is much more conversant than I am on the scientific issues. Read the whole thing.