A fundamental assumption of the AGW (Anthropogenic Global Warming) hypothesis is that when a new source of heat appears in the atmosphere, the average temperature of the entire atmosphere will rise. In particular, AGW states that increasing CO2 (carbon dioxide) concentration will absorb heat emitted by the earth's surface, thus creating a new heat source in the atmosphere, and this new heat source will cause global warming. As we have said before, this hypothesis has no empirical basis. The hypothesis may sound eminently reasonable, but that's what we demand of a hypothesis: it must be eminently reasonable so that we are motivated to test it. Until we have performed the experiments, however, a hypothesis remains a hypothesis.
The first house I lived in with a real fireplace was in Boston. It was −20°C outside in the winter of 2000. I built a big fire in the living room, telling my children that they would be extra-warm that night, because the fire was going to warm up the whole house. I piled on the wood, until the fireplace was a blaze of flame over a bed of coals. The living room was hot and dry. I was well pleased with my efforts. But when I put the kids to bed, I found that their room was cold, too cold for them to sleep comfortably. Before I lit the fire, their room had been warm.
This outcome was surprising to me, but I have observed the same thing many times since, also in our new house, where we have a wood stove in the living room. Indeed, if I want my children's bedrooms to be warm, I know I have to refrain from lighting a fire in the living room.
Either that, or I have to move the thermostat out of the living room.
The climate is a far more complex system than my house. It may be that more CO2 will warm up the world. On the other hand, it may be that more CO2 will bring about the next ice age. More likely, in my opinion, is that more CO2 will have no measurable effect at all upon the global temperature. I could make a compelling argument for any of these outcomes. But, as we have said already, a compelling argument doth not a scientific theory make.
UPDATE: For our warm-climate readers, see comments for explanation of "thermostat".