Dr. Roy Spencer writes on Facebook:
Wayne Rowley has asked me to explain how a cold atmosphere can warm the Earth’s surface (which is what happens in global warming theory), a question I’ve been asked many times in the last 20+ years.
First of all, the temperature of anything depends upon the rates of energy GAIN and energy LOSS. When those 2 are equal, temperature remains the same; if they are unequal, the temperature changes.
Everyone knows that increasing the rate of energy gain increases temperature: e.g. turn up the heat under a pot of water on the stove, or turn up the thermostat in your house in winter.
But you can also increase temperature by reducing the rate of energy LOSS: put a lid on the pot of water while keeping the flame under it constant, adding insulation to the walls of a heated house while keeping the rate of furnace heating the same.
Now, note that in these examples, the lid is *cooler* than the heated water, and the walls (in winter) are cooler than the heated home interior, yet they can make the warmer object even warmer still. Your clothes in winter (or summer) keep you warmer than if you had no clothes on, even though the clothes are cooler than your body temperature. The examples are literally endless.
So, for the atmosphere, the net flow of infrared radiation from the surface to the “cold” depths of outer space is greatly reduced by the atmosphere (the so-called “greenhouse effect”), keeping the surface warmer than if the atmosphere was not there, absorbing and emitting its own infrared radiation. (An interesting side effect is that while the greenhouse effect keeps the surface and lower layers of the atmosphere warmer, the upper atmosphere is actually made colder. The same happens if you add more and more insulation to the walls of a heated house.)
How does this apply to global warming? Adding CO2 to the atmosphere from fossil fuel burning slightly enhances the atmosphere’s ability to keep the surface warmer by reducing the rate of energy loss by the surface. The question is, by how much? The *direct* effect of a doubling of atmospheric CO2 is small, only about 1 deg. C. But indirect changes in the atmosphere resulting from that direct warming (“feedbacks”) can either amplify it or reduce it. I believe those feedbacks will limit the warming to considerably less that what we are being told by climate modelers.