A Simple Model for Why the Greenhouse Effect Warms the Surface
March 18, 2018While working on the answers to Judge Alsop's second question a very simple model that explains what is happening occurred to Eli about the third,
3. What is the mechanism by which infrared radiation trapped by CO2 in the atmosphere is turned into heat and finds its way back to sea level?The thought goes back to the early days of thermodynamics when it was realized that heat flows and interrupting or slowing a flow while maintaining constant delivery rates requires increasing the pressure head of the pump
Heat from the Sun flows to the Earth's surface at a constant rate determined by its ~6000 K black body temperature. The Earth's surface transforms the visible light from the sun into ~290 K IR emission which escapes to space. The flow of energy from the Sun to the Earth has to EXACTLY (them's Nikolov caps folks) match the flow of energy from the Earth to space.
Greenhouse gases function as a regulating valve. If their concentration increases, the valve restricts the energy flow and the Surface has to pump harder to maintain the flow. If the concentrations decrease the pressure the pump is delivering decreases.
The operating mechanism of the valve is simple. The higher the mixing ratio (concentration) of greenhouse gases, the more absorbing the atmosphere is at frequencies that the greenhouse gases can absorb. The optical density of the atmosphere at those frequencies sets the level at which each greenhouse gas can emit to space without being re-adsorbed, i.e. it sets the level below which emission is blocked. The rate of emission from the level at which the greenhouse gas IR emissions can reach space thus rises with concentration. Since the temperature decreases with altitude, the higher the effective level, the slower the emission, the more the valve closes.
OK, make a copy of this and take it to your festive dinner. Haul it out when Uncle Ralph starts.