There is something I don't quite understand about the Greenhouse Effect and Effective Temperature, so please bear with me as I lay out what I do understand and where I begin to stumble.
The effective temperature of the Earth is roughly 12 degree Fahrenheit, which means that in thermal equilibrium when it is emitting as much radiation as it is absorbing, the distribution of wavelengths and intensities of electromagnetic radiation an observer in outer space would measure would correspond to the temperature of a blackbody object at 12 degrees Fahrenheit.
Of course, the surface of the Earth is actually much warmer than 12 degrees Fahrenheit due to the Greenhouse Effect. If there are greenhouse gases in an atmosphere then some fraction of infrared radiation from the terrestrial surface is absorbed by the atmosphere and re-emitted toward the ground. This re-emission of infrared radiation to the ground is like a trap for energy and over time it will cause the surface temperature of the planet to rise. This is right about where I have actual questions:
If the terrestrial surface is warming it will have to emit even more infrared radiation back to space. Is the atmosphere (and greenhouse gases) now absorbing the same fraction of a now larger amount of infrared radiation? in order to eventually rebalance incoming and outgoing radiation? I was thinking that the surface will grow warmer and emit more radiation until the amount exiting equals the amount incoming, establishing the new thermal equilibrium.
I often get confused because most diagrams for the Earth energy budget and net radiation balance show more infrared leaving the surface than is ever supplied by solar radiation at the top of the atmosphere in the first place. Is this because they count percentages or units of radiation twice? because some is re-emitted back?
I have included a photo showing how the electromagnetic wave types are partitioned in the radiation balance. It is from NASA's CERES page. As you can see, more infrared is leaving Earth than enters as solar radiation and yet this is still a picture of equilibrium where the net radiation is zero.