sky holes
there are two major holes in the sky. ie two transparent windows in the absorption spectrum of the atmosphere. one is where sunlight shines through. the other one is in the far infrared where the earth's heat radiates away. that's no coincidence. the sun dumps heat energy onto the earth. if the earth is colder than the window the heat is trapped. and the earth warms until the heat can get out through this window. obviously, things are a lot more complex than that. but that's the general idea. now add lots of co2 to the atmosphere. co2 has an absorption peak right smack at the cold end of the window. which effectively shifts the window toward higher frequencies. in other words, the earth has to get warmer before the heat can escape. omg everybody panic. well not yet. once the earth gets hot enough to reach the window the heat can escape. no matter how much additional co2 you add. whew.