To a first approximation, the earth currently radiates out as low frequency thermal radiation the same amount of energy as it absorbs as high frequency solar radiation. (This ignores energy generated within the earth, which is also radiated away. But that amount is constant and is not relevant to my question. It also ignores energy stored or burned as fossil fuels.)
Let's assume that global warming will not change the amount of energy received from the sun and absorbed by the earth. (I realize that's not true. Global warming melts the ice caps, which reflect solar radiation. With the ice caps melted, the earth absorbs more of the solar radiation it receives.) But if we ignore the melting of the ice caps, the earth must receive and radiate away a fixed amount of solar radiation, which is independent of its temperature.
I would have thought that a warmer earth would radiate more thermal radiation than a cooler earth. But the argument above says that's not the case. How is this explained?