Re: What sealed system would that be?
If you treat the planet as as 'black box' sealed system with specified inputs and outputs, those inputs and outputs have to balance. There's pesky things called the laws of thermodynamics and the conservation of energy which kinda demand that. If the input (the sun) stays constant*, then the output (infrared radiation of heat) has to stay constant and has to balance it, or the planet gets hotter, until the ouptut does balance the input. (loss of heat through radiation being proportional to the temperature). Increasing the amount of gases in the atmosphere which absorb infrared radiation at the wavelengths they are emitted leads to heating of the atmosphere, as less of that heat energy escapes the 'sealed system'. That is what the poster was referring to, and to deliberately mis-represent it does your argument no favours.
*It is actually rising, as the hydrogen gets burned up and heavier elements serve as fuel instead, but over a MUCH longer timescale than we'll ever be around for
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