Heat is constantly lost to space as infrared radiation. The greenhouse effect slows this loss, and is driven by the output of "greenhouse gases" into the atmosphere. These gasses absorb the infrared that would be lost to space, retaining them inside the "closed system".

Greenhouse gasses have natural sources, but far and away the largest sources are manmade in the form of carbon dioxide as industrial byproduct, and methane as agricultural byproduct.

This is why experts are so concerned: stopping production of manmade greenhouse gasses will not immediately reduce the amount of those gasses in the atmosphere, so heating will likely continue for many years after peak output.