Humans, far too many of us on the planet and far too many of us consuming goods.
As I understand it, any gas made of differing atoms, CO2, Methane CH4, Sulphur Hexaflouride (SF6) are green house gases. I think the bond between the atoms is the energy absorber bit (any clever bods care to correct me). Methane is more absorbing gas than CO2 and SF6 is orders of magnitude worse than both but so far there is comparatively little in the atmosphere but its very persistent and as more 80’s/90’s/00’s vintage HV electrical switchgear is decommissioned more will escape.
Methane levels will increase dramatically as the tundra permafrost melts and the old frozen vegetation starts decomposing, also if the ocean temperature rises on certain bits of the continental shelf ridge bottoms the methane hydrates trapped by low temps and high pressure could degas and there are many millions of tonnes down there.
Volcanoes can warm and cool as the the CO2 emitted acts as a energy trap and the fine particulates that get lofted into the upper atmosphere in BIG eruptions acts as a sunshine reflector so cooling.