earth is in thermal equilibrium from a lot of years. It receives thermal energy from the sun by means of the radiation
Q/t is the heat transmitted in the unit of time, A the surface of earth
Earth emitts thermal energy with the same law (with infrared radiation) but T1 is now the earth temperature and T_2 is near zero.
Earth is cooling because it is losting its internal energy, due both to the initial high temeprature, both to radioactive phenomena, but it is warming because the emitted Ir radiation is partially hindered by water vapour and by greenhouse gas like carbon dioxide, NOx , freon, methane