"High emissivity" refers to a surface or material's ability to easily emit or release heat or infrared radiation. In simple terms, it means that the object can quickly and efficiently give off heat or energy. Full definition
Ceramic has a very high emissivity rating, meaning it produces a lot of infrared heat. (infraredsauna.com)
So the atmosphere is heated by IR from the sun and by convection of heated air (taking H2O with it, and some CO2 with that) from below and by OLR from surfaces with higher emissivities, topped by asphalt near 99 %. (wattsupwiththat.com)
The high emissivity of CO2 in the IR actually contributes to our radiative equilibrium temperature being another 20K or more lower than that but I'll wait until somebody is interested in implementing the computations in CoSy or puts a table, not a graph, of an actual measured mean spectrum in my lap. (wattsupwiththat.com)