http://eesc.columbia.edu/courses/ees/slides/climate/absorption.gif is probably a better picture,
This can't be right. The outbound, reradiated IR heats the plastic? No matter how thin? Bounces off? (There are hundreds of ridiculous diagrams in Google images depicting this.)
--actually, if IR radiating from earth surface hits an absorbent material (CO2, or plastic, or glass) then that material will then re-radiate that IR in both up & down directions. If up, then another absorb and re-radiate event likely. If down, the earth re-absorbs the heat. The net effect is not that the earth is entirely prevented from losing heat, it is merely a slowdown in the rate of heat-loss for a given earth-temperature. I.e. "insulation." The CO2 effect is as though the earth had reduced black body emissivity but same black body absorptivity. Mildly related: It is estimated that heat produced in sun core takes 200K years to escape, http://articles.adsabs.harvard.edu/full/1992ApJ...401..759M/0000760.000.html since the sun's gas is a thick layer of insulation that slows down radiative transfer.