Heating nonstable regime of resonanse absorption of intense radiation wide laser beam l= 10,6mm with atmospheric CO2 gase has been consedered. It is shown that it lead to threshold spectroscopic explosive absorption effect. The later consist of a rapid increasing of absorption coefficient Kn up to Kn+DKn, where DKn.Kn, because there is a strong selfheating of the beam channel during the time of interection. Explosive absorption developing time and the threshold beam parameters have been obtaind. Absorption coefficient for the case of atmospheric propogation of CO2 laser beam over the threshold regime of interaction have been calculated.