A reference book published by the Office of Naval Research, a department of the U.S. Navy, titled The Infrared Handbook was published in 1978. Atmospheric transmission data at sea level is contained in this book based on measurements that were taken in the 1950 time frame, much before any recent increases in the concentration of CO2 in the atmosphere. These particular measurements are over a path length of 300 meters, and cover the IR spectrum from short-wave infrared out to beyond 20 microns in the LWIR (see “Field Measurements of Atmospheric Transmission”).
In the LWIR absorption band of CO2 (center wavelength of 15 microns) the transmission measured is 0.0 due to CO2 absorption. That is, total 100% absorption over 300 meters at sea level in the spectral absorption band of CO2 that would capture the most “heat” being radiated by the earth’s surface.
Increasing the concentration of CO2 in the atmospheric mixture of gasses present in the 1950s will not increase the measured absorption in the CO2 LWIR band above the 100% level that was measured and reported in The Infrared Handbook. You cannot get more than 100% absorption.