CO2 is an important greenhouse gas and is is one of the master pieces in the energy balance of the Earth and, therefore, in its thermal structure. There are a large number of studies performed from the last century to the present day and they show how the CO2 concentration is being continually increased. The impact of this increase in the cooling and heating of the different atmospheric layers is clear. For instance, the trend of the temperature in the troposphere has been positive meanwhile, in the mesosphere, has been negative. Since the beginning of industrialization, mankind has been emitting increasing amounts of harmful volatile substances into the atmosphere changing its composition but we still have to answer what are the effects of these emissions on the different observed atmospheric changes. Regarding the energy balance, the most important cooling mechanism of the stratosphere, mesosphere and lower thermosphere is the CO2 15 um emission. The daytime heating rate is mainly induced by the near-infrared CO2 bands (2.7 and 4.3 um) and it is also important. It is worth to highlight that these infrared emissions come from ro-vibrational transitions of the molecule and they are in a non local thermodynamic equilibrium (non-LTE) in the middle and upper atmosphere. From the first in situ rocket measurements in the seventies until the most recent satellite measurements from solar occultation instruments like ACE or limb emission instruments like SABER, we have significantly gained knowledge about the CO2 abundance. It is worth to mention the great difficulty of analyzing the infrared emissions from the different CO2 bands and to derive its abundance from these emissions, primary coming from the complex non-LTE processes.

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