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Abstract: This paper discusses the effect of the greenhouse phenomenon and CO2 on global climate and suggests that numerical models that lack adequate knowledge of fundamental related factors cannot be used to extract “sound” conclusions. A very basic demonstration of this is done through a simple comparison between estimates of the forecast for global temperature increase obtained by various independent studies. Observing the global temperature and the CO2 atmospheric concentration though the geological aeons implies no obvious correlation. Physical observation on other planets like Mars and Venus, needing no numerical modeling, demonstrates the effect of the atmospheric-CO2 partial pressure on the temperature of the atmosphere. Moreover the CO2 role as a factor of danger or a benefactor for life is also addressed. On the other hand the role of the sun in the presently observed global warming has been greatly underestimated. Scientific evidence shows that the orbit of the earth and the Milankovitch cycles greatly affect the climate. A discussion follows pointing out the prime role that the sun should have on the earth's climate with regard to solar cycles’ activity and irradiance, cosmic rays and cloud formation. The conclusion drawn here is that a natural signal of solar forcing has been mistakenly overlooked for an anthropogenic change, maybe owing to their quite similar effects on climate. For the moment science does not really have a complete and total understanding of the factors affecting the earth's complex climate system and therefore no sound conclusions can be drawn. [Copyright &y& Elsevier]