At the end of Parshat Bereishet the Torah states, “And G-d saw that the wickedness of man was great upon the earth… I will blot out man, whom I created, from the face of the ground” (Ber. 6:5,7). G-d is stating clearly that man’s evil nature is the cause of the destructive flood. However, in Parshat Noach, at the conclusion of the flood, G-d says, “I will not continue to curse again the ground because of man, since the nature of man’s heart is evil from his youth; nor will I continue to smite every living being.” (Ber. 8:21). Here G-d is saying that man’s evil nature is the reason for not bringing another flood.
Abarbanel explains that man’s being “evil from his youth” is not a reason to be forgiven for transgressions. Rather, the punishments will take place over the course of a lifetime, beginning in his youth and extending into his old age, instead of being one cataclysmic event. Additionally, the verse concludes “…seedtime and harvest, cold and heat, summer and winter… shall not cease.” This indicates that future punishments will occur in the context of earth’s natural cycles of drought, famine, and extremes of heat and cold.
Prior to the flood, the earth’s rich physical environment led to gross materialism. Just as the first stage of an individual’s life is characterized by spiritual emptiness, so too the first stage in the development of mankind was characterized by spiritual emptiness, materialism and sensuality. The phrase “man's heart is evil from his youth” is a reference to this first stage. The flood obliterated this first stage and led to an attachment to spiritual pursuits, obviating the need for another flood.
The rainbow is then presented as the sign of the covenant that G-d will not bring another flood. Abarbanel points out a difficulty in understanding this narrative. First of all, a rainbow is a natural phenomenon which must have existed prior to the flood. If G-d is promising a new relationship with mankind, the sign should be something new as well. He answers that after the flood there was a change in the physical nature of the clouds themselves. During the flood they were so dense that no sunlight could penetrate to produce a rainbow. From this point onward they will no longer be so dense as to envelop the entire atmosphere in cloud. This would allow for the refraction of light necessary to produce the rainbow. This new atmospheric reality hints to the new status of mankind. There will no longer be one unified, immoral society. Some societies will sink into immorality, others will not. Just as rain will now fall in some areas and not in others, so too some regions will suffer due to the behavior of their inhabitants and others will not.
The rainbow is a reminder to mankind that G-d has modified nature and diluted the density of the atmosphere, precluding the possibility of another catastrophic deluge.