Jaguarbass,
I agree that the atmosphere would have changed.
Genesis 1:7 "So God made the expanse and separated the water under the expanse from the water above the expanse. And it was so."
The sky is the expanse. The separation was the canopy of water above the expanse.
The water under the expanse was that which came from springs that watered the earth in the form of a mist. The canopy acted as a barrier that held in the heat from the sun so that the earth was like a warm hot-house around the entire planet. It also kept out harmful radiation from the sun.
To recannopy the planet with a swaddling band of vaporous water, the ice must be melted. When the ice at the poles melt, the atmosphere will become heavier with evaporated waters, uplifted as water vapor. The earth will interact with volcanic activity to begin the process of creating the canopy to trap the moisture in the upper atmosphere. This is why no worldwide flood can ever occur again even though the polar ice caps will all become extinct.
By using mankind's own misuse of this planet, God brings his will into harmony. This planet was flooded long ago and the excess waters were frozen until such time in the future when those reserves would melt and be released into the atmosphere.