carbon dioxide is heavier than air, so how there is a layer of CO2 is forming around the atmosphere?
As far as I know there is no layer of CO2 forming around the atmosphere. Do you have any source to backup the claim?
Then what is the reason of Global Warming?
Global warming is a banned topic here, but the question is actually not about GW. You are misunderstanding the mechanism of the greenhouse effect. It doesn't require a layer of carbon dioxide anywhere, all it requires is a presence of the carbon dioxide in the atmosphere.
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