You said — Which is rather curious, when you recall that rainbows were first mentioned in conjunction with the flood. It is not as though the Genesis account said it had always rained, and then magically rainbows only appeared *after* the flood.
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And also, it’s something that has been postulated by those scientists who have been studying these things, having to do with the Flood, that the atmosphere was a lot denser, too — perhaps by a factor of two, with a lot more oxygen in it — making it a completely different atmosphere than we are accustomed to right now.
It would have been oxygen enriched, a more evenly-distributed climate, growing a lot bigger plants and/or animals, too, and no rain, as the Bible indicates.
And you’re right, the “sign of the rainbow” is something that was given after the flood as the promise from God that He would never bring a world-wide flood again. There’s a good scientific reason for that, too.
True and honest science does complement and is in complete accord with God’s own witness to us, in the Bible, of what actually happened.
Agreed on the atmosphere and so on, this would have kept out alot of the UV and had alot to do with the much longer life spans pre-Noah