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And maybe these cycles are how the planet self regulates?

The point is it doesn't self-regulate, which as the poster pointed out is why it veers precariously into and out of ice ages and thermal maxima.

If these fluctuations aren't part of a roughly stable pattern of self-regulation, why hasn't Earth gone into such an extreme ice age or thermal runaway that it it's unrecoverable and can't support life? Would you say it's externally regulated as opposed to self-regulating, maybe? Sheer luck?

Well, Earth’s orbit is roughly stable, solar output is roughly stable but both have long term oscillations but within somewhat stable parameters. That’s just luck.

There are some effects that cause unstable feedback loops though. Rising temperatures trigger several effects that lead to even more temperature rise. Reflective ice caps melt, tundra thaws releasing methane. Frozen methane in ocean sediments melts. Similarly our planet has been through several deep freeze events, far more severe than ice ages. If temperatures drop too much the ice reflects too much solar radiation leading to more freezing.

“We are lucky” is certainly a viable solution to the Fermi paradox.

Venus had thermal runaway and the precipitation there is lead rather than water.

Martian winters cause ~25% of the atmosphere to condense into the polar (dry)ice caps.

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