Re Snowball Earth, Constantin W. Arnscheidt and Daniel H. Rothman offer some ideas. The three factors are carbon, albedo, and surface irradiance from the sun. They're looking at irradiance.
These two're MIT but published to the Royal Society. I'd thought rather the point of Massachusetts was not to have to do that. Anyway.
Of carbon, albedo, and irradiance: albedo is a feedback-loop. The world gets whiter as it gets snowier. So for those looking into origins, they concentrate on carbon and irradiance. Carbon sequestration by plants used to be a hot topic but, I last heard, plants came 600+ million years earlier. These two researchers sketched up some equations for the factors mentioned. They found a short way to an ice age: 2 percent drop in incoming sunlight over a period of about 10,000 years
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This could be a solar-minimum; but these authors mainly look to volcano and cloud-formation. (We rule out meteors because they are one-and-done. Chicxulub certainly did not last as long as a million years.) For cloud-formation they're looking at photosynthesising life. Where they're looking, the Sturtian 717 Mya - "cryogenian" onset elsewhere - I'm not seeing life as a cloud-generator. In fact, I think clouds are more common when it is warmer.
BETTER IDEAS 2/15/24: The volcano lull.
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