Scientific Dispute
The argument against the hypothesis is evidence of fluctuation in ice cover and melting during "Snowball Earth" deposits. Evidence for such melting comes from evidence of glacial dropstones, geochemical evidence of climate cyclicity, and interbedded glacial and shallow marine sediments. A longer record from Oman, constrained to 13°N, covers the period from 712 to 545 million years ago — a time span containing the Sturtian and Marinoan glaciations — and shows both glacial and ice-free deposition.
There have been difficulties in recreating a Snowball Earth with global climate models. Simple GCMs with mixed-layer oceans can be made to freeze to the equator; a more sophisticated model with a full dynamic ocean (though only a primitive sea ice model) failed to form sea ice to the equator. In addition, the levels of CO2 necessary to melt a global ice cover have been calculated to be 130,000 ppm, which is considered by some to be unreasonably large.
Strontium isotopic data have been found to be at odds with proposed Snowball Earth models of silicate weathering shutdown during glaciation and rapid rates immediately post-glaciation. Therefore, methane release from permafrost during marine transgression was proposed to be the source of the large measured carbon excursion in the time immediately after glaciation.
Read more about this topic: Snowball Earth
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