Hyde et al. reply

We are not convinced that the data discussed by Schrag and Hoffman can be interpreted in only one way. With respect to the duration of glaciation, calculations1 suggest that the open-water solution could have persisted up to CO2 levels of about four times those at present. With buffering from the reactive ocean carbonate reservoir, the time required for degassing to raise atmospheric concentrations to this level would be about 0.2–0.6 million years (L. A. Derry, personal communication). As ice sheets would still discharge ground carbonate into the ocean, the buffering time would be extended. This timescale is consistent with range estimates derived from palaeomagnetic data2 and geochemical calculations3.

Our open-water results also agree with the environment of deposition of some glaciomarine sediments4. Iron is not a consistent feature of Neoproterozoic glacial sequences5. Meltwater from glacial calving could suppress water-column convection and decrease deep-sea oxygen; overturn6 could contribute to the low δ13C in postglacial sequences.

The idea that a modest CO2 increase would eliminate tropical glaciation in the open-water solution assumes a linear response, which is in contrast to many examples in the Pleistocene and the model used in our study1,7. Increases in CO2 produce virtually no meltback until a threshold level is reached1, whereupon a small increase causes rapid melting. Calculated ice-sheet retreat times of tens of thousands of years agree with sedimentological estimates8. Isostatic depression from the large ice sheets we describe could accommodate a thick layer of postglacial carbonate (only the thin bottom layer of which is cap rock in the strictest sense).

We are not as sanguine as Schrag and Hoffman about the ability of metazoans to survive under the extreme conditions of a hard snowball Earth. Whether life could survive on a few scattered volcanic islands is a matter of conjecture. A “thin-ice” scenario9 is not consistent with results10 indicating that such regions have temperatures substantially colder than those estimated in ref. 9 (with implications for sea-ice thickness). Although evidence for life extends almost to the oldest rocks, three billion years transpired before the appearance of metazoans. This vast time interval suggests that the environmental tolerance of metazoans is much narrower than that of their simpler and hardier colleagues. If deep waters were anoxic, they could not survive on deep-sea chemosynthetic communities either, as these organisms still require free oxygen.

Future data may call for the reassessment of our open-water scenario, but we consider that the hard-snowball scenario is not yet proven. We believe that the open-water solution is much more favourable for the survival of metazoans, allowing their remote progeny to continue this discussion.