How Should Snowball Earth Deglaciation Start.

The formation of melt ponds is pervasive on sea ice and ice shelves prior to their disintegration. Such process should be critical for the deglaciation of a snowball Earth but has never been considered in previous studies. Here we develop a module to explicitly track the initiation, growth and refreezing of melt ponds on ice. Incorporation of the module into a climate model indicates that it provides a strong positive feedback to the climate. Deglaciation of a snowball Earth will start when the annual mean equatorial surface temperature is only −7.7°C rather than 0°C. At this point, seasonal melt ponds start to appear in the mid‐latitude region. Its positive feedback induces the appearance of perennial melt ponds within the equatorial region and can increase the annual mean temperature there to 6.1°C in less than 10 years. Thus, our results indicate that the threshold CO2 required to deglaciate a snowball Earth will be greatly overestimated (by a factor of ∼4 for the model we use) if the annual mean surface temperature reaching 0°C is used as the criteria. The results also demonstrate unambiguously that the deglaciation of snowball Earth should start from the equator. We then speculate on what will happen to the tropical sea glacier after the surface melting starts. Plain Language Summary: The whole Earth was covered by thick ice during a snowball Earth event. When and where the deglaciation of such an event should start have not been answered. Previous studies judged whether atmospheric CO2 level was high enough to deglaciate a snowball Earth by checking whether the annual mean surface temperature at the equator approached 0 °C. However, seasonal melting of ice must have appeared at a much lower CO2 level in the subtropical region where summer is warm. Melt water reduces surface reflectivity and warms the surface, which will induce more melting and form a positive feedback loop. We find that this feedback is very strong; widespread melting occurs once the annual mean equatorial surface temperature approaches ‐8 °C, much earlier than thought before. Therefore, the threshold CO2 level that is required to deglaciate a snowball Earth was greatly overestimated in previous studies. Moreover, our results show that although melt water appears first in the subtropical region during summer, pervasive perennial melt water appears only around the equator due to stable high solar insolation there. This melt water is conjectured to break the thick sea ice near the equator and start the deglaciation of a snowball Earth there. Key Points: Melt ponds must be considered when studying snowball Earth deglaciationSnowball Earth deglaciation starts when the annual mean equatorial surface temperature reaches approximately −8°CSnowball Earth deglaciation starts from the equator not the subtropical region [ABSTRACT FROM AUTHOR]