What is Snowball Earth; cap carbonate?

Example 18.16 (How long to escape?)

The highest preferred estimate of today’s volcanic release is $0.26\,\mathrm{Gt}$ of carbon dioxide a year. Estimates of the mixing ratio needed to end a Cryogenian snowball range from 0.01 to 0.1. With $M_{\text{a}} = 5.1 \times 10^{18}\,\mathrm{kg}$, $\bar M = 28.97\,\mathrm{g}\,\mathrm{mol}^{-1}$ and $M_{\mathrm{CO_2}} = 44.01\,\mathrm{g}\,\mathrm{mol}^{-1}$, $x = 0.01$ takes $\frac{0.01}{0.99}\times1.519\times5.1 \times 10^{18}\,\mathrm{kg} / 0.26 \times 10^{12}\,\mathrm{kg}\,\mathrm{yr}^{-1} = 0.30\,\mathrm{Myr}$, and $x = 0.1$ takes $3.3\,\mathrm{Myr}$.