From these papers, one can trigger a full snowball Earth with a sufficient reduction in atmospheric CO2. (skepticalscience.com)
Of course if you are talking about snowball earth, that's another matter entirely. (julesandjames.blogspot.ru)
One or more ensued as the recovery from Snowball Earth episodes (a fascinating story, but there is no room to detail this here). (dotearth.blogs.nytimes.com)