This theory posits that, several times in Earth’s history, oceans froze, ice sheets covered the tropics, and global temperatures plummeted to −50°C, so that the entire Earth may have actually frozen over, resembling a ‘snowball’, and potentially causing some of the most severe crises in the history of life on the planet. Such episodes may have been triggered by a reduction in greenhouse gases in the atmosphere, principally CO₂ (carbon dioxide) and CH₄ (methane). This would have made the global climate colder, creating larger areas of ice and snow. This ice and snow reflects more solar radiation than does bare ground or liquid water, which creates a ‘positive feedback’. If the Earth ever became approximately half-covered by ice or snow, the feedback would become self-sustaining and glacial ice would rapidly spread to the equator.

http://www.snowballearth.org/overview.html Snowball Earth website.