The severity of the ice ages of the past two million years was eclipsed by glaciations in the distant past. The period between 820 and 550 million years ago saw intermittent ice ages in which even the tropics were covered by glaciers. According to the 'Snowball Earth' hypothesis, the Earth was completely glaciated, from the poles down to the Equator. Perhaps even the oceans froze, down to a depth of a kilometre or more. The planet was only saved from a permanent freeze by a massive injection of carbon dioxide from volcanoes, a 'greenhouse gas' that melted the ice. The problem with this idea is that life, already well-established, might easily have been driven to extinction on a Snowball Earth.
One way of explaining the coexistence of tropical ice-sheets and the continuation of life is to imagine a system of climate turned on its head, with the tropics colder than the poles. That way, glaciers could encircle the tropics, while life continued to flourish elsewhere, even at ice-free poles. This idea is perhaps even more bizarre than the Snowball Earth idea.
This is a preview of subscription content, access via your institution