For anyone with a passing interest in global climates, present & past, the UK's Birmingham University has put out an excellent article looking at the conditions that could have prevailed on a'snowball Earth'.
The term 'snowball Earth' refers to a time (about 630 million years ago) when our planet was in the grip of a major ice age & almost entirely glaciated. Birmingham researchers have come up with evidence (in the form of oxygen isotopes from pre-Cambrian rocks) supporting the contention that such icy conditions could - paradoxically - have co-existed with an ancient atmosphere rich in carbon dioxide. It does sound odd, doesn't it? But once the Earth's land masses had acquired even a partial coat of ice & snow, this would do two things: it would increase the planet's albedo (reflectivity), & it would also reduce the rate at which rocks weathered - & weathering removes CO2 from the atmosphere. At the same time, volcanoes are constantly adding CO2 to the air. So you could indeed end up with an ice-shrouded planet that also had an enhanced greenhouse atmosphere.
This isn't to say that the whole planet was covered with ice - many scientists suggest there must have been open water near the tropics, & indeed, it's hard to see how living things could have survived without such refuges. But condtions would have been very cold indeed over much of Earth's surface.
(Fairly predictably, at least one major daily newspaper in the UK has misinterpreted the research to mean that increasing levels of CO2 could trigger the snowball state, which is most definitely not what the Birmingham research suggests!)