I'm not aware of any time when that was a mainstream belief, but there might be a gap in my history-of-science knowledge. In any case, while we're currently in

I'm not aware of any time when that was a mainstream belief, but there might be a gap in my history-of-science knowledge. In any case, while we're currently in an "interglacial" period with respect to the Pleistocene glacial cycles, we're actually still in an "icehouse" period by the big-picture definition, since there are continental icesheets on Greenland and Antarctica. By contrast, a lot of the rest of the Phanerozoic has been characterized by "hothouse" climates, where there are no major continental icesheets anywhere on Earth. It's possible that anthropogenic CO2 emissions will prevent us from going back into a colder "glacial" period when we otherwise next would have ~80 kyr from now, but geologic time is long and there is a lot of it still in front of us.