During ice ages, the whole world doesn't freeze. In fact, much of it is quite nice. The Sahara desert shrinks and much of it becomes grassland; Australia becomes quite lush.  The actual "habitable" land remains about the same.  Northern Europe and northern North America are lost, but the oceans lower due to increased glaciation at the poles and there is the gain from the lost Sahara and Australian deserts.  The tropical parts of the Earth remain warm and balmy.