Are you sure? I'm not seeing that definition anywhere, and it looks like even in interglacial periods there's permanent ice in both hemispheres: https://en.wikipedia.org/wiki/Ice_age#Glacials_and_interglac...
> Individual pulses of cold climate within an ice age are termed glacial periods (glacials, glaciations, glacial stages, stadials, stades, or colloquially, ice ages), and intermittent warm periods within an ice age are called interglacials or interstadials.
You can actually see the definition (albeit a little verbose) as the first sentence of that article:
> An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers.
On a technicality, you can get me for not mentioning the snow capped mountains part, I'd concede on that. That part is actually news to me. All the same, the earth is colder than it usually is. [1]
An interesting thing I like to bounce around in my head: Could we live in the interior of the Pangean super continent if we had to? Interesting stepping stone between Earth's current, very mild climate, and trying to live in a place like Venus. Definitely would have to live like mole people.
We already avoid living in the interior of Australia, because living at temperatures of +50°C is just not very compatible with having body temperature slightly below +37°C. Same applies to the middle of Sahara desert. It's not impossible though, because the humidity is very low there, so, given a supply of water, you can cool by evaporation. At high humidity. you'd just die; such things happen during heat waves on the Indian subcontinent, for example.
However, there is no historical record of the wet bulb temperature outdoors exceeding human body temperature, anywhere, ever. So far.
Which is what it would take for the human body to be unable to remove excess heat in any fashion.
That’s not really saying much. The historical record is vanishingly small.
Looks like both sides are right in this case.
WP article: https://www.washingtonpost.com/climate-environment/2024/09/1... (https://archive.md/RM8ez)
> ...humans evolved during the coldest epoch of the Phanerozoic [the time period from 540 million years ago to the present], when global average temperatures were as low as 51.8 F (11 C).
> “We built our civilization around those geologic landscapes of an icehouse,” Judd [one of the study's authors] said.
Study (restricted): "A 485-million-year history of Earth’s surface temperature" -- https://www.science.org/doi/10.1126/science.adk3705
> Partitioning the reconstruction into climate states indicates that more time was spent in warmer rather than colder climates
Look at the graph - our time is on the very right. We humans developed and are still living in unusually cold times for this planet, historically.