Why do glacial periods end abruptly?
Notice the asymmetric shape of the Antarctic temperature
record (black line), with abrupt warmings shown in yellow
preceding more gradual coolings (Kawamura et al. 2007;
Jouzel et al. 2007). Warming at the end of glacial periods
tends to happen more abruptly than the increase in
solar insolation. Several positive feedbacks are respon-
sible for this. One is the ice-albedo feedback. A second
feedback involves atmospheric CO2. Direct measure-
ment of past CO2 trapped in ice core bubbles shows
that the amount of atmospheric CO2 decreased during
glacial periods (Kawamura et al. 2007; Siegenthaler et
al. 2005; Bereiter et al. 2015), in part because the deep
ocean stored more CO2 due to changes in either ocean
mixing or biological activity. Lower CO2 levels weakened
the atmosphere’s greenhouse effect and helped to
maintain lower temperatures. Warming at the end of
the glacial periods liberated CO2 from the ocean, which
strengthened the atmosphere’s greenhouse effect and
contributed to further warming.
end partial quote from:
https://www.ncei.noaa.gov/sites/default/files/2021-11/1%20Glacial-Interglacial%20Cycles-Final-OCT%202021.pdf
If I am understanding all this correctly when biological or other changes happen to species on earth it changes the CO2 amounts in the atmosphere and ocean. The lower the amounts of CO2 in the water and air the more likely it is to be in an ice age conversely the higher amounts of CO2 in the water and air the more likely it is to be like now.
So, with higher CO2 levels the oceans rise and with lower CO2 levels the oceans recede because so much of the water is then in ice near the northern and southern polar regions.
During Ice Ages humans have to move more towards the equator to stay warm enough to have civilizations more than small tribes to actually be functional as a larger civilization. Also, you can't really raise crops on glaciers unless you have a really advanced technology and greenhouses or something like that.
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