How Earth's general climate works to force planet-wide animal and human migrations
An enclosed spinning sphere, such as Earth....
O.K. An explanation before I continue.
Earth is an enclosed sphere. The enclosure is called gravity (at least for this post). In other posts, that enclosure might include the O-Zone, etc.
The Gravity enclosure tends to keep Earth in a relatively "smooth" sphere (at least so as we are taught). The faster a sphere spins with a reasonably flexible enclosure, the more of its fluid mass migrates to the outer circumference. Thus centrifugal force. The longer this happens the flatter the Earth tends to get.
As Earth warms from NATURAL causes and migrations throughout our Solar System, the Galaxy, and the Universe, we approach other space areas that heat and cool our Earth. Science has already proved all this, but they do not yet see it.
Earth contains life at present that thrives in -80 degree temperatures and in above +150 degree temperatures. Temperature/Climate changes (be they cooler or warmer) favor the one and not the other. It causes one to grow bigger and more populous. Thus, survival of the fittest.
As Earth naturally warms, the Polar Ice Caps melt, uncovering land mass. As the ice melts, the liquids now migrate towards the Equator, flooding equatorial land masses, exposing polar land masses.
The Equator life that is able to migrate (including humans) move towards the Poles. Massive wars escalate because the comfortable land-mass is small. Once most of the human population exterminated themselves via war; a "global" peace is reached for a period of time.
As millennia pass along, the poles begin icing up again and force warmer-life to once again migrate towards the equator. As the oceans begin freezing, mobile-life walks across the oceans as if it were land.
And thus the cycles of Earth's Climate. If we want to survive without the massive population-control wars, we should build massive floating cities with Iceberg technology. That is; where about 3/4 of these cities are underwater and 1/4 or so above water. Just a thought. We have the knowledge, skill and resources.