How would a generally colder climate effect the growth of horse-nomad cultures out of the Eurasian Steppe?
Could similar cultures have simply developed further south, or would it have put a hold on the series of horse nomad invasions that moved out from the Eurasian Steppes?
To set the scene - we are working off the following - Last glacial maximum came and went as normal something like 26000 years ago Warming continued as per our world until about 13000 years ago, when they slowed substantially but continued. Roughly four thousand years ago something akin to the Holocene climatic optimum https://en.wikipedia.org/wiki/Holocene_climatic_optimum began; leading to land that was previously could not be cultivated opening up in Eurasia, causing rapid population growth.
This is largely based off the idea that steppe nomad people potentially domesticated the horse far earlier than other people.
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My idea for a story takes place on an alternate earth, in which the last glacial period lasted into modern times. I'm given to understand aside from lower sea levels, differences in salinization would mean the moderating effects of ocean currents would be lessened. I'm going with the idea that most megafauna are still outcompeted by smaller animals/humans and most of them have died off, but there are still a few around. Mostly in the north.
Roughly 1500-1000 BCE global temperatures rose enough to open England to permanent habitation and Doggerland is still pretty much above sea level.
I imagine human civilization further south would have largely developed along similar paths to in our world, while anything north of the alps would be substantially different.
The Mediterranean would be smaller, only connecting to the Atlantic during recorded history. The Black Sea isn't connected yet.
My main interest is what would this have done to the cycle of steppe peoples periodically conquering their way out - I'm not super up on the history there except as it relates to European history.
But if anyone has any other general observances - the main time periods I'm concerned with would be our world equivalent of 1900-1920 - with the state of the planet being what it was around 6000 BCE, temperature wise, and I'm trying to work out the history that lead up to it.
Primary cultures are a Roman Empire that survived into modern times due to there being fewer barbarian migrations and a larger Italian heartland to hold onto - Sicily is still connected - and a British esque Kingdom that tamed a surviving mammoth population, that consequently broke and absorbed migrant waves instead of being subsumed by them.
Edit - this is more assuming the last glacial maximum came and went at the same time, however, it receded more slowly than in actual history.