How much of a threat would an unstable climate pose to modern agriculture and civilisation? Could we turn the soil in new regions into productive areas on par with those we lose?

We've seen before how the first agricultural societies such as in Jericho died out and later the impacts of the Bronze Age which even brought climate refugees. Even before those you have the drying out of the Sahara and Australia. It seems to go without saying that a severe unstable climatic period would btfo a settled people.

>man-made climate change isn't real REEEE

Don't care, this is about how humanity would fare in an unstable climatic period that marks human history before the rise of agriculture. Doesn't matter what it is but I presume it would be a bit stressful for people living in it going from forest to savannah or tundra in a few years.