I don't think it is the case that humans adapt well to a constantly changing environment. One of the remarkable things about the last 10,000+ years which has seen the huge expansion of humans on the planet is the extraordinarily constant state of the climate, to the extent that we have hardly had to adapt. In areas where ancient civilizations have expanded and vanished again, research has shown that it was often shifts in local climate that destabilized them.
Small perturbations on a global scale (massive volcanic eruptions) have caused all sorts of problems over the centuries (e.g. ~1816), but they quickly go away and life returns to normal. We have never had a long term global change to deal with (excluding the end of the last Ice Age as that marked the start of a stable period which saw civilization thrive).