You don't seem to understand rates of change. The rate of change was drastic when the ice age ended (which corresponds with your Doggerland example). Changes since then have been very slow. That period of slow rate of change corresponded with the beginnings of human civilization. Rates of change are once again picking up very fast.
You are correct that climate change can happen naturally (although that's not what is happening now). Tsunami's can also happen naturally...doesn't mean we should cause one by dropping a nuke in the ocean. Lightning can cause huge forest fires naturally...doesn't mean PG&E shouldn't have have maintained its powerlines better. Just b/c something terrible has the chance of eventually happening naturally doesn't mean we should allow it to happen artificially.