What if a large part of Earth's landmass had been lost a thousand years ago in some cataclysmic event?

What if a massive part of Earth had been lost to a cataclysmic event, like a massive earthquake, an asteroid impact, or some other cataclysmic event? Every continent lost a large part of its land, leaving massive gaps between countries. <br><br>How would we deal with such an event, in terms of politics/culture and economy? <br><br>Also, how would we adapt to the sudden lack of landmass ?