After 3.000 years of largely unchanged global mean sea levels (GMSL), the oceans have begun to rise from the 20th century onward, encroaching higher and higher onto the coastlines of continents and islands and threating the livelihoods of millions (if not billions) of people. The reason for this is twofold and directly connected to the global warming caused by increasing greenhouse gas concentration in the atmosphere: the water warms up and expands (thermal expansion), and the higher temperatures cause glaciers and ice shelves (primarily in Greenland and Antarctica) to melt.
