Many people might think that the sea has stayed the same level since the beginning of Earth, but in reality sea level has not always been the same. From researching today, we know that when we look at edges of continents submerged today were once dry in the past. And on the other hand, looking at many sedimentary rocks proves to us that the sea level has once covered land far passed the present shorelines. The submergence of part of a continent beneath seawater could result from either subsidence of the land, rise in sea level, or a combination of both. Therefore, it is difficult to determine exactly how sea levels has changed independently of the uplift and subsidence of continents.
From using the distribution of marine sedimentary deposits on continents, scientists can date back the sea level to about 140 million years ago. About 50 to 100 million years ago global sea level was about 200 meters higher than it is now. So you ask yourself…why does sea level rise and fall? If you think of the world ocean as a container, there are two big picture possibilities that come to mind: Either the container changed the amount of water it had or it changed the size of the container that holds the water. Either one of these possibilities could explain the increasing or decreasing of sea levels.
One easy explanation, which relates to today’s crisis is the fluctuation of the growth and melting of glaciers. 21,000 years ago (last ice age), the sea level was at least 100 meters lower than it is today because of all the water stored as ice on land. If the remaining glaciers melt, mostly covering Greenland and Antarctica, then the global sea level will rise an additional 80 meters.
Another explanation for the fluctuating sea levels is that the size of the actual ocean basin changes through time. If ocean-basin depths decrease, then the water spreads out onto previous dry land, therefore raising sea level without changing the volume of water in the oceans. This hypothesis of changing the size of the ocean basins is necessary because there are times in history when shoreline sedimentary deposits show changes in sea level without geologic evidence for the presence of glaciers. A third possibility to the rising of sea levels is plate tectonics. To link plate tectonics with sea level you need to make use of the fact that the elevation of seafloor relates to its age.