Final answer:
Fluctuations in sea level, or eustatic changes, are primarily caused by glacial oscillations and the expansion of ocean water as it warms. These changes have led to significant variations in sea level over geological time and are currently accelerating due to global warming.
Step-by-step explanation:
Worldwide fluctuations in sea level, known as eustatic changes, are primarily in response to changes in the volume of water in the oceans and the size of ice sheets. These fluctuations can be attributed to various factors such as glacial oscillations, which include the melting of glaciers and polar ice caps, and the expansion of water as it warms.
In the past, sea levels have experienced significant changes. For instance, during the last glacial maximum roughly 20,000 years ago, sea levels were about 120 meters lower than they are today. In the more recent history, from 1993 to 2010, sea level has risen at an accelerated rate of between 2.9 and 3.4 mm per year. This shift is largely due to global warming, which leads to meltwater from glaciers adding to ocean volume and warmer water occupying more space.
These sea level rises are a critical concern as they may inundate shorelines, reduce the size of islands, and in more severe cases, cause entire islands to disappear, affecting biodiversity and freshwater availability.