Ice-age cycles and glacial isostasy
Sea level has oscillated between roughly 120 meters below present and 5 meters above present over the past million years, driven by the growth and melting of continental ice sheets. During glacial maxima, so much water was locked in ice that sea level dropped enough to expose continental shelves worldwide, creating land bridges between islands now separated by shallow water. The English Channel was dry; humans could walk from Asia to North America.
These changes occur partly from the direct addition and removal of water from the ocean, and partly from isostatic adjustment of the crust. After glaciers melt, the crust beneath them rebounds upward over thousands of years, while peripheral areas that were forebulge sink. This means relative sea level change varies regionally even when global volume does not change.
Modern sea level rise and its drivers
Current sea level is rising at approximately 3-4 mm per year, significantly faster than the 20th-century average of 1.7 mm/year. The primary drivers are thermal expansion of warming ocean water (the same water molecule takes up more volume when heated) and melting of land-based ice, particularly from Greenland and Antarctic ice sheets. Thermal expansion accounts for about 1.5 mm/year; melting ice adds another 2 mm/year and is accelerating.
The ice sheet contribution is potentially dramatic because Greenland contains enough ice to raise sea level by 7 meters if fully melted, and Antarctica contains enough for 58 meters. Current melting rates suggest centuries to millennia for complete melting, but any acceleration would have severe consequences for coastal infrastructure and low-lying island nations. Regional sea level rise varies due to isostatic adjustment, ocean circulation changes, and gravitational effects of ice melt.