The Pulse of the Ocean
Sea level has risen and fallen by hundreds of meters over Earth's history, alternately flooding continental interiors and exposing vast continental shelves. These eustatic changes are driven by the competing effects of ice sheet growth, ocean thermal expansion, and the slow tectonic reshaping of ocean basins. Understanding past sea-level change is essential for predicting future coastal impacts as the climate warms.
Ice Sheets: The Dominant Driver
On timescales of thousands to millions of years, the growth and decay of continental ice sheets is the primary control on sea level. The Pleistocene ice ages saw sea level swing by 120-130 meters between glacials and interglacials. Melting the current Antarctic and Greenland ice sheets would raise sea level by about 65 meters, inundating most coastal cities worldwide.
Thermal Expansion & Steric Effects
Warming the ocean causes seawater to expand — a process called steric sea-level rise. Each degree of ocean warming raises sea level by roughly 0.4 meters when integrated over the full ocean depth. This effect is already contributing about 40% of present-day sea-level rise and will intensify as deep ocean waters gradually warm over coming centuries.
Tectonic Sea-Level Change
On timescales of tens of millions of years, changes in mid-ocean ridge spreading rates alter the volume of ocean basins. Young, fast-spreading ridges are hot and buoyant, displacing ocean water onto continents. This mechanism explains why Cretaceous sea levels were 100-250 meters higher than today, creating the vast Western Interior Seaway and Tethyan shallow seas that hosted rich marine ecosystems.