Ocean Warming and Sea-Level Rise

Ocean warming raises sea level mainly by increasing the volume of seawater and, indirectly, by promoting the loss of land-based ice. These are respectively thermal expansion and the addition of water mass from grounded glaciers and ice sheets.

Direct process: thermal expansion

As seawater warms, molecular motion increases and its density generally decreases. The same mass therefore occupies more volume, producing thermosteric sea-level rise without adding water to the ocean. The expansion depends on temperature, salinity and pressure, so equal warming does not produce equal expansion everywhere.

• Sea-level rise reflects the expansion integrated through the full depth and area of the warming ocean.
• Heat stored below the surface also contributes because ocean heat content, rather than surface temperature alone, controls total expansion.

Indirect process: addition of land ice

Ocean heat can melt the submerged fronts of marine-terminating glaciers and the bases of ice shelves. Melting or accelerated discharge of ice that was previously grounded on land transfers additional mass to the ocean, causing barystatic sea-level rise.

• Basal melting can thin ice shelves and weaken their buttressing effect, allowing grounded glaciers to flow faster into the sea.
• Melting floating sea ice or an already floating ice shelf causes little direct sea-level rise, but ice-shelf loss can indirectly accelerate grounded-ice discharge.

Global mean versus regional change

Thermal expansion and land-ice mass addition increase global mean sea level. Winds, currents and spatial differences in warming redistribute seawater and create regional departures, but redistribution alone does not substantially change the global mean.