Turning Ice Mass Into Millimetres of Ocean

The conversion is a single division: meltwater volume divided by ocean surface area. What makes it tractable is a convenient coincidence of units. One gigatonne is a trillion kilograms, and since fresh water has a density of 1000 kilograms per cubic metre, a gigatonne of ice becomes exactly one cubic kilometre of water. The ocean covers about 361.9 million square kilometres, so spreading that cubic kilometre across it raises the surface by roughly 0.00276 millimetres. Inverted, it takes about 362 gigatonnes of grounded ice loss to add one millimetre of global sea level, which is the number worth committing to memory when reading mass balance figures from Greenland or Antarctica.

Why Floating Ice Is Excluded

Arctic sea ice and Antarctic ice shelves are already afloat, and by Archimedes' principle a floating body displaces water equal to its own mass. When that ice melts, the resulting water fills precisely the volume the submerged portion had occupied, and the sea surface does not move. This is why the calculator assigns zero direct rise to sea ice despite its enormous extent. The exclusion is narrow, though: ice shelves act as buttresses that slow the grounded ice flowing behind them, so their collapse accelerates the discharge of ice that does raise sea level. The indirect effect is real even though the direct one is nil.

Thermal Expansion, the Other Half of the Problem

Roughly half of the sea level rise observed during the twentieth century came from nothing melting at all. Seawater expands as it warms, and the ocean has absorbed the overwhelming majority of the excess heat trapped by greenhouse gases. Current expansion contributes on the order of 1.3 to 1.5 millimetres per year. Because heat penetrates the deep ocean slowly and mixes over centuries, a substantial amount of expansion is already committed by warming that has occurred, and would continue for generations even if surface temperatures were stabilized immediately. The time horizon input here multiplies the annual expansion rate, which makes that commitment visible alongside the melt contribution.

Global Averages Versus What a Coastline Experiences

Eustatic rise describes the change in global ocean volume, but no coastline experiences the global average. Relative sea level combines that signal with vertical land motion, and the local term is often larger. Deltaic cities compacting under their own sediment load, or subsiding from groundwater extraction, can sink several millimetres per year and effectively double their exposure. Regions still rebounding from the weight of Pleistocene ice sheets can see relative sea level fall. Horizontal consequences depend on gradient: a coast sloping at half a percent retreats about 200 metres per metre of rise, while a five percent slope retreats only twenty. That geometry, not the vertical number alone, determines how much land is actually lost.