Sea ice vs. sea level -- footnote

When floating ice melts, does it affect water level?

† A previous version of this article (U.S. National Science Foundation: "America's Investment in the Future: The Importance of Sea Ice") contained the following erroneous statement: "melting sea ice also raises worldwide sea levels, with potentially significant effects for coastal cities and towns."

Actually, when floating sea ice melts, it does not significantly affect coastal sea levels. Melting ice on land affects coastal sea levels, but melting sea ice does not.

When floating sea ice melts, it increases in density (decreases in volume), causing the local sea level (the top of the ice!) to drop at that location. However, its displacement (which is the amount of other water it displaces, and which is measured in units of mass) does not change, so whether the ice is melted or frozen has no effect on sea level elsewhere.

This is most dramatically visible in icebergs. When frozen, water has reduced density, so an iceberg rises above the surrounding liquid water. Its top surface is a locally elevated sea level. When that ice melts, the locally elevated sea level falls, but it has no effect on coastal sea levels (elsewhere), because the iceberg's water has the same mass (displacement) regardless of its varying density and solidity.

The same thing happens when liquid surface water warms in the open ocean. Sea level goes up (slightly) locally, due to thermal expansion of the water, but it has no effect on coastal sea levels.

Density changes in lower layers of the ocean do affect coastal sea levels, and slight salinity reductions due to melting sea ice will eventually reach the lower layers, but it takes hundreds of years for surface heat and salinity changes to find their way down to the lower layers of the ocean, and even then the effect on sea level from salinity changes due to melting sea ice will be negligible. It is calculated that the eventual effect on sea level from the very slight reduction in salinity of the ocean due to melting sea ice will be equal to about 2.8 percent of the effect that the same amount of ice on land would have if it melted and drained into the ocean.