Environmental Science
How Glacial Isostatic Adjustment Shapes Coastal Sea-Level Projections
Quick fact
In parts of Scandinavia, the land is still rising so fast that relative sea level is falling, while along the U.S. East Coast, the land is subsiding, adding hundreds of millimetres to projected sea-level rise by 2100.
Why this is interesting
The ground beneath some coastal cities is rising or sinking, and the difference can be a foot of sea-level rise or more.
Read the full explanation
Understanding How Glacial Isostatic Adjustment Shapes Coastal Sea-Level Projections
Imagine Earth's surface as a memory foam mattress. When a heavy ice sheet presses down, the mattress (the crust) sinks into the soft foam (the mantle). When the ice melts, the foam slowly pushes the mattress back up. During the last ice age, huge ice sheets sat over North America, Scandinavia, and elsewhere. After they melted, the crust began to rebound, but it's still rising today in areas that were under the ice, while in a ring around those areas, a 'forebulge' is collapsing, causing the land to sink. This vertical movement adds to or subtracts from global sea-level rise. If the land is rising, the sea seems to be rising more slowly; if the land is sinking, the sea appears to rise faster. That's why sea-level projections are always local: they combine ocean volume change with how the land is moving.
A deeper explanation
The mechanism of GIA is driven by the slow flow of viscous mantle rock. When the ice is removed, the crust rebounds elastically at first, but the long-term response is controlled by the viscosity of the mantle, which causes a delayed, gradual adjustment that can last thousands of years. The response consists of two components: uplift in the formerly glaciated centres and subsidence in the surrounding forebulge. This vertical motion, called vertical land motion, occurs at rates ranging from a few tenths of a millimetre to about one centimetre per year. In coastal sea-level projections, the global mean sea-level rise (from thermal expansion and melting ice) is adjusted by the local vertical land motion, producing a relative sea-level curve. For example, on the U.S. Gulf Coast, the subsidence from GIA (plus other factors) can add several millimetres per year, which significantly increases the expected sea-level rise by 2100. Measurements from GPS stations and tide gauges are used to estimate these rates and improve projections. Because GIA is a slow, continuous process, it must be factored into any long-term coastal planning, infrastructure design, and flood risk assessment.