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While increasing sea levels are a primary driver of coastal effects, the movement of land also is important in some of the same coastal areas. “Relative” sea-level describes measurement of the ocean’s rise or fall compared to land. When land is sinking (or subsiding), it brings an apparent increase in sea levels, while land that is rising (or uplifting) leads to an apparent decrease. The general term for subsidence and uplift is “vertical land motion.” In some locations, vertical land motion occurs through natural processes. One of the most important of these is post-glacial rebound, also known as glacial isostatic adjustment, or GIA (see NASA’s Virtual Earth System Laboratory [VESL] for more details). At the close of the last ice age, some 10,000 years ago, the retreat of massive ice sheets from North America and the Eurasian continent lightened the load on the underlying mantle, deep below the Earth’s surface. The mantle is viscous, not solid, so it rebounds slowly, raising the rock layer above—the lithosphere—over thousands of years. This gradual lift, the recovery from the last ice age, continues today, altering the shape of ocean basins and leading to vertical land motion along many coastlines. While uplift occurs close to areas that were pushed downward by ice sheet loading 10,000 years ago, subsidence is occurring in areas that were once on the bulging edge, or forebulge, of those ancient ice layers. This can be seen along the eastern seaboard of the United States, where coastlines are sinking due to this forebulge collapse, leading to high rates of sea-level rise.
Subsidence also can result from human activity. Groundwater and hydrocarbon extraction can cause compaction of sediments. Subsidence related to groundwater withdrawal can be especially pronounced in river deltas with large populations and extensive agriculture. These effects are visible along the Gulf Coast of the United States (hydrocarbon extraction) and the Central Valley of California (groundwater extraction). Subsidence resulting from human activity also varies significantly over small spatial scales, which can be estimated using GPS observations and/or Interferometric Synthetic Aperture Radar (InSAR) analysis.