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Land subsidence is the downward movement of the ground; sea-level rise is an increase in ocean height. They are different physical processes, but a coastal community can experience both at once. When the land drops while the ocean rises, the local water level measured from that land rises faster than either process alone. Subsidence can occur gradually over a broad basin or suddenly in a collapse such as a sinkhole. Global mean sea level rises mainly because warming seawater expands and land-based ice melts.
What land subsidence means
The U.S. Geological Survey defines land subsidence as the gradual settling or sudden sinking of Earth’s surface because subsurface earth materials are removed or displaced. The key point is that the land surface moves downward. Subsidence is therefore a land-motion problem, not a change in the amount of water in the ocean.
Subsidence has several scales and speeds. A large aquifer system can compact by millimeters or centimeters per year across hundreds of square kilometers. A mine collapse or sinkhole can lower the surface abruptly in a small area. Both are subsidence, even though their causes, warning signs and hazards differ.
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A sinkhole is one possible form of subsidence: a localized collapse often associated with dissolution of soluble rock, underground voids or weakened material. Most subsidence is not a visible hole. Regional lowering caused by compaction of fine-grained aquifer sediments, drained organic soil or natural sediment settling can leave a broad, gently tilted surface with no dramatic cavity.
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How and why land sinks
Groundwater pumping and aquifer compaction
In many developed basins, pumping is the major human cause. An aquifer system may contain layers of clay and silt between more permeable water-bearing deposits. When wells lower groundwater pressure, the grains in compressible fine sediments bear more of the overlying load. The layers compact and the ground surface falls.
This is not simply a case of an empty underground space forming where water used to be. The USGS describes a reduction in pore space within the aquifer system. Some of that lost storage is effectively permanent: even if water levels later recover, compacted fine sediments may not return to their former thickness. A basin can therefore continue to lose elevation and storage after pumping is reduced, depending on the material and pressure history.
The USGS has estimated that more than 80 percent of identified subsidence in the United States was a consequence of human impact on underground water. That is a national estimate from 1999, repeated in later agency summaries; it does not mean groundwater causes all subsidence everywhere.
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Drainage and oxidation of organic soils
Peat and other organic-rich soils contain plant material that accumulated in waterlogged conditions. Drainage exposes the material to oxygen, allowing oxidation and decomposition. The soil loses mass and volume, and the surface drops. Repeated drying, cultivation and loading can make the lowering cumulative. Rewetting can slow further oxidation, but it does not automatically restore the elevation already lost.
Mining and extraction of fluids or minerals
Underground mining can remove physical support, allowing overlying strata to deform or collapse. Extraction of oil, natural gas or other underground fluids can also reduce pressure that helped support the rock framework. Removing subsurface minerals changes the distribution of load and can produce localized or regional deformation. The size and rate depend on geology, depth, extraction history and how the rock responds.
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Natural sediment compaction and loading
Young sediments can compact naturally under their own weight. Deltas, coastal plains and lake beds often contain water-rich deposits that settle as they consolidate. New sediment placed on a delta or floodplain adds load and can accelerate compaction. These processes can operate without pumping, although human construction and drainage may increase the rate.
Sinkholes, thaw and earthquakes
In karst terrain, groundwater can dissolve soluble rock and enlarge underground openings. The roof may eventually fail, producing a sinkhole. Thawing permafrost changes the ice and soil structure that supported the ground, leading to settlement or collapse. Earthquakes can permanently shift the surface vertically, sometimes abruptly. Erosion can remove supporting material, while tectonic movement can raise or lower broad areas. These natural mechanisms are why it is inaccurate to describe subsidence as always a groundwater problem.
What causes global sea-level rise?
Global mean sea level describes the average height of the world ocean. NOAA identifies two dominant warming-related contributions:
- Thermal expansion: seawater occupies more volume as it warms.
- Added water from land ice: glaciers and ice sheets lose mass, and that water enters the ocean.
Those mechanisms change the ocean, not the elevation of a particular beach, road or tide-gauge benchmark. The global average also masks regional differences caused by ocean circulation, winds, gravity and vertical movement of the land. NOAA notes that the ocean does not behave like a bathtub whose level rises identically everywhere.
Land subsidence versus sea-level rise
| Question | Land subsidence | Global sea-level rise | Local relative sea-level change |
|---|---|---|---|
| What moves? | The land surface moves downward. | The average ocean surface rises. | Water height changes relative to a local point on land. |
| Main mechanisms | Compaction, drainage, extraction, mining, collapse and natural vertical land motion. | Warming expands seawater; melting land ice adds water. | Global ocean change combined with local land elevation, circulation, gravity and other regional effects. |
| Where can it occur? | Inland or at the coast. | As a global average, with regional variation. | At a specific shoreline, harbor or tide station. |
| Useful measurements | GPS or CORS, satellite InSAR, repeat leveling, and ground or water sensors. | Satellite altimetry and ocean observations. | Tide gauges interpreted with independent land-motion measurements. |
A tide gauge records water height relative to its land-mounted benchmark. If the benchmark sinks, the gauge can show a rising relative sea level even when the ocean-only component is smaller. Conversely, uplift can offset ocean rise or make the local relative level fall. This is why a tide-gauge trend should not be described as an ocean-only measurement.
Why coastal communities can face both processes
At the coast, the quantity that matters for flooding is often relative sea level: the water surface compared with the ground where people, roads and buildings stand. If the ocean rises and the ground sinks, their effects compound. High tides reach farther inland, storm surge starts from a higher baseline and drainage systems lose elevation relative to the receiving water.
The contributions vary by location. One coast may have little subsidence and experience mostly the global ocean signal. Another may sit on compacting delta sediments, an over-pumped aquifer or an oil-producing basin where local land loss is much faster. A third may be uplifting because of tectonic motion or the slow readjustment of Earth after ice sheets melt, partly offsetting sea-level rise.
A 2021 review by Shirzaei and colleagues reported that some coastal subsidence rates associated with sediment loading and fluid extraction can be up to two orders of magnitude higher than the present-day rate of global sea-level rise. “Some” and “location-dependent” are essential qualifications: the comparison identifies hotspots and mechanisms, not a universal rate for every shoreline.
How scientists tell the processes apart
Measuring vertical land motion
- GPS and CORS: Continuously Operating Reference Stations provide fixed reference points from which height changes can be tracked.
- Satellite InSAR: Radar images collected repeatedly can reveal very small changes in the distance between a satellite and the ground across wide areas.
- Repeat geodetic leveling: Survey teams remeasure precise elevation differences along established routes.
- Ground and water sensors: Extensometers, benchmarks, well measurements and related instruments help connect surface movement with groundwater pressure and layer compaction.
Each method has a reference frame and limitations. GPS gives point measurements tied to a global or regional frame; InSAR maps relative changes and can be affected by vegetation, atmospheric conditions or loss of radar coherence; leveling is highly precise along surveyed lines but is labor-intensive. Combining methods produces a more reliable picture than relying on one instrument.
Measuring ocean and relative sea level
Satellite altimeters estimate ocean-surface height over broad areas. Tide stations measure local water height relative to a benchmark on land. A tide record becomes far more informative when paired with GPS or another land-motion record at the station. The difference helps separate water movement from sinking or rising ground.
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NOAA’s sea-level explainer describes 142 long-term coastal water-level stations with at least 30 years of observations being used to calculate mean sea-level trends relative to a fixed point on land. That inventory is an agency snapshot and can change as records are added or revised.
What the documented numbers show
United States affected area
A USGS assessment published in 2018 found that more than 123,000 square kilometers of land and waterways in over 50 areas of the conterminous United States had been permanently lowered by withdrawal of subsurface fluids. This is a U.S. affected-area estimate, not a worldwide total, and “permanently” refers to the documented lowering in those areas.
San Joaquin Valley example
In California’s San Joaquin Valley, the land surface subsided roughly 9 meters between 1925 and 1977 because of groundwater withdrawals, according to a USGS overview. It is a historical, local example of severe aquifer-system compaction, not a typical annual rate or a prediction for every farming basin.
Why rates cannot be applied everywhere
Subsidence depends on sediment type, pumping or drainage history, extraction depth, natural loading and the time period selected. A millimeter-per-year regional trend, a sudden sinkhole and a multi-meter historical lowering describe different phenomena. Reports should specify the place, dates, measurement method and whether the value is a maximum, an average or a cumulative change.
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Flood and infrastructure planning
Engineers need relative elevation: the height of a road, levee, foundation or drainage outlet compared with expected water levels. A plan based only on global sea-level rise can understate risk where the ground is sinking. Conversely, attributing every local change to subsidence can lead to the wrong intervention if the dominant signal is ocean warming or circulation.
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Water-management decisions
Where pumping drives compaction, managing withdrawals, moving production to less compressible zones and recharging aquifers can reduce future deformation. These actions cannot instantly recover pore volume already lost in fine sediments. The appropriate response depends on local hydrogeology and must be evaluated with groundwater levels and deformation data.
Diagnosing a local problem
A useful investigation asks three separate questions:
- Is the ground elevation changing, and at what spatial scale and rate?
- Is groundwater pressure, drainage, extraction, mining, thaw or another mechanism changing at the same time?
- What does the local water-level record show after accounting for land motion?
Answers from only one record can mislead. A falling benchmark, a declining well level and a rising tide-gauge trend together support a subsidence explanation; a tide-gauge trend without measurable local land motion points toward a larger ocean or regional signal, although measurement uncertainty remains.
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- “Sea-level rise is just land sinking.” No. Global mean sea level changes because the ocean expands and gains water from melting land ice; sinking land changes the local relative level.
- “Every sinking place has a sinkhole.” No. Broad aquifer compaction, organic-soil oxidation and natural sediment settling usually produce no open hole.
- “Groundwater is always the cause.” No. Mining, oil and gas extraction, drainage, earthquakes, erosion, permafrost thaw, natural compaction and tectonic movement can all contribute.
- “Recovering groundwater reverses all subsidence.” Not necessarily. Compacted fine sediments can lose pore space and storage that do not fully recover.
- “A tide gauge measures the ocean alone.” No. It measures water relative to land, so vertical motion of the benchmark is part of the record.
Bottom line for interpreting a sinking coast
Land subsidence is downward land motion caused by compaction, removal or displacement of material. Sea-level rise is primarily a change in ocean volume and height caused by warming and land-ice melt. The hazard people experience at a shoreline is relative sea-level change, which combines both signals along with regional ocean and gravity effects. Separating them requires land-motion observations such as GPS, InSAR or leveling alongside tide-gauge and satellite-ocean records. The distinction is not merely semantic: it identifies whether the most effective response concerns groundwater and land management, climate-driven ocean change, or both.
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