Relative Sea Level Change (RSLC) combines the movement of the land and the rising of the ocean. The rate is not a single number but a regional measurement that varies across the state because it is driven by two distinct scientific processes: the geological movement of the landmass and the volumetric increase of the ocean. The resulting combined rate is accelerating, creating an urgent challenge for the low-lying peninsula.
Measuring the Rate of Relative Sea Level Change
The most current measurements of Florida’s rate of sea level rise are captured through a combination of high-precision instruments. Tide gauges, which have decades of historical data, measure the sea surface height relative to fixed points on the land, providing the local or relative rate of change. Satellite altimetry, in use since the early 1990s, measures the absolute height of the ocean’s surface from space, which helps scientists isolate the actual water volume increase.
The observed rate of relative sea level rise in Florida is generally greater than the global average. For example, Key West has a long-term historical average trend of approximately 2.64 millimeters per year (mm/yr). In Southeast Florida, the rate has accelerated sharply. Tide gauge data from areas like Virginia Key show a rise of about one inch every three years over the last decade, a rate significantly faster than the long-term average.
The Geological Reality: Florida’s Natural Subsidence
The “sinking” part of the equation is due to the natural and human-induced movement of the Florida landmass itself. The state sits atop an enormous platform of porous, soluble limestone that forms karst topography. Rainwater absorbs carbon dioxide from the atmosphere and soil, creating a weak carbonic acid that slowly dissolves this underlying limestone.
This constant, slow dissolution creates underground voids, caves, and fissures, which can lead to the gradual subsidence or sudden collapse of the overlying sediment, commonly seen as sinkholes. This geological process causes a small, long-term rate of land subsidence, generally less than 0.5 mm/yr in stable areas.
Human activity significantly exacerbates this natural rate in localized areas. Excessive groundwater withdrawal, particularly for municipal and agricultural use, lowers the water table. This reduction in hydrostatic pressure removes buoyant support for underground cavities and causes sediments to compact, leading to local sinking rates of up to 6 mm/yr in parts of the Tampa Bay area. Subsidence in coastal cities like Miami Beach is also linked to the compaction of unconsolidated sediments and reclaimed marshland under the weight of urban development.
The Oceanic Component: Global Sea Level Rise
The majority of the relative sea level change is driven by the global increase in ocean volume. This rise is caused by two main factors: the thermal expansion of seawater and the melting of land-based ice. As the ocean absorbs over 90% of the Earth’s excess heat, the water physically expands, increasing its volume.
The melting of land ice, such as glaciers and ice sheets in Greenland and Antarctica, contributes new water mass to the ocean basins. This added water, combined with thermal expansion, results in the global average sea level rise that has accelerated to approximately 3.4 to 3.6 mm/yr since the early 1990s.
Florida’s rate is also amplified by regional oceanographic dynamics, particularly the Atlantic Meridional Overturning Circulation (AMOC). A healthy Gulf Stream typically pulls water away from the U.S. Southeast coast, effectively lowering the sea surface height along Florida. However, a documented slowing of the AMOC causes water to “stack up” against the coast, contributing to the higher rates of sea level rise observed in Southeast Florida.
Translating Rates into Physical Changes
The combination of sinking land and rising ocean translates into tangible physical consequences, such as the increased frequency of “sunny day” or nuisance flooding during high tides even without rain. In Miami, areas that once flooded once or twice a year now experience a dozen or more inundation events annually. Local projections estimate sea levels could be 10 to 17 inches higher than 2000 levels by 2040.
The Biscayne Aquifer, the primary source of drinking water for millions in South Florida, is vulnerable to saltwater intrusion. As the sea level rises, the heavier saltwater pushes inland through the permeable rock, contaminating freshwater wellfields and forcing municipalities to relocate water supply infrastructure. By 2011, approximately 1,200 square kilometers of the Biscayne Aquifer were already intruded by saltwater.
The rising ocean also compromises gravity-flow drainage systems. These systems rely on a slope to move stormwater out to sea, but the higher sea level reduces the hydraulic gradient. This diminishing efficiency means that coastal flood control structures may have their capacity reduced by as much as 65 to 70 percent by 2040 in some areas, leading to more frequent and prolonged inland flooding.