The idea that Puerto Rico is “sinking” involves a complex interplay of geological forces and global climate trends. To understand how the island’s elevation is changing relative to the ocean, it is necessary to examine two distinct scientific factors. The first involves the movement of the Earth’s tectonic plates, which determines the vertical stability of the landmass itself. The second factor is the accelerating rise of the global sea level, which affects all coastlines equally. Analyzing both the local land motion and the oceanic changes allows scientists to determine the precise rate of relative sea-level change experienced by the island’s coastal communities.
The Tectonic Setting
Puerto Rico is located within a broad, seismically active boundary zone where the North American Plate interacts with the smaller Caribbean Plate. This geological context is the primary driver of long-term vertical movement of the island’s landmass. The North American Plate is moving westward and colliding with the Caribbean Plate in a process known as oblique subduction. This complex interaction creates the Puerto Rico Trench to the north and the Muertos Trough to the south.
The island sits on the Puerto Rico-Virgin Islands microplate, which is being squeezed within this larger plate boundary zone. The long-term effect of these tectonic forces is the vertical land movement (VLM) observed across the island. Continuous pressure from the converging plates causes localized areas of slight uplift or subsidence. This movement is a slow, ongoing process that is only detectable through highly precise geodetic measurements.
Vertical Land Movement Data
Scientists use advanced geodetic techniques to measure the subtle vertical movement of the land. Continuous Global Positioning System (CGPS) stations and Interferometric Synthetic Aperture Radar (InSAR) track changes in land elevation with millimeter accuracy. These measurements show that the movement is highly localized and generally small, often falling below the observation uncertainty of approximately 0.5 millimeters per year. Some coastal areas show slight uplift, which helps to counteract the effects of rising sea levels.
For instance, geodetic analysis in the San Juan area suggests a small uplift rate of around 0.02 millimeters per year, while Isla Magueyes has shown an uplift closer to 0.49 millimeters per year. These positive movements indicate that the land itself in these specific locations is slowly gaining elevation. The largest localized vertical movements, sometimes reaching 1 to 2.3 millimeters per year, are typically found in the southwest part of the island, such as the Lajas Valley. The measured vertical movement is a result of deep tectonic processes, contrasting sharply with the notion of rapid, island-wide sinking. Even where subsidence occurs tectonically, the rates are very low, measured in fractions of a millimeter annually. This data confirms that the landmass of Puerto Rico is largely stable.
Global Sea-Level Contribution
The primary factor contributing to the perception of the island sinking is the global sea-level rise. This worldwide phenomenon is driven by two main processes linked to the warming climate: the thermal expansion of seawater and the addition of meltwater from continental ice sheets and glaciers. As ocean temperatures increase, the water itself expands, taking up more volume. The global mean sea level has been rising at an accelerating rate over the past few decades. Recent satellite altimetry data show the global average has increased to approximately 3.6 millimeters per year since the 1990s.
This global rise affects all coastlines, including Puerto Rico, independent of any local land movement. For Puerto Rico, the rate of relative sea-level rise (RSLR) has been observed to steepen significantly in recent decades. Tide gauge data indicates that the linear increase in sea level has accelerated from about 1.75 millimeters per year to as much as 7.25 millimeters per year in some areas. This dramatic increase is largely attributed to the global warming signal.
Combined Effects on Coastlines
The relative sea-level rise experienced by coastal communities is the sum of the local vertical land movement and the global sea-level rise. Since Puerto Rico’s landmass is mostly stable or even slightly uplifting, the observed RSLR is overwhelmingly dominated by the global oceanic rise. For example, a location with a slight uplift of 0.5 millimeters per year would still experience a net rise of approximately 3.1 millimeters per year if the global sea level is rising at 3.6 millimeters per year. This combined effect has practical consequences for the island’s low-lying coastal zones and built infrastructure. Accelerated RSLR increases the frequency and severity of coastal flooding, especially during storm surges and high tides.
Consequences of Rising Sea Levels
The continuous encroachment of the sea leads to saltwater intrusion, contaminating freshwater aquifers near the coast, which threatens local water supplies. Approximately 60% of the island’s beaches experienced erosion between the 1970s and 2010, a trend exacerbated by the rising water levels. Furthermore, rising water levels increase the vulnerability of roads, ports, and other infrastructure located near the shoreline. The primary threat is not the land sinking, but the ocean rising, requiring proactive planning for relocation or protective measures.