The calculation of sea level rise is a complex process. It represents a global metric informing coastal planning and climate policy. Determining this change requires measuring two distinct phenomena: the change in the total volume of ocean water, which involves both mass and heat, and the resulting change in the height of the ocean’s surface. These interconnected factors necessitate the use of different, specialized methodologies to achieve an accurate global picture of sea level change.
The Physical Contributors to Sea Level Volume
The fundamental inputs driving sea level change are physical processes that alter the total volume of the ocean. These changes fall into two main categories: thermal expansion and the addition of water mass.
Thermal expansion, or thermosteric sea level rise, occurs because water expands as it warms. When the ocean absorbs excess heat, water molecules vibrate more rapidly, decreasing density and increasing total volume. This volume increase, which is not the addition of new water, historically accounts for about one-third of observed global sea level rise. Since the ocean absorbs over 90% of the excess heat trapped in the Earth system, this expansion is a major component of the calculation.
The second mechanism is the addition of physical water mass, known as eustatic sea level rise. This mass input comes almost entirely from the melting of land-based ice, primarily mountain glaciers and the ice sheets of Greenland and Antarctica. As this ice melts and flows into the ocean, it increases the total mass of the global ocean system. This process is accelerating, making mass input from melting ice sheets an increasingly dominant factor in modern sea level calculations.
Global Measurement Using Satellite Altimetry
To calculate the global average rate of sea level rise, scientists rely heavily on satellite altimetry. This technique allows for the precise measurement of Absolute Sea Level (ASL), which is the height of the ocean surface relative to the center of the Earth. Satellite missions, such as the Jason series, have consistently collected data since the early 1990s, forming the backbone of the global sea level record.
The satellite altimeter operates by emitting a microwave radar pulse down toward the sea surface and then measuring the time it takes for the pulse to reflect and return. This precise round-trip time is converted into a distance, known as the range, between the satellite and the water. By continuously tracking the satellite’s exact position in space relative to a geocentric reference frame (geodesy), scientists subtract the range from the satellite’s altitude to calculate the sea surface height.
These measurements are taken over nearly the entire globe every ten days, providing a vast dataset. The raw measurements are corrected for atmospheric effects, such as water vapor and ionospheric delays, as well as for the influence of ocean waves. This methodology allows researchers to determine the mean sea surface height to an accuracy of a few centimeters, revealing the steady acceleration of the global average sea level.
Calculating Relative Sea Level (Local Rates)
While satellite altimetry provides Absolute Sea Level (ASL), coastal communities experience Relative Sea Level (RSL). RSL is the height of the water relative to the elevation of the adjacent land. The calculation of RSL is distinct because it must incorporate Vertical Land Motion (VLM), the movement of the land itself. This local land movement can either amplify or diminish the effects of global sea level rise.
RSL is calculated by adjusting the global or regional ASL rise rate by the local VLM rate. Land can subside, or sink, due to factors like sediment compaction, groundwater extraction, or tectonic plate movements, causing the observed RSL rise to be faster than the global average. Conversely, land can rise, or uplift, in areas experiencing post-glacial rebound, which slows the apparent rate of sea level rise.
Historical tide gauges measure RSL relative to a fixed benchmark on the land and are the primary tools used to monitor local rates over decades. To separate true sea level change from the VLM component, tide gauge records are often paired with nearby Global Navigation Satellite System (GNSS) receivers. The GNSS data precisely tracks the movement of the land benchmark, allowing scientists to isolate and remove the VLM signal from the tide gauge record to derive a local ASL trend.
Modeling Future Sea Level Scenarios
Calculating future sea level rise requires shifting from measurement to complex numerical modeling, as a simple linear extrapolation of current trends is inadequate. Scientists employ Earth System Models (ESMs) to predict how the various contributing factors will evolve under different climate conditions. The two main components integrated into these models are the prediction of future thermal expansion and the mass loss from land ice.
To account for political and economic uncertainties, future projections are developed as a range of outcomes based on standardized greenhouse gas emission pathways, known as Shared Socioeconomic Pathways (SSPs). These scenarios range from a low-emissions pathway to a high-emissions pathway, each leading to a different calculated amount of expected ocean heating and, consequently, thermal expansion.
The models also incorporate specialized ice sheet dynamics to predict the future mass loss from Greenland and Antarctica. This mass loss is the largest source of uncertainty in long-term projections. By combining the calculated outcomes for thermal expansion, glacier melt, and ice sheet loss for each emission scenario, scientists provide decision-makers with a range of plausible future sea level scenarios necessary for risk assessment and infrastructure planning.