Climate change causes global mean sea level to rise primarily through two major physical processes, thermal expansion of seawater and the addition of water to the oceans from melting land based ice. As ocean water warms, it expands, increasing ocean volume even when the total mass of water remains essentially unchanged. Glaciers and ice sheets contribute differently because their meltwater transfers water that was stored on land into the ocean, increasing ocean mass.
Smaller contributions come from changes in terrestrial water storage, including groundwater depletion, reservoir storage and other movements of water between land and ocean. Sea level is therefore not controlled by a single process. It reflects interactions among ocean temperature, ice mass, atmospheric circulation, precipitation, evaporation, groundwater, land motion and ocean dynamics. Climate change affects several of these components simultaneously, producing a sustained rise in global mean sea level as the climate warms.
Thermal expansion is one of the fundamental mechanisms connecting ocean warming with sea level rise. Water does not expand equally at all temperatures and depths, so the contribution depends on where ocean heat is stored. The upper ocean has warmed substantially, but heat also penetrates into deeper layers, where warming contributes to expansion. Ocean salinity affects density as well, so changes in freshwater input and circulation can modify regional sea level through changes in seawater density.
Ocean currents and atmospheric pressure patterns redistribute water geographically, meaning that local or regional sea level can rise faster or slower than the global average. Consequently, a coastal location can experience substantial sea level change even when its local ocean temperature does not follow the global average exactly. Measurements therefore distinguish global mean sea level from regional and local relative sea level.
Melting land ice is another major source of sea level rise. Mountain glaciers contain freshwater stored on land, and when glaciers lose mass through increased melting or reduced snowfall, some of that water eventually reaches the ocean. The Greenland and Antarctic ice sheets contain much larger quantities of frozen freshwater.
Their contribution to sea level can occur through surface melting and runoff, but also through increased ice discharge into the ocean when glaciers flowing from ice sheets accelerate and lose mass at their margins. Ice floating in the ocean behaves differently because melting floating ice has little direct effect on sea level through displacement, although changes in floating ice shelves can affect the flow of grounded ice behind them. This distinction between grounded and floating ice is fundamental when explaining why the loss of continental ice contributes to sea level rise while the melting of already floating sea ice has comparatively little direct effect on global mean sea level.
Climate change can also alter sea level through changes in the amount of water stored on land. Human extraction of groundwater for irrigation, drinking water and industry can move water from underground reservoirs into rivers and eventually the ocean. Some of this water may be temporarily retained in soils, vegetation or inland reservoirs, producing smaller changes in the opposite direction. Large dams can therefore affect sea level by storing water on land, although their contribution varies over time and geographically.
Changes in precipitation and evaporation can also modify terrestrial water storage. These effects are smaller than the principal contributions from ocean warming and land ice for long term climate driven sea level rise, but they matter for precise global water budgets. Sea level science consequently considers the entire hydrological cycle rather than treating the ocean as an isolated reservoir.
Sea level does not rise uniformly everywhere because ocean dynamics and land movement create large regional differences. Ocean circulation transports heat and water, atmospheric winds redistribute surface water, and changes in ocean density influence the height of the sea surface. Glacial isostatic adjustment also affects relative sea level because land that was depressed by ancient ice sheets continues to rise in some regions while surrounding areas can experience relative subsidence.
Tectonic uplift and subsidence can alter local coastal elevation as well. Human activities such as groundwater withdrawal can cause additional land subsidence in some coastal areas. The sea surface can therefore appear to rise rapidly relative to a particular coastline because the ocean is rising, the land is sinking, or both. Tide gauges measure relative sea level at specific coastlines, whereas satellite altimetry provides broad measurements of changes in ocean surface height.
Rising sea levels increase the baseline from which coastal flooding occurs. A storm surge that previously remained below a particular elevation can reach further inland when the average sea surface is higher. Higher sea levels can increase coastal erosion, saltwater intrusion into freshwater aquifers and estuaries, and the frequency of high tide flooding. Low lying islands, deltas and coastal cities can face particular challenges because small increases in water level can affect extensive areas of flat terrain.
The effect depends on local topography, coastal defences, sediment supply, tidal range, wave climate and land motion. Sea level rise therefore does not mean that every coastline will be permanently submerged at the same rate. Instead, it changes the probability and extent of coastal inundation and interacts with existing geographical and engineering conditions.
Sea level responds slowly to changes in climate because oceans and ice sheets have enormous thermal and physical inertia. Even if atmospheric warming were stabilised, the ocean would continue exchanging heat with the atmosphere and ice sheets and glaciers would continue responding to the altered climate for many years, decades or longer. This means sea level is partly committed to continued change because the climate system does not instantly reach equilibrium after atmospheric greenhouse gas concentrations stop increasing.
The amount of future sea level rise depends strongly on future greenhouse gas emissions, ocean warming and the long term response of glaciers and ice sheets. Higher warming generally produces greater long term sea level rise, with ice sheet behaviour introducing important uncertainties over longer timescales. This long response time makes sea level rise a particularly important example of climate change whose consequences persist well beyond the period in which emissions occur.
Scientific understanding of sea level change combines satellite altimetry, tide gauges, ocean temperature measurements, gravity observations, glacier surveys, ice sheet observations and climate models. Satellite missions can measure changes in the height of the ocean surface across large areas, while satellite gravity measurements can detect changes in the distribution of water and ice mass. Ocean profiling systems provide information about temperature and salinity changes throughout the ocean.
Researchers combine these independent measurements to estimate the components of global sea level change and test whether the observed rise is physically consistent with warming, ice loss and changes in terrestrial water storage. The overall evidence shows that global mean sea level has risen substantially since the nineteenth century and that the rate of rise has accelerated in recent decades. Future sea level change will vary by region, but continued warming is expected to increase the long term rise and associated coastal risks.
