Sea levels rise primarily because warming causes seawater to expand and because glaciers and ice sheets lose mass that subsequently enters the ocean. These are the two dominant mechanisms behind modern global mean sea-level rise. Thermal expansion occurs because the average distance between water molecules and the volume occupied by seawater increase as temperature rises, although the magnitude depends on temperature, salinity and pressure.
Land ice contributes through surface melting, iceberg calving followed by eventual melting and changes in ice flow that transfer ice mass from land into the ocean. Additional contributions arise from changes in terrestrial water storage, including groundwater depletion and reservoir storage. Local relative sea level can rise or fall for additional reasons involving land motion, including tectonic subsidence or uplift, glacial isostatic adjustment and human induced ground subsidence. The observed sea-level trend is therefore the combined result of changes in ocean temperature, ocean mass and the vertical position of the land.
Thermal expansion is a major component of sea-level rise because warmer water occupies more volume. The effect is not uniform throughout the ocean because seawater's thermal expansion coefficient varies with temperature, pressure and salinity. Warm surface waters generally have different expansion characteristics from cold deep waters, and heat transported into the deep ocean can contribute to sea-level rise there as well. Scientists estimate steric sea-level change from measurements of temperature and salinity throughout the ocean.
The term steric sea level encompasses changes in ocean volume associated with changes in temperature and salinity, although temperature driven expansion is the dominant component under current climate change. Ocean warming therefore contributes to sea-level rise even when the total mass of water in the ocean remains unchanged. This mechanism demonstrates why reducing sea-level rise requires addressing the underlying energy imbalance and not merely preventing additional freshwater from entering the ocean.
Glaciers are important contributors because they store freshwater on land and respond relatively quickly to climatic changes compared with the largest ice sheets. When a glacier loses more mass through melting and iceberg calving than it gains from snowfall and other accumulation, its net mass decreases. Meltwater eventually reaches the ocean through rivers, groundwater pathways or direct runoff, increasing ocean mass.
Glaciers occur on every continent except Australia and collectively contain enough ice to make a significant contribution to potential future sea-level rise, although their total volume is far smaller than that of the Greenland and Antarctic ice sheets. Glacier mass balance is affected by temperature, precipitation, elevation, debris cover, albedo, ice dynamics and ocean conditions for marine terminating glaciers. Their response is not instantaneous because glaciers possess thermal and dynamic inertia, but sustained negative mass balance leads to continued contribution to sea-level rise.
The Greenland and Antarctic ice sheets contain enormous quantities of land based ice, making their long term behaviour particularly important for future sea levels. Greenland loses ice through both surface mass balance processes and the discharge of ice into the ocean from outlet glaciers. Surface melting produces runoff, while changes in snowfall alter accumulation.
Antarctica behaves differently because much of its surface remains extremely cold, and its mass balance is strongly influenced by snowfall and the dynamics of marine terminating glaciers and ice shelves. Ice shelves float in seawater and therefore their direct melting does not substantially raise global sea level, but they can act as buttresses that slow the flow of grounded ice into the ocean. If ice shelves thin or collapse, some glaciers can accelerate, transferring more grounded ice into the ocean. The response of large ice sheets involves complex feedbacks and remains an important area of climate research.
Changes in terrestrial water storage can also alter sea level. Groundwater pumping removes water from underground reservoirs for agriculture, industry and domestic use, and some of this water eventually reaches the ocean. Irrigation can redistribute groundwater and surface water, changing how much water remains stored on land. Reservoir construction has the opposite effect for the water that is retained behind dams, temporarily preventing some water from reaching the ocean.
However, reservoir storage is not necessarily permanent because sediment accumulation, evaporation, release and changing management alter the amount of water retained. Changes in soil moisture, snowpack and wetlands can also shift water between land and ocean. These processes are smaller than the major thermal expansion and land ice contributions at the global scale but are essential when researchers attempt to account for the complete global sea-level budget.
Local sea-level rise can be amplified by vertical land motion. Subsidence occurs when land elevation decreases relative to a stable reference frame, which can happen through sediment compaction, extraction of groundwater or hydrocarbons, loading by infrastructure, tectonic processes or other mechanisms. Coastal megacities built on young sediments can be especially vulnerable to rapid relative sea-level rise when natural and human induced subsidence combine with global ocean rise. Conversely, land can rise through tectonic uplift or glacial isostatic adjustment.
During the last ice age, enormous ice sheets depressed portions of Earth's crust. After the ice melted, the crust began and continues to rebound in many regions, while surrounding areas can experience compensating subsidence. This ongoing adjustment changes local relative sea level independently of contemporary climate forcing and must be included in coastal projections.
Ocean circulation and changes in the distribution of water also create regional differences in sea level. Winds can pile water against coastlines, while currents, atmospheric pressure and changes in ocean density alter the height of the sea surface. When land ice melts, the redistribution of mass changes Earth's gravity field and rotation slightly, producing a geographically varying sea-level fingerprint. Water added to the ocean does not therefore spread instantaneously into a perfectly uniform global layer.
Ocean dynamics redistribute it over time, producing regional patterns of higher and lower change. Coastal geometry can further amplify water levels through tides, storm surge and wave setup. These factors explain why local observations can diverge considerably from the global mean even when the underlying global trend is well established.
The different causes of sea-level rise operate on different timescales and respond differently to climate forcing. Thermal expansion follows ocean warming and can continue as heat penetrates deeper. Glacier mass loss can respond over decades to changes in temperature and precipitation. Large ice sheets can respond over centuries or longer, with some processes capable of accelerating once particular physical thresholds are crossed. Terrestrial water storage can change relatively rapidly in response to human water management.
Land motion can occur continuously or episodically through geological and human processes. Scientists therefore use sea-level budgets, satellite observations, tide gauges, gravimetry, ocean temperature measurements, ice mass observations and climate models to separate these contributions. The principal global mechanisms are thermal expansion and loss of land ice, while local experience depends additionally on ocean dynamics and vertical land movement.
