Sea-Level Rise and Melting Ice: Why the Oceans Are Rising

Sea-Level Rise and Melting Ice: Why the Oceans Are Rising

Editorial advisor: Mr. John Smith, Specialist Environment, PhD in Environmental Engineering.

Global mean sea level has risen by more than 20 centimetres since 1900, and the rate of rise has accelerated — from about 1.4 millimetres per year in much of the twentieth century to more than 3.7 millimetres per year in recent years, according to satellite measurements. Twenty centimetres may sound modest, but for low-lying coasts, deltas, and small island states, it is already enough to worsen flooding, erode shorelines, and contaminate freshwater.

Sea-level rise is also unusually long-lasting. Even if warming stopped tomorrow, sea level would continue to rise for centuries as oceans slowly warm and ice sheets continue to respond. Decisions made now therefore shape coastlines for generations beyond those who make them.

The two main causes

The first cause is thermal expansion. As ocean water warms, it expands. Because oceans are vast and deep, even a small average warming produces a significant volume increase. Thermal expansion accounts for roughly half of observed sea-level rise to date, and it will continue as heat already trapped in the climate system slowly mixes into the deep ocean.

The second cause is melting land ice. Glaciers and ice caps in mountain regions, the Greenland ice sheet, and the Antarctic ice sheet all lose mass as temperatures rise. When land ice melts, water that was stored on land flows into the sea, raising sea level directly. This is different from melting sea ice in the Arctic: sea ice already floats, so its melting does not raise sea level directly, though its loss accelerates warming by exposing dark ocean that absorbs more sunlight.

A third, smaller contribution comes from changes in land water storage — groundwater extraction, reservoir filling, and other human activities — but thermal expansion and land-ice loss dominate.

Greenland, Antarctica, and the question of speed

Greenland holds enough ice to raise global sea level by about seven metres if it melted entirely. Antarctica holds far more — around 58 metres — though most of it is considered stable on century timescales. The question is not whether these ice sheets will melt completely in the near term — they will not — but how fast they lose mass in the coming decades and centuries.

Greenland is losing mass through surface melting in summer and through glaciers flowing faster into the sea as warming ocean water undermines their fronts. Antarctica's loss is concentrated in West Antarctica, where warm ocean water melts ice shelves from below. Ice shelves themselves float, so their melting does not raise sea level directly, but they buttress the land ice behind them. When a shelf thins or collapses, the land ice behind it flows faster into the sea.

Scientists are particularly concerned about tipping points in West Antarctica, where some glaciers rest on bedrock below sea level that slopes downward inland. Once retreat passes a certain point, it may become self-sustaining even if warming stops. The timing and thresholds of such processes remain uncertain, which is why sea-level projections carry a wide range, especially for high-emission futures and for centuries beyond 2100.

Why sea-level rise is not uniform

Global mean sea level is an average. Local sea-level change varies considerably. Ocean currents, winds, and the gravitational effects of ice loss redistribute water unevenly: as Greenland loses mass, its gravitational pull on nearby ocean weakens, so sea level actually falls near Greenland but rises more than average far away, including along the eastern United States. Land itself also moves: some coasts are sinking because of groundwater extraction or natural subsidence, especially in deltas, which worsens relative sea-level rise even if the ocean itself rose uniformly.

That is why local projections, not just the global mean, matter for planning. A city built on a subsiding delta may experience twice the global rate of relative rise. Conversely, some places experiencing land uplift from the rebound after the last ice age may see slower relative rise for now.

What rising seas already do

The most immediate impact is not permanent inundation but more frequent nuisance flooding — high tides that flood streets, damage roads, and overwhelm drains — and more damaging storm surges. A storm surge that rides on a sea 20 centimetres higher reaches further inland and higher up defences. Saltwater intrusion contaminates drinking water wells and farmland in deltas and islands, reducing crop yields long before land is permanently lost.

Coastal erosion accelerates as higher water reaches dunes, beaches, and cliffs more often. Wetlands and mangroves, which buffer coasts against storms, are squeezed between rising seas and development inland, losing the space to migrate. Coral reefs, already stressed by warming and acidification, offer less protection as they degrade.

Small island developing states face an existential version of these pressures. For low-lying atolls, where the highest land may be only a few metres above sea level, even moderate rise threatens freshwater, agriculture, infrastructure, and ultimately habitability. Their leaders have been among the most vocal in international climate negotiations, precisely because the stakes are not abstract.

How far could seas rise?

Projections depend on future emissions. Under low-emission pathways, sea level is projected to rise by roughly 0.3 to 0.5 metres by 2100 relative to recent levels. Under high-emission pathways, projections rise to roughly 0.6 to 1 metre by 2100, with larger rises possible if ice-sheet processes prove faster than currently modelled. Beyond 2100, the divergence grows: low emissions limit multi-century rise to a few metres at most over millennia, while high emissions could commit the planet to many metres over centuries to millennia as ice sheets continue to respond.

These long timescales are hard to grasp, but they are the reason sea-level rise is often described as irreversible on human timescales. We cannot quickly reverse thermal expansion or regrow ice sheets once lost. Mitigation now limits the eventual total; adaptation must cope with the rise already committed.

Adapting coasts

Coastal adaptation takes several forms. Protection includes seawalls, levees, and restored mangroves and wetlands that absorb wave energy. Accommodation includes raising buildings, flood-proofing infrastructure, and updating building codes and drainage. Planned retreat — moving people and assets away from the most exposed areas — is the most difficult option politically and emotionally, but in some places it is the only durable one.

Effective adaptation requires honest risk communication. Building new homes in areas projected to flood regularly within a mortgage lifetime, without disclosing that risk, transfers costs to future owners and taxpayers. Insurance, planning, and public investment decisions made today will determine whether coastal communities adapt in an orderly way or face repeated, costly disasters.

Sea-level rise is slow compared with a storm, but it is relentless. It does not announce itself with a single event; it accumulates tide by tide, storm by storm, until places that were once safe are no longer so. Recognising that slow accumulation early is what makes adaptation possible.

Sources

• IPCC, Sixth Assessment Report, Working Group I — https://www.ipcc.ch/report/ar6/wg1/
• NASA, Sea Level — https://science.nasa.gov/climate-change/sea-level/
• NOAA, Sea Level Rise — https://www.noaa.gov/climate
• IPCC, Special Report on the Ocean and Cryosphere — https://www.ipcc.ch/srocc/
• National Snow and Ice Data Center — https://nsidc.org/