Physical Impacts: Sea-Level Rise and Melting Ice

Physical impacts of climate change include rising sea levels, melting glaciers and ice sheets, shrinking Arctic sea ice, warming and acidifying oceans, thawing permafrost and more frequent extreme heat and heavy rain. In brief: global mean sea level has risen by about 20 cm since 1900, and the rate has accelerated to a few millimeters per year. Two main causes are the thermal expansion of warming seawater and the melting of land ice from glaciers and the ice sheets of Greenland and Antarctica. Melting sea ice does not add to sea level, but its loss accelerates Arctic warming.

The effects on ice and sea level are among the most visible signs of climate change and have long-lasting consequences for coasts and water supply. This guide explains why sea level rises, what is happening to ice, what is projected, compares the contributions in a table, works through calculations, outlines the history of monitoring and corrects common mistakes.

Why is sea level rising?

There are three main reasons. First, thermal expansion: water expands when it warms, and the oceans have absorbed more than 90% of the extra heat trapped by greenhouse gases. Second, the melting of mountain glaciers adds water to the oceans. Third, the loss of ice from the Greenland and Antarctic ice sheets, through melting and the discharge of icebergs, is a growing contributor. Smaller contributions come from changes in the storage of water on land, such as groundwater pumping. According to the IPCC, global mean sea level rose by about 0.20 m between 1901 and 2018, and the rate increased from about 1.3 mm per year in 1901–1971 to about 3.7 mm per year in 2006–2018. More recent satellite data show a rate above 4 mm per year.

Sea-level rise is not uniform. Land that is sinking (subsidence), for example in deltas such as the Mississippi and the Ganges, and ocean currents and winds, make the local rise greater in some regions. Land that is rising after the ice age, such as parts of Scandinavia, can have a falling relative sea level. The rise increases the risk of coastal flooding during storm surges, erosion, loss of wetlands and salt contamination of freshwater and soils.

What is happening to ice?

• Mountain glaciers: almost all of the world’s glaciers are losing mass, with significant retreat in the Alps, Andes, Himalaya and Alaska. Over time, this affects the water supply of rivers fed by meltwater, with first more flow and later less.
• Greenland ice sheet: it has lost mass at a rate of the order of 200–300 billion tonnes per year in recent decades. Its complete melting would raise sea level by about 7 m, though this would take centuries to millennia.
• Antarctic ice sheet: it holds enough ice for about 58 m of sea-level rise. Loss is concentrated in West Antarctica and parts of the East, where warm ocean water melts floating ice shelves from below, which allows glaciers behind them to speed up. The Thwaites Glacier is closely watched.
• Arctic sea ice: the summer minimum extent has fallen by about 12–13% per decade since satellite records began in 1979. Floating ice does not change sea level when it melts, because it already displaces its own weight of water, but the loss of the bright ice exposes darker water that absorbs more heat (ice-albedo feedback).
• Permafrost and snow cover: permafrost thaw and shorter snow seasons in the Northern Hemisphere.

What are other physical impacts?

The oceans are warming and absorbing about a quarter of the carbon dioxide emitted, which makes them more acidic (ocean acidification). Heat waves are more frequent and intense, and the heavy rainfall is more likely in many regions because warmer air holds about 7% more water vapor per °C. Droughts are becoming more severe in some regions, and the weather patterns of storms are changing. The changes in monsoon rainfall and in the strength of the largest tropical cyclones are also being studied.

What do projections say?

Projections depend on future emissions. The IPCC’s 2021 report estimated that by 2100, global mean sea level would be higher by about 0.28–0.55 m under a very low emissions scenario and by about 0.63–1.01 m under a very high emissions scenario, relative to 1995–2014. Higher values cannot be ruled out if ice sheets lose mass faster. Sea level would continue rising for centuries even if warming were stabilized, because the deep ocean and ice sheets respond slowly. Low-lying coasts, river deltas and small island states, such as Tuvalu and the Maldives, face the greatest risks. Many large cities, such as Jakarta, Miami and Dhaka, are exposed.

Contributions to sea-level rise compared

Worked example: sea level and ice

1. Converting ice to sea level: the ocean covers about 361 million km², which is 3.61 × 10¹⁴ m². A layer 1 mm thick over this area has a volume of 3.61 × 10¹⁴ × 0.001 = 3.61 × 10¹¹ m³, about 361 billion tonnes of water. So 361 billion tonnes of melted ice raises sea level by 1 mm.
2. Greenland loss: if Greenland loses 270 billion tonnes per year, the contribution is 270 ÷ 361 = about 0.75 mm per year.
3. Accumulating rise: at a rate of 3.7 mm per year, 30 years bring 3.7 × 30 = 111 mm, or about 11 cm. If the rate rises to 6 mm per year, 30 years bring 18 cm.
4. Thermal expansion (simplified): an upper ocean layer 700 m thick warms by 0.5 °C. With an expansion coefficient of about 2 × 10⁻⁴ per °C, the layer expands by 700 × 2 × 10⁻⁴ × 0.5 = 0.07 m, or 7 cm. The coefficient is higher in warm water and lower in cold water, so this is only an estimate.
5. Land flooded: on a coastal plain with a slope of 1 in 1,000 (a rise of 1 m for each 1,000 m), a sea-level rise of 0.5 m moves the shoreline inland by 0.5 × 1,000 = 500 m. On a plain with a slope of 1 in 5,000, it moves by 2,500 m.
6. Sea ice extent: if the September extent is 6.0 million km² now and was 7.0 million km² in 1990, the decline is 1.0 million, or 1 ÷ 7 = 14%.

How have changes been monitored?

Tide gauges have measured sea level at ports since the 18th and 19th centuries, but are affected by land movement. Since 1992, satellite altimeters (TOPEX/Poseidon and later Jason and Sentinel missions) have measured sea level across the open ocean to within a few millimeters. Satellites have measured the area of Arctic sea ice since 1979, and the GRACE satellites launched in 2002 measure changes in gravity that show ice and water mass changes. The Argo network of about 4,000 floats has measured ocean temperature and salinity since the early 2000s. The IPCC assessments combine these data. The finding of accelerating ice loss from Greenland and Antarctica since the 1990s has drawn particular attention.

Which mistakes are common?

Mistake 1: Believing that melting sea ice raises sea level

Floating ice displaces its own weight of water, so melting changes the level almost not at all. Land ice, which sits on rock, adds water to the oceans when it melts.

Mistake 2: Thinking that sea level rises equally everywhere

Local factors such as land movement, currents and gravity alter the local rise.

Mistake 3: Overlooking thermal expansion

A large part of the rise comes from the expansion of seawater, which is not visible as melting ice.

Mistake 4: Assuming that sea level will stop rising when warming stops

The oceans and ice sheets respond slowly, so the rise is expected to continue for centuries after temperatures stabilize.