Cause 1: thermal expansion
Almost every liquid grows a little bigger when it is heated, because its particles move faster and spread out. The sea is no different. The oceans have taken in about 90% of the extra heat trapped by greenhouse gases, so the water is expanding.
A simple estimate: rise = layer depth × expansion factor × temperature rise. For sea water the factor is about 0.0002 per °C. So a 1000 m layer that warms by 1 °C rises about 1000 × 0.0002 × 1 = 0.2 m.
Cause 2: ice that melts on land
Ice sheets (Greenland, Antarctica) and mountain glaciers rest on land. When they melt, the water runs to the sea and adds to it. About 360 billion tonnes (360 Gt) of melted ice raises the whole ocean by about 1 mm. If all of Greenland melted, the sea would rise about 7 m; all of Antarctica, about 58 m. That would take hundreds of years or more, but even small melts matter for coasts.
Why floating ice does not matter
Sea ice and ice shelves already float. A floating object pushes aside water equal to its own weight (Archimedes). When the ice melts, the water it makes takes exactly the space it was pushing aside. So the level stays the same. This is like an ice cube melting in a glass.
But floating ice still matters for warming: it is white, so it reflects sunlight (the ice-albedo feedback).
Effects and responses
Higher seas bring more flooding at high tide, stronger storm surges, salty water in rivers, wells and soil, and loss of beaches, mangroves and low islands. Rise is not even everywhere; land that sinks (for example from pumping groundwater) feels it more.
People respond in two ways: reduce the cause (cut greenhouse gases) and adapt (sea walls, mangrove belts, raised homes, early warning, planned moves).
Try it: ice cube in a glass
Fill a glass to the brim, add an ice cube, and mark the water level. Wait until it melts and check the level. Next, put a few ice cubes on a plate tilted over the glass (land ice) and let them melt into it. Which one raises the level? Compare with the 3D steps 2 and 3.
Key formulas and definitions
- Thermal expansion rise ≈ layer depth × 0.0002 per °C × temperature rise
- 360 Gt of melted land ice ≈ 1 mm of sea-level rise
- Floating ice melting: no change in sea level
- Rise × time: total rise = rate (mm/year) × years
Worked examples
1. A 1000 m deep layer of sea warms by 1 °C. The expansion factor is 0.0002 per °C. Find the rise.
Rise = 1000 × 0.0002 × 1 = 0.2 m = 20 cm.
2. Melting land ice adds 1080 Gt of water. About 360 Gt raises the sea by 1 mm. How much is the rise?
1080 ÷ 360 = 3 mm.
3. The sea rises 4 mm each year. How much does it rise in 25 years?
4 × 25 = 100 mm = 10 cm.
4. A beach step is 30 cm high. At 4 mm a year, in how many years will the sea reach its top (ignoring storms)?
30 cm = 300 mm. 300 ÷ 4 = 75 years.
Common mistakes
- Thinking melting sea ice raises sea level. It floats already, so the level does not change.
- Forgetting thermal expansion. Even with no ice melting, warm water alone makes the sea rise.
- Mixing up land ice and sea ice. Only land ice (ice sheets, glaciers) adds new water.
- Believing the sea rises evenly everywhere. Local land movement, winds and currents change it.