As Earth warms (what changes climate), the oceans rise for two reasons. Satellites have measured the global sea level since 1993, and tide gauges go back much further.
Key idea
Sea level rises because seawater expands as it warms, and because ice on land (glaciers and the Greenland and Antarctic ice sheets) melts or slides into the sea. Global mean sea level rose about 20 cm from 1901 to 2018, and about 10 cm since 1993 alone, at a rate that has more than doubled (IPCC 2021; NASA). Melting sea ice, which already floats, adds almost nothing.
Why it works
Water, like most materials, takes up more space when warmer: seawater expands by roughly 0.01 to 0.03% for each degree, depending on its temperature. Spread over an ocean thousands of meters deep, that adds up. Land ice is new water for the ocean: when it melts or flows into the sea, the ocean gains that water. Floating ice is different. By Archimedes' principle, floating ice already pushes aside its own weight of water, so when it melts it fills almost exactly the hole it made. Since 1971, about half of the rise has come from warming water and most of the rest from melting glaciers and ice sheets (IPCC 2021).
Worked example: A melting ice cap
An ice cap on land holds 1000 km³ of ice (density 0.917 g/cm³). If it all melted, about how much would global sea level rise? The ocean covers about 361,000,000 km².
- Water from the ice: $1000 \times 0.917 = 917$ km³.
- Spread over the ocean: $917 \div 361{,}000{,}000 \approx 2.54 \times 10^{-6}$ km.
- That is about 2.5 mm. Greenland's ice, about 2.9 million km³, would add about 7 m.
Answer: About 2.5 mm
Worked example: Warming water
The top 700 m of the ocean warms by 0.5 °C, and seawater expands by 0.0002 of its height per °C. About how much does sea level rise?
- Rise = depth × expansion per °C × warming = $700 \times 0.0002 \times 0.5$.
- $= 0.07$ m, about 7 cm.
Answer: About 7 cm
Common mistake
Thinking melting sea ice raises sea level. Sea ice already floats, displacing its own weight of water, so melting it barely changes the level, just as melting ice cubes don't make a full glass overflow. Only ice that sits on land (glaciers, Greenland, Antarctica) adds water to the ocean. Check: ask whether the ice is floating or resting on land.
Projected rise by 2100 (compared with 1995–2014) is likely about 0.3 to 0.6 m if emissions are cut deeply and about 0.6 to 1.0 m with very high emissions (IPCC 2021). The rise continues for centuries, because the deep ocean and the ice sheets respond slowly.