Why does the sea rise when the world warms, even before the ice melts?

The sea rises for two separate reasons, and melting ice is only one of them. The other is simple physics: warm water takes up more room than cold water. This is called thermal expansion. Between 1993 and 2018, it caused 42 percent of global sea level rise. Melting glaciers and ice sheets added another 44 percent. Melting sea ice barely counts, because floating ice already displaces its own weight in water.

How do we know? The oldest tool is the tide gauge, which tracks the height of the sea against a fixed mark on land. Amsterdam's record goes back to 1700, and about 1,450 stations have records worldwide. Since the 1990s, satellites have added a precise global view.

Together they show the sea rising 15 to 25 centimetres between 1901 and 2018. That is faster than at any time in at least 3,000 years. The pace was 2.3 millimetres a year from the 1970s, and 4.6 millimetres a year over 2013–2022. The ocean responds to warming with a delay of decades. So the rise will keep speeding up until 2050, whatever happens to emissions.

Local numbers differ, because the land itself moves. Some coasts sink, others rise as land rebounds after the ice ages, so the local rise can be higher or lower than the global one.

After 2050, choices matter. With very deep emissions cuts, the projections give 30 centimetres to 1 metre more by 2100. With high emissions, 50 centimetres, or even 1.9 metres. Low-lying islands face the earliest risk. Atolls rise on average only 0.9 to 1.8 metres above the sea, and at current rates the Maldives would become uninhabitable by 2100.