Chapter 7 · Try it yourself

The Ice That Decides the Coastline

An ice cube melting in a glass of water does not make the glass overflow. So why do scientists worry so much about melting ice? Because it matters enormously where the ice sits. Melt each kind of ice below, and see which ones move the waterline.

Part 1: Four glasses of seawater

Each glass is 10 cm by 10 cm and holds the same seawater. Glasses a, b and c each hold 100 g of ice. The dashed line marks the starting water level; the scale on the right is in millimeters. Level changes are drawn 5 times taller than real so you can see them.

Sea-level gauge: how much each glass rose

    Nothing melted yet.

    Part 2: Where the new coast would be

    Raise the sea by

    Map of a made-up coast.
    • Today’s sea
    • Newly flooded
    • Land (darker is higher)
    • Building
    • New shoreline

    Illustrative: a made-up gentle coast about 4 km deep from beach to hills, with a low river valley. Real coasts differ a lot.

    The coast today

    Buildings flooded
    0 of 40

    Shoreline moves inland
    0 m

    So far: global sea level has risen about 20 cm since 1901, and satellites have measured about 10 cm of that since 1993.

    It is speeding up: about 1.3 mm a year in the early 1900s, 3.7 mm a year over 2006–2018, and 5.9 mm in 2024, an unusually warm year.

    Where it comes from: usually about two-thirds from melting ice on land (mountain glaciers, Greenland and Antarctica) and one-third from seawater expanding as it warms. In 2024 that flipped, with about two-thirds from warming water.

    Simplified model. Glasses: 100 g of ice in a 100 cm² glass; meltwater from floating ice is fresh and fills about 2.6% more volume than the seawater the ice displaced (Noerdlinger and Brower, 2007), giving about +0.2 mm here; grounded ice adds its full 100 cm³, +10 mm; glass d warms about 11 cm of seawater from 5 to 25 °C, about +0.5 mm. All the world’s sea ice and floating ice shelves together would raise sea level only about 4 cm (Noerdlinger and Brower, 2007). Coast map: illustrative elevations, no land sinking or rebound, and the sea rises evenly everywhere (in reality it does not). Scenarios: IPCC AR6 (2021) projections for 2100 relative to 1995–2014; about 3.3 m from the marine-based part of the West Antarctic Ice Sheet (Bamber and colleagues, 2009); about 58 m for all Antarctic ice (BedMachine Antarctica, Morlighem and colleagues, 2020). Observations: IPCC AR6 (20 cm since 1901; 1.3 mm/yr 1901–1971; 3.7 mm/yr 2006–2018) and NASA (2024: 0.59 cm; about 10 cm since 1993; usual split two-thirds ice, one-third warming).