AP® Environmental Science · Unit 9 · Topic 9.5
Sea Level Rise and Melting Ice
Sea level rises from two separate processes: ocean water physically expanding as it warms, and land-based ice (glaciers, ice sheets) melting and adding new water to the ocean. Floating sea ice melting does neither.
Sea level rise has two independent causes, and they add together. The first is thermal expansion: as ocean water absorbs heat, its molecules spread out and the water itself takes up more volume, raising the surface even with no new water added. The second is land ice melt: glaciers, ice caps, and ice sheets sitting on Greenland or Antarctica contain water that has never been part of the ocean. When that ice melts and runs off into the sea, it adds genuinely new volume. Floating sea ice, like the Arctic's summer ice, is different. It is already sitting in the ocean, displacing its own weight in water, so melting it just converts ice back into the water it was already displacing. No new volume, no sea level change.
To find how much land ice melt raises sea level, convert the melted ice mass to a volume of water, then spread that volume across the ocean's surface area. Divide volume by area and you get a rise in height, usually a few millimeters for thousands of gigatons, because the ocean's surface area is enormous. This calculation treats the ocean as a flat bathtub, which is a simplification: it ignores water being absorbed into currents, redistributed by wind, or pulled by gravity toward or away from melting ice sheets.
That simplification is why real sea level rise isn't uniform. Gravity from a massive ice sheet pulls water toward it; when that ice sheet shrinks, nearby seas can actually drop while far-off coastlines rise more than average. Ocean currents, land subsidence, and local geology add more variation. This unevenness matters directly for coastal cities: a one-meter global average rise over a century can mean several meters of effective flooding risk for a sinking delta city like Jakarta or Dhaka, where land is also sinking from groundwater extraction, compounding the ocean's rise.
Key terms
- thermal expansion
- The increase in volume of ocean water as it warms, raising sea level without adding any new water.
- land ice
- Ice sitting on land, such as glaciers and ice sheets, whose melt adds new volume to the ocean.
- floating sea ice
- Ice already floating in the ocean that displaces its own weight in water, so its melting causes no sea level change.
- ice sheet
- A massive, continuous layer of land ice covering a large area, like those on Greenland or Antarctica.
- land subsidence
- The sinking of land surface, often from groundwater removal, which worsens local flooding on top of sea level rise.
Watch it explained
- How it worksWalk me through thermal expansion and land ice melt each adding to sea level separately.
- Worked exampleFind the sea level rise when 3000 gigatons of land ice melts into a 361 million square km ocean.
- Going furtherWhat happens to a low-lying delta city as sea level rises 1 meter over a century?
- Common mistakeWhy isn't sea level rising by the same amount everywhere on Earth?
Check yourself
End of topic.