Climate change, melting ice and sea level rise

One implication of climate change is sea level rise. Sea level is the average height of the ocean relative to the land, between the high and low tides. The rising global temperature is causing both land ice and sea ice to melt.

Land ice and sea ice are not the same. They form differently, and the consequences of their melting affect the planet in different ways.

Rights: Christine Zenino, Creative Commons 2.0

Greenland ice shelf

An ice shelf extends from the land and floats on the water.

Land ice – glaciers, ice shelves and ice sheets

Glaciers, ice shelves and ice sheets are land ice. They are all masses of frozen freshwater and are named for their different sizes and locations.

Glaciers are bodies of ice formed from snow that has accumulated and been compacted over long periods of time. They form in mountainous areas and high latitudes. Glaciers are found on every continent except Australia.

Ice shelves are masses of thick glacial ice that extend from the coastlines and float on the sea. Ice shelves are only found in Antarctica, Greenland and the Arctic.

Ice sheets are large masses of glacial ice on the land. To be called an ice sheet, ice must cover more than 50,000 km². Ice sheets are only found in Antarctica and Greenland.

To put this all in context, there are several glaciers that make up the Ross Ice Shelf, which is part of the larger Western Antarctic Ice Sheet.

Rights: Public Domain

Antarctic ice sheets and ice shelves

The Antarctic ice sheets are shown in white and are on the land. The ice shelves are shown in grey and are on water.

Sea ice

Sea ice is frozen seawater. It forms, grows and melts in the sea. Sea ice forms more slowly than freshwater ice due to a combination of factors. First, the freezing point of saltwater is lower than freshwater. The seawater temperature must get down to -1.8°C. It often takes longer to reach this temperature because, as seawater cools, it sinks. The top 100–150 m of seawater often needs to cool to -1.8°C for ice to form.

Sea ice is found in both the Arctic and Antarctic regions, with some areas covered in sea ice year round. Sea ice forms during the winter, and some of it melts during the summer. It is often covered by snow.

Rights: National Institute of Water and Atmospheric Research (NIWA)

Pancake Ice

This image shows the RV Tangaroa sailing through pancake ice, which is sea ice that has formed in this particular shape through the wave action of the sea.

Melting ice and sea level rise

Melting land ice leads to sea level rise, whereas melting sea ice has minimal impact. To understand why this is, imagine a jug of ice and water. As the ice warms and melts, the total volume of water in the jug does not change, so the water level stays the same. It’s the same with sea ice – when it melts, it does not change the volume of water in the sea.

Melting land ice is different. It adds water to the sea. This is similar to adding more water to the jug of ice and water. The volume of water in the jug increases – and will overflow if too much is added.

Although melting sea ice does not cause sea level rise, it does have other implications for the global climate. Sea ice has a light-coloured surface and reflects some of the sunlight that hits it. When sea ice melts, it exposes the darker sea surface, which absorbs solar energy (heat). This causes further temperature rises and causes more ice to melt.

Thermal expansion and sea level rise

Warming seawater also causes sea level rise. Water expands when it warms up – heat energy makes its molecules move around more and take up more space. Because the molecules are more spread out, the density decreases.

Climate change – additional implications

The ocean is a complex and continuous body of water that covers two-thirds of our planet. Melting land and sea ice affect many of the properties that drive the ocean’s chemical, physical and biological processes. Read about the properties – and the impacts climate change will cause – in the articles Ocean temperature, Ocean density, Ocean salinity and Ocean motion (currents and circulation).