Global average sea level is currently rising at a rate of about 3mm per year.

This is primarily caused by ocean warming and the melting of ice sheets on Greenland and Antarctica. The melting of this land-based ice in particular is gathering pace.

As these vast, frozen lands melt and flow into the sea, certain parts of the world will face an increased risk of flooding.

But the science behind how they affect sea level is complex.

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Sea level rise is counterintuitive

You might expect that sea levels would rise most where the ice melts. However, a range of complicated influences mean this isn’t the case.

A number of factors can bring about regional variations in sea level, such as changes in ocean circulation or changes in land water storage.

But the ice sheets are set to play an increasingly big role over the next 100 years and beyond. The way they interact with gravity, the Earth’s crust and its rotation mean they affect the oceans in a counterintuitive way.

Here’s how it happens.

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Gravity

Gravity describes an object with mass attracting other objects to it. An ice sheet has a lot of mass, and so has a large gravitational pull on the water around it. This means the water level near an ice sheet is higher than it otherwise would be.



As the Greenland and Antarctic ice sheets melt, the pull of gravity is reduced. This causes a drop in sea level around the ice sheets. The water has to flow somewhere, so the local drop is then balanced by a rise in sea level on the other side of the Earth.



This change in the Earth’s gravity field results in a higher than average sea level rise at low latitudes, as the ice melts primarily near the poles.

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Ice melt from the last ice age affects today’s sea level

In addition, the solid Earth is still adapting to changes that happened at the end of the last ice age, around 10,000 years ago. Big lumps of ice had pressed down on areas of the Earth’s crust, particularly across the Northern hemisphere. As these melted, the crust started to rise again.

“It’s like a foam bed. When you sit up in the morning, the foam lifts up, so where there was a hole before, it’s now basically back in shape,” says Nasa scientist Dr Eric Larour.

This process, called glacial isostatic adjustment, takes place over many thousands of years.

As the land surface slowly rises upwards, the local sea level goes down in relation to it. The sea level fall in Scandinavia, for instance, is associated with this change.

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Ice melt changes how the Earth wobbles

As ice melts and water enters the oceans, the mass of the planet is moved around and its shape is changed. This is among the factors that alter the way the Earth wobbles on its axis. This can also impact regional variation of sea level rise, although not by as much as the other factors mentioned above.

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The broader picture

Ice sheets are important in explaining regional sea level variation. But they should be seen as part of a wider picture.

There are other processes at work that will contribute to the uneven rise in sea level. Changes in ocean currents, temperature and salinity, the melting of mountain glaciers, changes in land-water storage, and weather patterns will all play a role too.

Sources:

• Dr Matthew Palmer, UK Met Office

• Dr Aimée Slangen, NIOZ Royal Netherlands Institute for Sea Research