Del

 2-2011

Rising sea levels – but where?

Even if the Greenland Ice Sheet melts sufficiently to raise the average global sea level by one metre, Norway will barely be affected. It is poor countries that will be hardest hit by rising sea levels.

Av Jan Mangerud

It is highly likely that global warming will result in rising sea levels, both because of the expansion of seawater as it warms, and because the meltwater produced as ice sheets shrink more rapidly will run into the sea. It is quite certain that the rise in sea level will not be uniform throughout the world.
 
A number of factors influence sea level regionally. At an international conference, glaciologist Jonathan Bamber of the University of Bristol described how he had distributed a questionnaire to researchers working on the Greenland and Antarctic ice sheets and asked them for estimates of the scale of ice melt and sea level rise. He received answers from 14 of them, and the average of their estimates, or qualified guesses, was that they expected sea level to rise by 32 centimetres by 2100 as a result of ice melt. When water from smaller glaciers and the thermal expansion of water was included, the estimate rise was 64 to 86 centimetres – but this is the global average.

Changing gravity field

The Earth’s gravity field keeps the water of the oceans in place, and prevents it from being hurled into space as the planet rotates. The gravity field is formed by both the deeper masses of the Earth and the surface continents. It is stable in the short term, because the Earth itself is stable. Only the glaciers and ice sheets change much on a decadal time scale.
 
We can consider the moon for an example of how variations in gravity affect sea level. When the Moon is directly above us, its gravity field results in the local rise in sea level we call high tide; when the Moon is further away and its gravitational force is lower, we experience low tide.
 
The Greenland Ice Sheet contains a huge mass of ice, and its gravitational field has an effect similar to that of the Moon. As ice melts, the ice sheet will lose mass and its gravitational field will become weaker. The result will be a drop in sea level around Greenland – a form of “low tide” – while sea level in other parts of the world will rise even more than it would simply because of the additional water. The effects will be much more long-lasting than those of the Moon, which of course produces high tide and low tide twice every day.
 
In other words, the paradoxical result will be that as the Greenland ice melts, world sea level will rise because more water enters the oceans, but the sea level around Greenland itself will drop because the island loses mass and its gravity is weakened. The map shows the expected pattern of sea level rise around the world if melting of the Greenland Ice Sheet results in an average sea level rise of one millimetre per year, or 10 centimetres in 100 years. We can see that there will be no sea level rise at all around Greenland – in fact, the sea level will sink, but this is not apparent from the simplified map.
 
In this scenario, the sea level will not rise along the Norwegian coast either. The greatest rise will, most unfairly, be in poor, low-lying countries, for example Pacific islands and along the coasts of Asia and South America. Only a belt between Africa and South America and across the northern Pacific Ocean will experience a sea level rise of one millimetre a year, the average rate for the Earth as a whole.
 
If the Greenland ice melts at a higher or lower rate, the pattern of sea level rise will be the same, but the actual figures will be different. Norway’s situation is particularly favourable because the land is still rising after being depressed during the last Ice Age.

Antarctic ice melt will give a different pattern

If smaller glaciers and ice caps melt, the result will be a similar pattern of sea level rise, since most of them are situated in northerly latitudes, especially in Alaska and Arctic Canada.
 
Norway’s glaciers fit the same pattern, but are far too small to have any substantial effect. Melting of ice in Svalbard, Iceland and the Russian Arctic islands would be more important.
 
On the other hand, if the Antarctic Ice Sheet melts, the continent’s gravitational field will be weakened and the sea level in the Southern Ocean will drop. The rise in sea level will be in middle and northern latitudes, where it will also cause problems for Norway.
 
Scientists are less certain about what to expect in Antarctica, although recent measurements do suggest that the huge Antarctic Ice Sheet is also shrinking. However, projections of what will happen in Antarctica are made more difficult by the fact that a rising temperature will give more snowfall, which may make up for the increase in ice melt. If the entire Antarctic Ice Sheet should melt, the sea level would rise by 60 to 70 metres, but this is not something that we need to worry about for the next million years.

Bad news for poor countries

If all the ice sheets and glaciers in both the Northern and the Southern Hemisphere lose some of their mass, which is perhaps the most likely scenario, the outlook is not promising, according to estimates by Jonathan Bamber and Riccardo Riva. They included the effects of changes in the Earth’s axis of rotation as a result of the redistribution of mass, in addition to changes in the gravitational field.
 
They found that in this case, sea level will rise most in a wide belt near the equator including most of the world’s poor, low-lying coastal states, while northwestern Europe and North America will be less hard hit.
 
One clear conclusion of almost all scientific papers on changes in sea level is that we still know too little about the many processes involved to make reliable projections for several decades ahead. We can learn about the processes by studying deglaciation after the last Ice Age. The development of satellite technology has hugely increased opportunities to study the changes that are occurring, and the quality and range of satellite observations are steadily increasing.

Sist oppdatert: 01.06.2011


People from Greenland and Pacific islands, plus other threatened island and Arctic communities, contribute to the Many Strong Voices programme.

They are joining together to learn from each other about tackling climate change, including dealing with melting ice and sea level rise.

Learn more about the Many Strong Voices programme at Manystrongvoices.org
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