Vikie S, of Portobello, asks :-

Jeff Miller, of Palmerston North, asked:-

Seeing the reclamation work being carried out in Otago Harbour made me wonder, where does all the sea water go when we are constantly filling the sea with land?

Can reclamation of coastal areas world-wide account for any part of rising sea levels?

James White, a geologist at the University of Otago, responded.

For centuries the Dutch have been noted for taking new land from the sea, or "reclaiming" it, and the process continues around the globe today. Forty percent of Japan's industrial areas are built on land "reclaimed" from the sea, as is 20 per cent of the total area of the nation-city of Singapore.

Despite the extensive infilling of shallow coastal areas to make new land now and in the past, the volume of seawater displaced by land reclamation, even when looked at across the globe, is a tiny fraction of the ocean's volume.

The volume of ocean waters displaced worldwide by reclamation is not well known, but can be estimated. One of the highest estimates of the land area "impoldered", a term from the Dutch which also includes reclamation from freshwater swamps and bogs, is 500,000 square kilometres throughout human history. To obtain a volume from this area, assume that the water displaced by reclamation averaged 0.01 km deep (10 metres, which is certainly an overestimate), and we have a very generous estimate of 5,000 cubic kilometres. If we compare this with the ocean's volume of 1,335,000,000 km3, its proportion of the ocean's volume is 0.00037 per cent, or less than 4 one-millionths, over all of humanity's history. That's about the same proportion as a small grapefruit in an olympic swimming pool.

For comparison, the volume of water stored onland in the Antarctic and Greenland ice caps is over 30,000,000 cubic kilometres; still only a few percent of the ocean's volume, but thousands of times more than humanity's coastline modifications throughout history.

And reclamation doesn't always involve adding new material to the ocean. A significant amount of sediment for land reclamation is dredged from the ocean's foreshores. Coastal sand mining, which is the extraction of sand from the seafloor, is an international issue (coastalcare.org). Like reclamation, this is not significant in terms of the ocean's volume, but it has many environmental effects, and even political ones. Singapore has been characterised as a "sand pirate" that builds new land with sand taken from neighbouring Indonesia and Malaysia.

So to sum up, land reclamation can have many effects, but significant infilling of the ocean isn't one of them.