Monday, March 30, 2009

Rising Sea Levels?

Experts, i.e. those who study sea level changes, not those who think they do like Al Gore, are likely to know what they are talking about when it comes to predicting the future of our coastal cities.

As a geologist I have my own take on the subject. Since the last ice age began to recede there has been a significant global rise in sea levels. This has tailed off in recent centuries as there is simply very little ice left to melt compared with the huge thicknesses of ice that accumulated on the continents during each phase of the four ice ages. The only significant ice sheets left are Greenland and Antarctica and these are tiny compared to the huge sheets that covered the northern hemisphere during the peaks of the ice ages. (To illustrate, the Matterhorn in Switzerland was completed covered by the Alpine Ice Sheet, while the peak of Everest was likewise shaped under moving ice sheets in the Himalayas).

The Greenland ice sheet is but a remnant of its former area and thickness, so it is reasonable to state that there is relatively little left to melt.

Much of the Polar ice that can melt is resting on water. It is, to put it another way, already a part of the oceans.

So consider this simple experiment. Take a full glass of iced water. The ice cubes float on the water and just like icebergs, stick out above the rim of the glass. Let the glass warm up and watch the ice melt. No water flows over the side of the glass! When all the ice has melted there remains a full glass of water. Go back to the reason why icebergs float - ice is less dense than water.

Apply this to the oceans and it becomes obvious that whatever happens to the polar ice regions there will be no effect on sea level change.

But this, of course, is too simple.

Why? Because some parts of the Earth's surface are rising, some are falling, mostly due to tectonic forces within the Earth's crust. Relative sea level changes can therefore be measured and found to be different all over the globe. An unscientific quick look at the problem can easily focus on those areas that are drowning and draw the wrong conclusions.

Another factor that influences many coastal cities is subsidence due to additional loading (buildings in London, for example) or to extraction of water from deep wells (a problem in the Texas Gulf Coast). Such subsidence has nothing to do with global sea level changes.

Which is why I trust a real expert like Nils-Axel Mörner.