Retorts

Raising the Sea Level

A tsunami up to four metres high could devastate low-lying parts of Wellington, if there is a major earthquake on the West Wairarapa fault line.

The chance is up to 30% in the next 100 years (less a month or so already.) The full force of a tsunami is estimated to take up to 29 minutes to hit Lyall Bay, says a report compiled by Dr Wayne Hastie, senior advisory officer at Wellington Regional Council, from research by DSIR.

A distant earthquake -- or ice change in Antarctica, I suggest -- could also affect New Zealand.

Some years ago, the Dominion science reporter quoted NASA scientists saying they have observed changes in Antarctic ice sheets, which could have serious worldwide implications. According to my encyclopedia, the West Antarctic ice sheet is a moderate-sized ice sheet, 100 miles long by 50 miles wide at longitude 85^o East on the Antarctic circle, 67^o South. However, a tabular iceberg of nearly this dimension, actually 100 by 35 miles, had already been spotted floating in the ocean, by the 1960s publication date of that encyclopedia. The Dominion said that Antarctic ice changes could send ice chunks crashing into the sea, causing a rapid and dramatic sea level rise.

The energy released if 1,000 cubic miles of ice (50 by 20 miles by 1 mile thick) should fall 1 mile vertical distance into the sea would be 2.7E19 joules. This is equivalent to the energy released by a magnitude 10 earthquake (Richter scale.)

The ice-cube effect, when a huge chunk of ice falls into the drink, raises sea level in the glass or globe. This means that global sealevel over the world's oceans area of 140 million square miles, would rise by nearly 0.9 cm in 24 hours. However, nearer to the icefall, sealevel rise would be earlier and momentarily greater. If an icefall displaced water in a band of ocean from longitude 0^o to 180^o East and 47^o to 67^o South, (Antarctica, Indian Ocean, to Stewart Island), the sealevel rise would be 13.5cm.

If a tsunami from an ice fall coincides with ordinary high tides or spring tides and adverse weather (low barometric pressure), then the Carter Fountain at Wellington's Oriental Bay may well be submerged by waves.

The New Zealand Climate Report (1990) suggests that sea level is expected to rise permanently by 12-29 cm if climate warms 1^oC by 2050 AD. It could mean a tidal inundation scenario -- "goodbye to every coastal settlement," Trevor Chinn, quoted in the Dominion.

The "big one" could strike in my lifetime, or an Antarctic icefall could be triggered by man's increased production of greenhouse and ozone-depleting gases in the twentieth century.

D S McDonald, Wellington