Introduction

I taught an introductory course in Geology at the University of the South Pacific in 1977. Each of the countries that participated in USP was invited to send 2 students. They had varying interests, and it was amusing to watch how they woke up when we were teaching geology relating to their own job. Some were interested in gold mining, others in highways and landslides, some in coastal erosion, others in active volcanoes. It was rather a surprise when the sole student from Tuvalu approached me one day and said "Sir, this is all wasted on me. My island is just made of sand." Any news from Tuvalu always struck a chord from that moment.

Since then, of course, Tuvalu has become "hot news" as the favourite island to be doomed by sea level rise driven by global warming, allegedly caused in turn by anthropogenic carbon dioxide. If you look up Tuvalu on the internet you are inundated with articles about its impending fate. Tuvalu has become the touchstone for alarm about global warming and rising sea level.

The geological background

There may have been good reason to think that Tuvalu was doomed anyway. Charles Darwin, who was a geologist before he became a biologist, gave us the Darwin theory of coral islands which has been largely substantiated since his time. The idea is this: When a new volcano erupts above sea level in the tropical ocean, corals eventually colonise the shore. They can grow upwards and outwards (away from the volcanic island) but they can’t grow above sea level. The coral first forms a fringing reef, in contact with the island. As it grows outwards a lagoon forms between the island and the living reef, which is then a barrier reef. If the original volcano sinks beneath the waves a ring of coral betrays its location as an atoll.




But besides the slow sinking of the volcanic base there are variations of sea level due to many causes such as tectonics (Earth movements) and climate change. If sea level rises the coral has to grow up to the higher sea level. Many reefs have managed this to a remarkable extent. Drilling on the coral islands Bikini and Eniwetok shows about 1500m thickness of limestone and therefore of subsidence. Coral cannot start growing on a deep basement, because it needs sunlight and normally grows down to only 50 m.

If the island is sinking slowly (or relative sea level rising slowly) the growth of coral can keep up with it. In the right circumstances some corals can grow over 2 cm in a year, but growth rate depends on many factors.

Sometimes the relative subsidence is too great for the coral to keep pace. Hundreds of flat-topped sea mountains called guyots, some capped by coral, lie at various depths below sea level. They indicate places where relative sea level rise was too fast for coral growth to keep pace.

Sea level and coral islands in the last twenty years

What about the present day situation? The alarmist view that Tuvalu is drowning has been forced upon us for twenty years, but the island is still there. What about the changes in sea level?

Rather than accept my interpretation, look at the data for yourself. First take a regional view. For a number of well-studied islands it can be located at:




http://www.bom.gov.au/pacificsealevel/picreports.shtml

The Tuvalu data is provided at:

http://www.bom.gov.au/ntc/IDO60033/IDO60033.2009.pdf