Tsunamis help atolls to beat rising seas

New Zealand and Australian scientists predict a surprise reprieve for low coral atolls that many fear will disappear under rising oceans because of global warming.

Auckland University geographers Paul Kench and Scott Nichol, and Australian professor Roger McLean have found that coral islets in the Maldives in the Indian Ocean built up in a way that kept them ahead of rising sea level in the past, and should be able to keep it up in the future.

Ironically, events such as the Boxing Day tsunami, which seemed to doom the islands when it swept over them, are part of a natural process that is stripping sediment off the surrounding reefs and dropping it on the islands.

The Royal Society of New Zealand has given Dr Kench's group $7000 to go to the Maldives next month to measure the effect of the tsunami on 13 tiny uninhabited islands that are less than 600m long and no more than 1.5m high.

The team made detailed measurements of the islands for three successive years from 2002 to last year and found they were "highly mobile" around the edges, with sediment washed by cyclones from one side of an island to the other.

But Dr Kench said reports that such islands might have been wiped out by the tsunami were unlikely to be true.

"We suspect that we might not observe that much change at all. We know the wave rode up over those islands, but the tsunami in the Maldives was only 1m to 1.5m high. It was more like a very rapid tide coming up.

"There might be massive alterations round the edges, and we might think some of the sand on the [coral] reef and beaches has been swept to the top of the island.

"But we have always proposed that, because they are susceptible to storms overtopping them that will put sand on the top, and the islands can adapt by growing upwards, which gives them a nice mechanism for withstanding rising sea level."

In fact, Dr Kench says that islands on coral atolls are built by sediments from their surrounding reefs and never grow higher than the height to which storm waves can carry the sediment. They have kept up with past sea level changes that were far bigger than anything we are likely to see in the near future.

The sea level rose by about 140m, to around its present level, when the second-to-last ice age ended between 140,000 and 130,000 years ago.

It dropped back slowly to about 130m below the present level at the peak of the last ice age about 30,000 years ago, then rose quickly to its current level 12,000 years ago.

Since then the sea has not changed much. In the Maldives, the present islands built up mainly between 5500 and 4000 years ago, and the sea may have risen by only about 2.5m since then.

On this scale, Dr Kench says, the predicted rise of less than 1m because of global warming this century is "unlikely to physically destabilise the reef islands".

In the February issue of the US journal Geology, he and Dr Nichol and Dr McLean argue that the islands "have existed more than 5000 years, are morphologically resilient rather than fragile systems, and are expected to persist under current scenarios of future climate change and sea-level rise".

The deputy director of Waikato University's International Global Change Institute, Dr Richard Warrick, warned this might not be true for densely populated islands such as the Maldives capital of Male, where people disrupt natural processes.

"If you have a storm and it dumps metres of debris in your roads and around your house, the tendency is to clear it away," he said.

Dr Kench conceded that waves washing over an island would not be much fun for people living on it, even if the waves did wash sediment on to the island to keep its head above water.

"This doesn't mean communities living there will not need to make plans," he said. If they know how their islands will respond - they will change but not catastrophically - they can make planning decisions about where they put people We can direct developments to islands which are more stable.

"Simply saying they won't be there in 100 years doesn't help anybody and doesn't help how to cope with it."

Islands reprieved?

* The United Nations predicts that a rise in global temperatures of between 1.4C and 5.8C in the rest of this century will lift sea levels by between 9cm and 88cm.

* Previous studies have warned that this may flood coral atolls, which are only a few metres above sea level.

* Now NZ and Australian scientists say the atolls have kept building up through much bigger sea level rises in the past, and should be able to do so again.