While Singapore hopes never to go to war, it cannot avoid taking on another threat to the country's existence: climate change.

Sea levels will rise, posing a grave threat to the low-lying island, said Prime Minister Lee Hsien Loong yesterday.

The only question is when. He estimates it will cost $100 billion or more, over 100 years, to protect the country against rising sea levels.

Like the Singapore Armed Forces (SAF), climate change defences are vital to Singapore's existence.

Singapore is already experiencing some of the effects of climate change, including more intense rainfall and prolonged dry spells.

By 2100, it could see a rise in daily mean temperatures by as much as 4.6 deg C, and more extreme and intense weather, which could threaten water, food and energy supply.

PM Lee noted that current projections are that sea levels will rise by up to 1m by the end of the century, but scientists' estimates have been going up.

Professor Benjamin Horton, chair of Nanyang Technological University's Asian School of the Environment, said that to mitigate flood risk from sea level rise, potential solutions must support the long-term resilience and sustainability of communities and the environment.

This, Prof Horton said, requires robust, accurate local projection of sea-level rise. "Singapore must invest in the science of sea-level rise. Science first, responsible, cost-effective adaptation follows."

Much of Singapore lies only 15m above the mean sea level, with about 30 per cent of the island less than 5m above this level.

The Centre for Climate Research Singapore has found that in the rare scenario of high mean sea levels, high tide and high surge occurring at the same time, sea levels could rise by almost 4m above the current mean and overwhelm the island's low-lying coastal areas.

PM Lee said the centre is working with its counterparts in neighbouring countries to study in greater detail how climate change is affecting the region.

"They are finding that Singapore, being near the equator, is more vulnerable to climate change than the global model suggests," he added.

Climate change plans must be kept flexible and implemented progressively, he said.

"But we must start now and sustain the effort, as the Dutch have done over the centuries, and as we have done with the SAF," he added.

"We must make this effort. Otherwise one day, our children and grandchildren will be ashamed of what our generation did not do."