SINGAPORE - Around $100 billion or more may be needed over the long term to protect Singapore against rising sea levels, one of the many threats posed by climate change and one which the country is especially vulnerable to, said Prime Minister Lee Hsien Loong on Sunday (Aug 18).

There are good engineering solutions to the problem and these could include reclaiming offshore islands and connecting them with barrages, or building polders to protect the coastline. But they come at a cost.

"How much will it cost, to protect ourselves against rising sea levels? My guess is probably $100 billion over 100 years, quite possibly more.

"If we only have 10 years to solve the problem, we won't have the time or resources to do it," he said during his National Day Rally speech.

"But because this is a 50- to 100-year problem, we can implement a 50- to 100-year solution."

Climate change is one of mankind's gravest challenges, he stressed.

Singapore is already feeling the impact of a warming planet, such as through hotter days and heavier rainstorms, and the situation will likely worsen.

Risks and consequences include more extreme weather, new diseases, food shortages and even war.

Climate change defences should be treated like the Singapore Armed Forces - with utmost seriousness, he added.

This called for working steadily at it, maintaining a stable budget year after year, keeping an eye on the target and doing so over many years and several generations.

"That way we can afford it, and when we need it, we will be ready," said PM Lee.

"Both the SAF and climate change defences are existential for Singapore.

"These are life and death matters. Everything else must bend at the knee to safeguard the existence of our island nation."

While Singapore hopes to never go to war, having to deal with the effects of climate change is a sure thing.

Sea levels will rise, the only question is when. He 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.

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

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

Singapore's National Sea Level Research Programme aims to develop more robust sea level rise projections. The country is also boosting its resilience to floods and will spend another $400 million over the next two years to upgrade and maintain its drains.

Other major and large-scale engineering options are also being explored.

PM Lee said that climate change plans must be kept flexible and implemented progressively.

"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 said.

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