IN SHORT

• 267 million people worldwide live on land that’s currently less than two metres above sea level. 
• Insurers are heavily influencing change around sea-level-rise risk mitigation.
• Cutting-edge data and modelling are helping communities select the interventions most suitable for their area and increasing awareness of the issue of sea-level rise. 

Sea levels have changed dramatically throughout our planet’s history. The United Nations’ Intergovernmental Panel on Climate Change (IPCC) says that since the end of the last ice age, sea levels have risen around 120 metres. So, why are we concerned about a relatively small rise of another 55 to 200 centimetres — according to the IPCC — by the end of this century? 

‘There was that 120-metre rise following on from the last ice age. Then, for the last 6,000 years, sea levels have remained relatively stable,’ says Dr Kathleen McInnes, senior research scientist at CSIRO. 

‘The problem is that it’s in that small window — in that 6,000 years — humans settled, and they settled mainly in coastal zones.’ 

A 2021 paper published in Nature Communications found that 267 million people worldwide currently live on land less than two metres above sea level. 

‘The beginning of the Industrial Revolution saw sea levels once again beginning to rise, and the rise is human induced,’ McInnes says. ‘The concern now is that we can’t just pick up our things and move away from the coast. We’ve got very established, very vulnerable cities.’ 

What is worth saving? 

Currently, sea levels are rising 3.6 millimetres annually, McInnes says. While this may seem inconsequential, the cumulative effects of rises year-on-year are felt intensely during particular events. Climate change and sea-level rise isn’t linear. Instead, as both temperatures and sea levels increase, they combine to create exponential growth in the incidence of storms (and related storm surges), higher tides and flooding. Storms and high tides can also coincide, further increasing the sea level for those periods. 

‘These can be tidal extremes, storm surges or high-wave events, and so on,’ McInnes says. ‘Also, the combination of sea-level rise with a riverine flood event combines to increase flooding in coastal areas.’ 

Inhabited areas that previously had low sea-level variability naturally built infrastructure around that small range, she explains. As sea levels rise, that infrastructure no longer does the job. Flood mitigation, then, becomes a matter of cost benefit. Is the village, town, suburb or city worth saving? In some places, seawalls have been the go-to solution to mitigate flood risk. However, as 50- and 100-year storm and flooding events occur much more frequently, our existing seawalls may no longer be fit for purpose.  

For instance, seawalls protect 360 km of the Massachusetts coastline. Authorities anticipate a 25–36-centimetre sea-level rise in the region by 2050. If the area experiences a storm similar to the 2015 North American blizzard again, researchers say the seawalls would need to be at least 90 centimetres taller than they currently are to prevent flooding. This would mean that seawalls in some places would need to more than double in height. But what may appear to be a simple fix is anything but. 

‘The engineering has to be different for the higher seawall,’ says Dr Peter Sousounis, vice president and director of Climate Change Research at Verisk Extreme Event Solutions.

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