Scientists have reconstructed nearly two centuries of relative sea level rise to show how gradually sinking land is compounding the impact of climate change in many parts of the Wellington region.

Their just-published findings suggest that, had it not been for the uplifting effect of New Zealand’s most powerful earthquake in modern history, local sea levels might have been higher than they are today.

While future projections have traditionally been based on the planet’s oceans rising, recent research has shown how New Zealand’s rising and sinking coastlines are also a major part of the picture.

Together, scientists say these two factors could make once-in-a-century coastal storms an annual event in our two biggest cities and other places as early as 2040.

Rewi Newnham, a professor of physical geography at Victoria University, said local processes like tectonic vertical uplift could be highly variable over time, making it challenging to predict how sea levels could change in decades and centuries to come.

“In the Wellington region, for example, recent satellite-based measurements show that much of the coastline has been sinking, in some parts at a rate comparable to global sea-level rise,” Newnham said.

“But we also know from geological evidence and recent earthquakes that the coastline can also be uplifted and in fact the geological evidence points to long-term net uplift.”

In their study, published in the journal Marine Geology, Newham, lead author Daniel King and other researchers turned to fossil marine micro-organisms preserved in saltmarsh sediments at Pāuatahanui Inlet to piece together 170 years of change.

Their results revealed how the local sea level abruptly dropped by a metre as the mammoth, magnitude 8.2 1855 Wairarapa Earthquake violently forced up the coastline.

Wellington, Auckland and many other centres could see more than half a metre of sea level rise by 2060 - and as much as 1.2m by 2100, recent research indicated. Photo / Mark Mitchell

Newnham said the marsh sediments had since accumulated at rates higher than would be expected from sea level rise alone, pointing to the effect of ongoing subsidence.

“The earthquake-related uplift and subsequent subsidence are not unexpected, but by estimating the rates of relative sea level rise associated with these local processes, we have been able to show that net subsidence has prevailed over this critical decadal-centennial scale in the recent past.”

That carried important implications for the future.

“The most important message is that net subsidence is likely to prevail over the coming decades for much of the Wellington region, if we use the equivalent period in the most recent past as a guide,” Newnham said.

Another key takeaway, however, was that a healthy saltmarsh environment could provide an effective natural barrier to sea level rise - even where the coastline was sinking.

“Without this dynamic natural barrier, coastal flooding would have posed a much bigger threat to Pāuatahanui village, nestled in behind the saltmarsh,” Newnham said.

“Of course, just how long this natural protection can continue to keep pace with accelerating sea level rise remains to be seen.”

Rising seas are among the largest climate risks to New Zealand, with an estimated 750,000 Kiwis and 500,000 buildings near rivers and coasts already vulnerable to inundation.

Rough estimates have put the cost of losing properties and assets in coastal and floodplain areas at about $145 billion - posing wide-ranging implications for the banking, insurance and property sectors.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.