Sea levels will rise 30cm around New Zealand by 2050 and threaten coastal properties and infrastructure in low-lying areas, a new report by Government's environment watchdog warns.
Parliamentary Commissioner for the Environment Jan Wright's report, released this afternoon, says rising oceans caused by a warming atmosphere will have a significant impact on many New Zealanders within their lifetimes.
Dr Wright looked at around 200 years of scientific study of sea levels for the peer-reviewed report.
She said three factors were causing ocean levels to rise - seawater expanding as it became warmer, retreating glaciers, and shrinking ice sheets in Greenland and Antarctica.
Her report said sea levels around the world had risen 20cm in the last 100 years.
Drawing on research from the Intergovernmental Panel on Climate Change (IPCC), the commissioner warned of a further rise of 20cm to 40cm by 2050. This would occur regardless of action taken on reducing greenhouse gas emissions, meaning New Zealand would be forced to adapt.
"A rise of 30cm may not sound like much, but its impact will be very costly for many landowners," Dr Wright said.
The impacts of a rise in sea level were most evident during storm surges, when wind and waves piled up water against the coast.
The commissioner said a storm surge in downtown Auckland in January 2011, which flooded roads, homes and shops, was worse than a similar size storm in 1936 because the sea level was 11cm higher.
She cited Niwa research which showed that in 30 years, this level of flooding will occur in Auckland once every decade.
After 2050, sea level rise will depend on global actions to reduce emissions. The report said under current settings, a sea level rise of up to a metre could occur.
Flooding incidents such as the storm surges in Auckland in 2011 would became yearly if no action was taken.
Local and central government would have to prepare for increased costs because roads, buildings and wastewater systems in low-lying areas would be affected by rising seas.
The report also said some areas of Christchurch had experienced an effective sea level rise of 50cm because of land dropping after the Canterbury earthquakes. Flood insurance was becoming more costly and more difficult to get in the city.
Councils were obliged to plan for the impacts of rising sea levels, but it was difficult for them to introduce changes which could lower the value of private property.
This was evident in the Kapiti Coast, where plans by the district council to put a coastal erosion warning on Land Information Memorandum (LIM) reports were fiercely opposed by 1800 affected homeowners.
Similar conflicts were "understandable and inevitable", the report said.
Dr Wright said reducing greenhouse gas emissions was the only way to avoid larger rises beyond 2050.
"A rising sea will be with us for a long time to come - one way or another we will have to adapt. But how high and how fast the water rises will be influenced by the speed at which the world including New Zealand reduces greenhouse gas emissions."
"Scientists have been studying this for over a hundred years, and the body of evidence is now irrefutable. The climate is changing and causing the sea to rise."
A second report released next year will look at which parts of the country were most vulnerable to sea level change.