Rocks Rd is one of the most scenic commutes in the country. But the road is now fighting a losing battle...

Rocks Rd in Nelson is one of the most scenic commutes in the country. The iconic curves of the stanchion and chain fence frame the idyllic harbour, as the sun bounces playfully off the waves.

Drivers have plenty of time to take in the view, because traffic on the road is notoriously bad. When NZTA announced plans to fast-track a shared walking and cycle way, the community greeted it with relief.

Then half a year later, NZTA abruptly pulled the pin. It no longer had complete confidence in the sea wall which protects the road.

Once perched well above the dangers of the ocean, the road is now fighting a losing battle. Every time a storm hits, the waves attack the sides, hammering into the concrete and splashing aggressively over the top. It's only a matter of time until it fails.

In Dunedin, the sea walls are already proving inadequate. Swells regularly overpower it, flooding the low-lying suburb. Three thousand homes are in the direct path of destruction, less than 50cm above sea level.

Waitati resident Cushla McCarthy's home was flooded and her insurance cut. Her house is uninhabitable. She's forced to live in her storage shed.

Twenty years ago, melting arctic ice and rising seas were primarily seen as an issue for the survival of Polar Bears. Now, it's a very human threat.

In New Zealand, homes and coastal communities are at risk. Elsewhere in the world, critical food supplies are already being hit, and entire island nations could be uninhabitable.

A new report on oceans and the cryosphere (frozen areas of Earth) by the Intergovernmental Panel on Climate Change is the most detailed insight yet into just how melting ice will impact our future.

Arctic sea ice is reducing by 13 per cent per decade, a rate that is unprecedented in the last 1000 years.

Antarctic ice sheets are melting at an increasing rate; they lost three times more mass in the last 10 years compared to the decade prior.

One of the worst case scenarios involves widespread permafrost melting. That's a major issue because permafrost in Siberia is covering huge peat bogs, which contain huge stores of natural methane. If those melt, they could release tens to hundreds of billions of tons of greenhouse gases into the atmosphere by 2100, causing potentially irreversible effects.

Even under the most optimistic scenario, glaciers will reduce 18 per cent by 2100 (36 per cent under current conditions), with some smaller glaciers expected to reduce by as much as 80 per cent, or even disappear completely.

That melting ice is running off into the sea, causing the Southern Ocean to rise by about 1cm per year. By 2100, the report estimates global sea level rise of 43cm even in a best-case scenario where major work is done to curb emissions. A future without serious mitigation could see as much as 84cm, and some estimates go as high as a metre.

Even what may seem like a small rise will have an outsized impact. The report predicts a major increase extreme sea level events, as well as more intense rainfall and tides. Significant wave heights will increase across southern Pacific Ocean.

Climate change may also be responsible for an increase in Category 4 and 5 tropical cyclones, though there was a lower level of confidence in that prediction.

New Zealand was one of dozens of locations around the world where historically 1-in-100-year sea level events will become annual within the next 30-60 years.

University of Otago professor Cliff Law, who acted as an expert reviewer on the report, said that figure "should ring alarm bells".

"Coastal zones are where we interact with and gain most benefit from the oceans, yet these regions are in the front line of climate change, being impacted by a number of different stressors," he said.

The level of the sea is only one piece of the puzzle. The health of the ocean itself is also a major focus of the IPCC.

It is 'virtually certain' the global ocean has warmed unabated since 1970. More than 90 per cent of excess heat produced due to human-driven climate change has been absorbed into oceans, and the rate of warming has doubled in the last 25 years.

The Southern Ocean accounted for two thirds of heat gain. That has led to a massive increase in marine heatwaves.

Between 84-90 per cent of marine heatwaves, like the one which caused extreme temperatures this summer, are attributable to human activity.

Twenty to 30 per cent of all human Co2 emissions since 1980 have been absorbed by the ocean, causing a slow acidification. These unprecedented conditions, combined with rising sea temperatures, is causing habitat changes for a number of fish species, and have already resulted in reduced fishery catch.

A worst-case scenario prediction has the global biomass of marine animals reducing by 15 per cent, and maximum catch potential for fisheries cut by a quarter.

Indigenous populations in the Arctic have already lost herding, hunting, and fishing areas as a result, and many smaller coastal communities have had to plan for total relocation.

Many low-lying islands, including entire countries, like Kiribati, are likely to be uninhabitable by 2100 due to ocean changes, but habitability thresholds remain extremely difficult to assess.

MAN-MADE PROBLEM, MAN-MADE SOLUTION

Major coastal areas, including most large cities around the world, will need to invest heavily in coastal protection measures within the next few years to plan for the next few decades, the report said.

Land which has been reclaimed from the sea is at particular risk, such as the low lying South Dunedin, and the Wellington CBD.

That means more sea walls, like in Nelson and Dunedin, but also dikes, surge barriers, early warning systems, and flood-proof buildings. Even where that infrastructure already exists, much of it will need to be backed up or improved. Globally, that's going to run up a bill of hundreds of billions of dollars per year.

For major world cities, many of which already have some form of coastal protection infrastructure in place, that might be a cost effective solution. But poorer areas and rural areas with smaller population will find it harder to afford, and in many cases it is the very people with the highest exposure who have the least financial ability to adapt, either by moving or by building reinforcements.

The report urges that restoring coastal vegetation is crucial. Not only is it a carbon sink, but ecosystems like seagrass meadows and mangroves are effective in providing storm protection and improving water quality.

While the sea is a looming threat, it could also be a solution in itself. Azura, a device developed in New Zealand, has the capacity to generate electricity from the power of waves. Oil rigs can be converted into offshore wind farms, and research into algae-based biofuels has been promising.

Victoria University of Wellington Professor James Renwick said the only guaranteed solution was "'unprecedented transitions in all aspects of society."

"If the wholesale transition to renewable energy does not start by 2020, we will be well on the way to two degrees of warming and beyond. Such a warm future would bring serious disruption to global food security and water availability and would displace hundreds of millions of people. The economic and human costs are virtually incalculable."

 - Stuff

JOEL MACMANUS

Last updated 10:57, Sep 26 2019