Photo: RNZ Pacific/ Lydia Lewis
The government wants blue carbon counted towards the country's efforts to slow climate change.
Its final Emissions Reduction Plan for 2026-2030, due in December, is expected to mention the potential to harness the ocean, including using seagrasses, mangroves and other marine species as carbon sinks.
The draft version of the plan earlier this year highlighted the role of "alternative" sinks such as blue carbon as a potential boost to climate efforts beyond the most popular way of sucking in carbon produced by cars, trucks, factories, and farming - planting trees.
But counting blue carbon could cut both ways.
While seagrass, mangroves, kelp and the like have potential to store carbon, other human activities such as bottom trawling release carbon.
New Zealand's enormous exclusive economic zone contains a huge amount of carbon in its ocean sediments, and releasing even a tiny proportion could undo any other climate benefits.
Blue carbon is any carbon stored in the ocean and coastlines, from seaweed to sediment on the sea floor.
NZ First campaigned on trying to get blue carbon officially recognised in the country's climate accounts, and NZ First/National's coalition deal includes "progressing work to recognise other forms of carbon sequestration, including blue carbon".
But, like any carbon sink, blue carbon only counts towards a country's climate efforts if people deliberately do something to increase how much is stored. Natural stores or releases of carbon (like wildfires) do not count.
And, according to the draft Emissions Reduction Plan, before blue carbon could qualify for payments under New Zealand's Emissions Trading Scheme, more studies are needed studies exactly how much carbon is sucked in by planting marine species, protecting estuaries and the like.
The potential for oceans to help stabilise the climate is enormous, Rebecca Mills, managing director of carbon impact consultancy The Lever Room, says.
"There's a dominant narrative that we're a small country so it doesn't matter what we do on the world stage, whereas actually if you look at our marine space we're responsible for a big chunk of the planet. The ocean plays a major role in regulating our climate and can be both a source and sink for carbon."
Mills says oceans have bought us precious time to cut fossil fuel emissions by absorbing both heat (about nine tenth of the heat humans have added to Earth so far) and carbon dioxide, preventing the land surface we live from the feeling anything like the full effects of our greenhouse gas emissions.
They could do more. Marine plants are only part of it, she says. "There's also things like ocean-based renewable energy, low carbon ocean-based food...."
It is a potential the government is keen to harness - at least in theory.
Its decision to fast-track sand mining off the coast of Taranaki saw one major company give up on establishing offshore wind generation here.
Photo: 123RF
The draft of its carbon-cutting plan highlights the role the likes of coastal mangroves and seagrasses could play but notes more evidence is needed.
Some researchers are working on getting the proof, like Dr Robert Hickson of Blue Carbon Services, who is using a Government Smart Ideas grant to study how much carbon could be stored by kelp cultivated on mussel farms.
He is currently studying mussel farms growing kelp in Cook Strait.
Hickson wants to build a model calculating how much of the carbon from dropped leaves and other detritus shed by kelp is munched by the mussels, bacteria and other critters - versus how much survives to reach deep water, where it could stay for hundreds or thousands of years. Below 1000 metros there is not much mixing, and carbon can be safe for a long time, he says.
But it is a tough journey for carbon-rich materials to get there.
"It's a little bit like running a gauntlet," he says, "between where it's produced on the coast where the water's shallow and there's every critter conceivable to break it down ... into the open ocean down into deep water, the question we are trying to determine is what percentage does survive until it can get down into deep water."
He says the biggest carbon-storing potential is probably from deep water mussel-and-kelp farms, 10km offshore and near deep water, to shorten the journey the carbon must take before its safe from being demolished and released back to the atmosphere.
Rich carbon stores at risk
Counting blue carbon could cut both ways.
Marine scientist Geoffroy Lamarche, chief science adviser to Parliament's environment commissioner, says New Zealand has 2.2 billion tonnes of carbon in its ocean floor - and some of these rich deposits of carbon in risk being released by bottom trawling.
The Parliamentary Commission for the Environment's office commissioned research in 2022 estimating not only how much blue carbon was in New Zealand's territory, but also any overlap between rich carbon deposits in sediment and areas open to fishing and bottom trawling.
"You've got a lot of carbon that is stored in marine sediment. It's reasonably well acknowledges that the biggest risk is bottom trawling," Lamarche says.
Fiordland is particularly rich in carbon but less at risk from trawling, while the Hauraki Gulf is both rich in carbon and more vulnerable to disturbance, he said.
Run-off of carbon-rich soil from land around the gulf finds its way into the harbour, where it can be released when the floor of the seabed is disturbed, he says.
"In the Hauraki Gulf the risk is high because you have a lot of carbon sequestered and there are still some fishing and disturbances," he says.
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