Scale of planting to meet zero carbon goal ‘unachievable'
Dunedin City Council zero carbon manager Jinty MacTavish. File photo: Peter McIntosh
It is "unachievable" for Dunedin to plant a path to its zero carbon goals, city councillors have been told.
At a workshop yesterday, Dunedin City Council zero carbon manager Jinty MacTavish told councillors sequestration — capturing and storing atmospheric carbon dioxide, often with trees or vegetation — needed to be combined with a community-wide effort to reduce emissions.
Staff had carried out modelling to provide an idea of the trees required to meet the council's goal, although she warned councillors the results were "incredibly theoretical and incredibly indicative".
Speaking to the modelling, senior zero carbon policy analyst Rory MacLean said the scale of planting needed to meet the city's zero carbon goal was "unachievable".
The council had aimed for Dunedin to be a net zero-carbon city by 2030, excluding biogenic methane, but conceded in January this was unlikely, regardless of the level of investment.
Mr MacLean said more than 100,000ha of indigenous planting would have been required to meet the goal by 2030 — "and that's just imaginary numbers really".
If the goal was shifted to 2035, 27,000ha of indigenous plantings or 16,500ha of exotic plantings were needed for the city to become net zero as there was more time for the trees to grow.
"At present there's about 17,000ha of commercial exotic forests in Dunedin, so you're talking about a doubling or almost tripling of the land area covered by forests."
In an "accelerated ambition" scenario, 20,000ha of indigenous plantings or 10,000ha of exotic plantings would meet the 2035 goal.
Still, this was an "enormous" land area to be converted to forestry, Mr MacLean said.
"One of the assumptions in this is that all the plantings happen this year, which obviously would not happen.
"So if you're actually looking to do this, the land area would be even larger because you would need to space out the plantings over multiple years."
Ms MacTavish said decreasing emissions would reduce the amount of land required for planting.
"The intention of this was just to show that this needs to be a whole of community effort rather than something that the DCC alone would take on if it were to be achieved."
In 2021-22, the most recent year the council had full data for, Dunedin's forests absorbed 493,000tonnes of CO₂.
"It's not insignificant, and that sets us apart from other cities that don't have the large land area that Dunedin has. "
Forests were the only type of sequestration included in the Emissions Trading Scheme and emerging methods of absorbing carbon — such as blue carbon (wetlands) or increasing soil carbon — were not easily measurable and verifiable, she said.
A report on carbon renewals would go to council later this month.
ruby.shaw@odt.co.nz
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