A new report has pointed to an overlooked menace hampering New Zealand's big climate change goals – possums and deer.
An analysis by Forest and Bird, released this morning, suggests browsing pests could be eroding the carbon-storing potential of New Zealand's podocarp forests by millions of tonnes each year – and calls for more control.
Planting more permanent native forest is a key part of the Climate Change Commission's vision for a decarbonised New Zealand.
Amid the country's steep and erosion-prone land, the commission recommended 300,000ha of it could be planted over the next decade, pointing out that the $5b to $15b cost would soon be outweighed by the carbon benefits alone.
Already, the country's native forests are storing billions of tonnes of carbon dioxide equivalent.
But Forest and Bird's report found that, in the West Coast's kāmahi-podocarp forests alone, some 3.4 million tonnes of CO2 storage were being lost every year because of browsing from deer, goats, chamois and possums.
That was equivalent to the country's domestic air travel emissions for all of 2018.
The report also estimated that controlling these feral animal pests to the lowest possible levels would increase the carbon sequestration of native ecosystems by 8.4 million tonnes of CO2 per year, which is equivalent to nearly 15 per cent of New Zealand's 2018 net greenhouse gas emissions.
"The possums, deer, wallabies, goats, pigs, chamois, and tahr that were released into the wild have been chomping their way through native forests, shrublands, and tussocklands," Forest and Bird chief executive Kevin Hague said.
"This has destroyed the natural ability of native ecosystems to be the best carbon sinks on land."
"Our native ecosystems in their natural state are superb at locking in and storing vast amounts of carbon. But they need to be healthy and free from introduced animal pests to do that."
Hague said the range of browsing pests was "out of control" across the country, with both deer and goat numbers increasing significantly over the past decade."
"When native forests collapse, huge volumes of carbon dioxide are released as trees die and rot. Our largest forest type is presently bleeding 3.4 million tonnes of CO2 every year," Hague said.
"Impacts are multiplied if more than one invasive species of browser is present.
"They are killing our native habitats and causing native forests, shrublands, and tussocklands to release carbon instead of holding it. By eating seedlings and killing young trees these introduced pest animals also consume future generations of forest, and our future carbon sinks."
Hague said acting now to turn around the destruction caused by browsing pests would restore natural carbon sinks and protect native plants and wildlife.
"This work needs to be over and above New Zealand's climate commitments to eliminate fossil fuel emissions and substantially cut agricultural emissions as part of our fair share of global efforts to help keep warming below 1.5C," he said.
"It could even help make Aotearoa carbon positive within a few decades."
The report called for increased control, coordination, and research to reduce browsing pests, and restore the carbon sequestration of native ecosystems.
"Also, farmland that is currently being retired and allowed to regenerate as well as newly planted permanent native forest sinks will need protection from browsing mammals too or all that work will be wasted."
The new analysis comes days after a landmark report found that treating climate, biodiversity and human society as "coupled systems" would be key to successfully mitigate the effects of climate change.
The report, produced by the UN's Intergovernmental Panel on Climate Change (IPCC) and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), found that previous policies had largely tackled biodiversity loss and climate change independently.
Among the interventions they suggested were stopping the loss and degradation of carbon-and species-rich ecosystem, including forests and wetlands, and coastal ecosystems such as mangroves, salt marshes, kelp forests, and seagrass meadows.