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A study has found New Zealand's native forests are absorbing more carbon dioxide (CO₂) than previously thought. Study leader, NIWA atmospheric scientist Dr. Beata Bukosa says the findings could have implications for New Zealand's greenhouse gas reporting, carbon credit costs, and climate and land-use policies. The research was in Atmospheric Chemistry and ÌÇÐÄÊÓƵics.

She says forests—both native and exotic—play a vital role in absorbing CO₂ through photosynthesis, but previous studies may have underestimated the amount of carbon taken up by New Zealand's mature indigenous forests, which were thought to be roughly carbon neutral.

Using advanced modeling and NIWA's supercomputer, the researchers examined a decade of atmospheric data, from 2011 to 2020, to better estimate the amount of CO₂ absorbed by New Zealand's land ecosystems. The NIWA team worked with collaborators at GNS Science and Manaaki Whenua as well as other New Zealand and overseas universities and institutes.

The team used an inverse modeling technique—this combines atmospheric greenhouse gases with a model showing how air is transported through the atmosphere to identify CO₂ sources and sinks—and compared the results against New Zealand's Greenhouse Gas Inventory as well as "bottom-up" models.

While the Inventory applies a combination of field inventory, modeling, and remote sensing to quantify forest carbon stocks and stock changes, the 'bottom-up' models use calculations based on ecosystem processes, land use and climate across the country, says Dr. Bukosa.

"It was thought that some areas and land use types were in a near-balance state with the absorption and release of CO₂. Earlier estimates of how much carbon was removed by New Zealand land ecosystems ranged from a net 24 to 118 million tonnes a year," says Dr. Bukosa. "Our research found that New Zealand's natural environment absorbed approximately 171 million tonnes of CO₂ annually."

She says the largest differences between earlier estimates and the new findings came in the South Island. "This was especially in areas dominated by mature native forests and certain grazing lands. We also found seasonal variation, as during autumn and winter, less CO₂ is released into the atmosphere than earlier estimates suggested."

The research follows a pilot study in 2017, which also suggested indigenous forests in Fiordland might be absorbing more CO₂ than expected.

"That study was based on only three years of data, and we weren't sure if it was just a transient effect related to the climatic conditions, or if the effect was confined to Fiordland. Our new study shows the carbon sink is more widespread than we thought, particularly across the South Island, with greater uptake of CO₂ extending up the West Coast," says Dr. Bukosa.

"With improvements in our modeling techniques, and data coverage, we've now shown the extra carbon uptake has persisted for at least a decade. More research could help us understand exactly why our method has shown such a difference in the carbon source and sink balance compared with other methods."

Inverse modeling provides an independent estimate of emissions that can complement inventory-based approaches for emissions reporting, she says. "New Zealand was the first country to develop the capability to infer national CO₂ emissions from atmospheric data and has since supported other countries to develop similar capability."

Dr. Andrea Brandon, a Ministry for the Environment principal scientist who co-authored the study, said the findings help build a clearer picture of the role New Zealand's natural systems play in absorbing emissions from the atmosphere. However, further work will be needed before they can be included in official emissions reporting.

"We continually improve the Inventory—New Zealand's annual record of emissions and removals—as new science and evidence comes to light. This ensures we have robust information so that we continue to meet our international reporting obligations.

"The findings from this study indicate there may be additional carbon uptake somewhere in the system that we are currently not tracking. We need to identify what we are missing so that we can further refine our Inventory methods to capture it," she said.

Dr. Bukosa says the results have important implications for New Zealand's tracking of carbon emissions and climate policies. "We need to better understand why our native forests are absorbing more CO₂ than expected, and what this could mean for our efforts to reduce greenhouse gas emissions and achieve our domestic and international targets."

The research was part of a NIWA-led, Endeavor program called CarbonWatch NZ, which ended last year. NIWA principal scientist Dr. Sara Mikaloff-Fletcher led CarbonWatch NZ and says the team is now looking to extend this work to definitively solve the puzzle of the difference in carbon between inventory methods and atmospheric measurements.

"This research suggests that we could make the most of opportunities to slow climate change through changes to land management. Projections suggest New Zealand will need 84 million tonnes of emissions reductions on top of what can be done at home to meet its 2030 international commitments under the Paris Agreement," says Dr. Mikaloff-Fletcher.

"In addition to reducing the need for overseas offsets, better management of our native forests and other lands could enable New Zealand to be long-term stewards of our carbon sinks and offer magnificent biodiversity co-benefits."