Native forests absorbing more carbon dioxide
New Zealand’s forests and other land areas may be absorbing up to 60% more carbon dioxide than has been calculated, with much of this uptake likely occurring in native forests, NIWA scientists have discovered.
New research led by NIWA atmospheric scientists Drs Kay Steinkamp and Sara Mikaloff-Fletcher, indicates that New Zealand’s forests absorb much more than previously thought, with much of the uptake occurring in the southwest of the South Island.
Carbon dioxide is a primary greenhouse gas and responsible for most of the human-induced warming in the atmosphere. Globally, carbon sinks, such as oceans and forests, have helped mitigate the effects of climate change by absorbing about half the carbon dioxide emitted by human activities over the past few decades. New Zealand’s forest carbon uptake played a key role in meeting our commitments under the Kyoto Climate treaty and is expected to play an important role in meeting our COP21 commitments.
The results of the research have just been published in the scientific journal Atmospheric Chemistry and Physics.
A new approach to combining sources and sink data
Dr Mikaloff-Fletcher and her team used an “inverse” modelling approach to estimate the amount of carbon uptake. This is done by measuring the carbon dioxide present in the atmosphere at a network of sites, and then using high resolution weather models to determine what parts of New Zealand the air has passed over before reaching the site. Simulations from a land model, run by partners at GNS Science, and ocean carbon data provide additional information. From there, the team calculates the best combinations of sources and sinks to match the data.
This project included data from NIWA’s clean air station at Baring Head, near Wellington, its atmospheric research station at Lauder in Central Otago, and measurements taken from a ship that collects observations on a line between Nelson and Osaka, Japan.
“The inverse approach integrates information about carbon dioxide sources and sinks from atmospheric data, ocean data and models,” Dr Steinkamp says.
“The story the atmosphere is telling us is that there’s a big carbon sink somewhere in the South Island, and the areas that seem to be responsible are those largely dominated by indigenous forests. However, we cannot rule out an important role for carbon uptake in the hill country or from pasture from our current data. ” Indigenous forests cover about 6.2 million hectares in New Zealand.
Dr Mikaloff-Fletcher says that was a very surprising result mainly because strong carbon sinks are expected when there is a lot of forest regrowth.
“Carbon uptake this strong is usually associated with peak growth of recently planted forests and tends to slow as forests mature. This amount of uptake from relatively undisturbed forest land is remarkable and may be caused by processes unique to New Zealand or part of a wider global story."
Carbon uptake could be up to 60% higher
The National Inventory method reported by Ministry for the Environment reports annually on New Zealand’s carbon uptake. This internationally standardised methodology puts the amount of carbon being absorbed by all New Zealand forests at 82 teragrams (Tg) CO2 (A teragram is one millon metric tons) total over 2011-2013, the period studied by Dr. Mikaloff Fletcher’s team.
Once accounting rule differences are corrected for, the new NIWA measurement approach finds that actual carbon uptake could be up to 60% higher.
The inventory-based method estimates carbon uptake using measurements of tree growth taken from about 100 sampling areas, and extrapolates this to the entire country using statistical techniques and modelling. There is still considerable work to be done in comparing the two independent approaches.
“We need to find out definitively what processes are controlling this unexpectedly large carbon uptake, in order to understand the implications for land management and climate treaties. We need additional measurements to tell us if this is unique to the southern half of the South Island or holds across a wider range of New Zealand.”
Dr Mikaloff-Fletcher says the ability of forests to absorb carbon is a powerful tool to help address the challenge of climate change.
Next steps include incorporating data from NIWA’s newest atmospheric CO2 observing site, Maunga Kākaramea/Rainbow Mountain in the central North Island, deploying two new atmospheric CO2 observing sites and a major improvement to model resolution. This will start to shed light on what’s happening in the North Island and the Canterbury plains.