Capturing carbon to slow climate change


New research tests the air to estimate the carbon sink potential of forests and landscapes. It reveals that the ability of New Zealand’s land biosphere to absorb carbon could be 50 per cent more than currently estimated.

About half of all the carbon dioxide that has been emitted to the atmosphere from human activities over the last few decades has been absorbed by Earth’s oceans and land biosphere. These carbon sinks have dramatically slowed climate change compared to what we would have experienced without them.

Despite the importance of carbon sinks in slowing climate change and meeting our international treaty obligations, we don’t yet have a solid sense of how much carbon dioxide is absorbed by forests, soils, pasture and other land areas.

The amount of carbon absorbed by our forests is currently determined by measurements of live trees and dead wood made at a national network of forest plots, which are scaled up to the national level using modelling techniques. This approach is powerful in that it allows a clean separation between different types of land use. Yet there is considerable uncertainty associated with upscaling measurements at sample plots to the national level, and many processes that are not explicitly measured may be missed.

Forests planted since 1990 are counted as carbon credits in international climate treaties. Forest carbon uptake is particularly vital to New Zealand’s emissions targets, offsetting just over 30 per cent of our emissions in 2014 according to estimates from the Ministry for the Environment.

Scientists and policymakers are keen to improve the data and models on which the estimates are based. Atmospheric observations could be the key to validating and improving these estimates.

Sniffing the air

An alternate ‘inverse’ approach is being applied for the first time in New Zealand to refine estimates of carbon uptake. As air passes over our landscape, the amount of carbon dioxide in the air goes up or down due to carbon exchange with the underlying land. It is possible to estimate the total amount of carbon that has been absorbed or emitted by combining atmospheric carbon dioxide measurements from a network of sites around New Zealand with atmospheric models that describe the pathway of air arriving at the sites.

NIWA researcher Dr Sara Mikaloff-Fletcher is using this approach to estimate our national carbon sinks. A core-funded project analysed data collected by NIWA’s three national atmospheric carbon dioxide monitoring stations, looking for telltale changes in atmospheric carbon dioxide.

“The inverse modelling technique is like smelling an amazing BBQ somewhere in your neighbourhood. If you sniff the air in a few different places, and notice the direction the wind is blowing from at each spot, you’ll probably be able to work out where the BBQ is."

“A network of atmospheric monitoring sites ‘sniff’ for changes in atmospheric carbon dioxide and atmospheric models to work out which direction the wind was blowing when each measurement was taken. These pieces of information can be combined to work out the carbon uptake from different regions around New Zealand needed to match the data.”

An atmospheric model is run backwards in time to understand what parts of New Zealand the air passed over before it arrived at the station.

Mikaloff-Fletcher explains: “For each measurement being used in the inverse model, we release 1,000 particles at the site and run the model backwards in time for four days. This tells us all of the different ways air might have travelled to the observing site at that particular measurement time.”

This research also integrates land model simulations produced by GNS Science, and observations of sea surface carbon dioxide, to provide a ‘first guess’ about what the air-land and air-sea exchange.

“The inverse approach integrates information about carbon dioxide sources and sinks from atmospheric data, ocean data and models.”

The results are reported in a research paper submitted to Atmospheric Chemistry and Physics, "Atmospheric CO₂ observations and models suggest strong carbon uptake by forests in New Zealand".

Bigger than it looks

The most stunning discovery is that the carbon carrying capacity of New Zealand’s landscape could be much greater than expected from the national inventory report or the land model used in the study.

On the annual scale, the terrestrial biosphere in New Zealand is estimated to be a net CO₂ sink, removing 98 (±37) teragrams (Tg) CO₂ per year (a teragram is one million metric tons) from the atmosphere on average during 2011–13. This sink is much larger than the reported 27 Tg CO₂ per year from the national inventory for the same time period.

Mikaloff-Fletcher says the team can partially reconcile the difference by factoring in forest and agricultural management and exports, fossil fuel emission estimates, hydrologic fluxes and soil carbon change.

“But there are still a lot of processes that may not be captured by the national inventory report."

“The results suggest that indigenous forests on the South Island may be a much more vigorous carbon sink than previously thought. Uptake by grasslands, hill country, and soils may also play a key role."

“The inverse model reveals strong terrestrial land fluxes from the South Island of New Zealand, especially in western regions covered by indigenous forest, suggesting a surprisingly high carbon uptake there.”

“There might be some unique terrestrial processes happening inside the Fiordland forest that we don’t yet know about,” she says.

What next?

The Intergovernmental Panel on Climate Change recommends using inverse methods to refine and compliment carbon sink calculations from the national inventory reporting done by the Ministry for the Environment. New Zealand has a natural advantage for the application of these techniques.

“By the time air reaches New Zealand it is well mixed and contains very little influence from other countries,” Mikaloff-Fletcher says.

This map shows how well CO2 emitted or absorbed by different parts of New Zealand can be detected by our three observing stations. Colours indicate frequency with which CO2 from the originating location is observed by one or more of the three stations. [Image: NIWA]
Now the proof of concept has been piloted, Mikaloff-Fletcher is planning a range of sophisticated improvements.

“We need two things to give New Zealanders policy-relevant information about our carbon sinks; better information about where and why our forests and land areas are absorbing so much carbon dioxide, and less uncertainty in the inverse estimates.

“The former is particularly important, since that’s where we might be able to give real advice about how land management decisions could impact on climate and treaty obligations.”

The initial study was based on only two observing sites; Baring Head, near Wellington, and Lauder in Cental Otago.

“Even from these two sites, it is possible to observe much of New Zealand, because we have measured carbon dioxide there continuously for years and because the winds are always changing. This afternoon, our Lauder station might be seeing signals from Fiordland, but tomorrow it could be telling us something about the Canterbury Plains.”

However, with just two sites, Mikaloff Fletcher says they can only reliably estimate the total carbon uptake from relatively large spatial regions.

“We have already established a new carbon dioxide observing site at Rainbow Mountain, near Rotorua, and the measurement record there is long enough that we can begin using it in the inversion later this year.

“Our first analysis of the Rainbow Mountain data suggests that it has an exciting story to tell us about forest carbon uptake in the central North Island.“

Mikaloff-Fletcher hopes to deploy at least two more atmospheric sensing sites. The white space in the graphic on the left indicates the regions not currently being ‘sensed’ by the three existing stations. The expanded national carbon dioxide observing network will improve ability to pinpoint sink locations and reduce uncertainty in estimates.

“The next step will be to improve the resolution of the meteorology we use to drive the model, which will dramatically reduce the uncertainty of our estimates. This is particularly important for New Zealand, because of the way the winds interact with our complex topography.

“All this doesn’t amount to much without building close links to New Zealand’s land carbon uptake community. The inverse method’s strength is also its weakness. The atmosphere records total carbon exchange, so all processes are captured.

“It is very difficult to isolate processes using this technique, which you need in order to understand what the results mean for land and carbon management decisions and treaty obligations. We work with land modellers and the carbon accounting community, and these collaborations need to strengthen if we are to understand why New Zealand is absorbing so much carbon.”

It’s estimated that natural and planted forest covers about 37 per cent of New Zealand’s land area. Indigenous forests cover approximately 6.2 million
hectares. Scrublands cover 2.7 million hectares. [Photo: Dave Allen, NIWA]


What a difference research makes

News that our carbon sink could be bigger than estimated is significant information for those discussing climate change policy and negotiating international agreements. Mikaloff-Fletcher’s long-term work will provide more accurate assessments of the importance of non-Kyoto factors in determining our contribution to climate change, and perhaps to our targets.

She warns that the first study produced a carbon-carrying capacity bigger than expected.

“It’s produced results that are bigger than we expected, and with less certainty than we need. While that suggests the current models could be improved, our techniques need improving as well.”

“The research report has stirred a lot of interest among scientists and policymakers. Everyone has an interest in being more accurate about the role of landscape in the carbon cycle.”

Counting trees

To determine carbon stocks in forestland and change in those stocks over time, the Ministry for the Environment uses inventory-based methods, and conversion equations and models.

About one hundred standard-sized sampling areas (forest plots) are located on a sampling grid set across forestland. The plots are permanent and monitored over time.

Measurements are taken of both living trees and dead wood, along with other plot specific data. Allometric equations and modelling techniques are used in the calculations for the amount of carbon held in natural and planted forests respectively.

New Zealand has estimated the carbon carrying capacity of forests with the help of an airborne remote sensing technology called LiDAR. The tool measures distance and surface properties of targets by illuminating them with a laser and recording the reflections. Analysis of the data allows researchers to estimate the forest structure and ground elevation.

Forests are complex three-dimensional structures containing large variations in the amount, orientation, distribution and clumping of vegetative tissues and non-vegetative elements. That means the complex interaction of LiDAR’s electromagnetic radiation with vegetation canopies needs to be unscrambled with the assistance of other statistical techniques.

New Zealand’s LiDAR data is made up of 70m-wide sweeps across forests established before and after the crucial Kyoto 1989/1990 creation dates. The measurements were taken in 2008 and 2010. Ground-based sampling updates growth rates.

How much is green?

It’s estimated that natural and planted forest covers about 37 per cent of New Zealand’s land area. Indigenous forests cover approximately 6.2 million hectares. Scrublands cover 2.7 million hectares. The combined area of indigenous forest and scrub covers more than five times the area that is under plantation forest, and contains a significant stock of carbon locked up in trees, understory, forest floor and soil.

Vegetation survey

The National Vegetation Survey Databank (NVS) records over 94,000 vegetation survey plots. This provides a 50-year record of indigenous and exotic plants across New Zealand habitats, particularly native forest and grasslands.

The data has been used to meet reporting requirements for the Convention on Biological Diversity, Framework Convention on Climate Change, and Resource Management Act. It has also been used to assess the impacts of climate change on indigenous ecosystems, the storage of carbon in indigenous ecosystems, and setting restoration goals in degraded areas.

The New Zealand Forest Service, Department of Lands & Survey, and the DSIR Botany Division conducted the original surveys. On-going surveys and research by the Department of Conservation, regional councils, universities, private consultants and Landcare Research are continually providing new data to NVS.

Note: This feature originally appeared in Water & Atmosphere, October 2016
Research subject: Climate change