What is a national GHG inventory?
The Ministry for the Environment defines New Zealand’s Greenhouse Gas Inventory as “an annual report of all human-induced emissions and removals of greenhouse gases in New Zealand”. The Ministry must produce the inventory every year to support national commitments under the UNFCCC and the Kyoto Protocol.
The inventory reports on six direct groups of greenhouse gases across six sectors.
INVENTORY GREENHOUSE GASES
- carbon dioxide
- nitrous oxide
- sulphur hexafluoride
- industrial processes
- solvent and other product use
- land use, land-use change and forestry
Agriculture sector emissions in the New Zealand inventory include only methane and nitrous oxide; changes in soil carbon stocks currently are not estimated for agriculture as these are considered to be too uncertain at this stage. Read more on soil carbon
For the inventory, two key types of information are necessary:
1. Activity Data, such as the total number (and weight and type) of animals, or total amount of oil, gas and coal consumed; and
2. Emission Factors that describe the amount of greenhouse gas emissions associated with an activity, e.g. the emission from the burning of one barrel of oil, the average emissions per adult milking cow, or from one kilogram of nitrogen fertiliser deposited on soil.
Total emissions are then calculated by multiplying the emission factor with activity data. Different levels of complexity are used to describe activities (at national and regional scales), and to differentiate emissions generated by different activities, processes, and animal classes.
Read more about farm scale emissions assesment
Emissions from an average cow or sheep depend on the animal’s productivity and feed input. This makes it important to underpin the national agricultural emissions inventory with actual measurements of New Zealand animals in New Zealand grazing systems.
The Intergovernmental Panel on Climate Change (IPCC) has developed good practice guidelines for national greenhouse gas inventories.
There are different ‘tiers’ of complexity depending on the sophistication of available data.
Tier 1 inventory
‘Tier 1’ methodologies are for countries that do not have the capacity to collect detailed emissions data. The ‘Tier 1’ methodology for agriculture, for instance, does not require any knowledge of the sheep population structure (numbers of ewes, rams, lambs, etc) and simply assumes a single emission factor per (average) sheep.
Tier 2 inventory
Given the importance of agriculture to the New Zealand economy, we have robust systems for collecting data such as animal numbers, classes and productivity, and hence our inventory can be more sophisticated.
In the IPCC guidelines, ‘Tier 2’ methodologies for agricultural emissions hinge on estimating dry matter intake, which varies during the year with breeding cycles and lactation status. Methane and nitrous oxide emissions are directly linked to the number of animals and the amount they eat to give them the energy they need.
New Zealand has devised its own approved Tier 2 methodology for pasturefed cattle, sheep and deer, to reflect emissions under New Zealand conditions and farming practices, based on the range of measurements outlined in this fact sheet.
The detailed methodology and calculations are published by the Ministry for Primary Industries. Read more about methodology and calculations (external website)
Examples of data used in the New Zealand agricultural inventory methodology
- regional productivity
- animal age and breeding status
- differing energy requirements for milk production, conception/gestation and live weight gain
- adjustment of energy requirements for rising one-yearolds (since they spend part of the first year of life being milk-fed)
- specific energy requirements for velvet production in deer
- monthly pasture dry matter digestibility and metabolisable energy content
- amount of nitrogen content in urine relative to the amount of nitrogen content in dung
- proportion of total annual nitrogen excretion in manure management systems (for dairy)
Why do inventory numbers change?
Direct measurement techniques like those described above underpin the development of emissions factors and calculation methods used in inventories. For example, nitrous oxide emissions have been found to vary considerably with soil type and rainfall around New Zealand, but a balance needs to be struck between reflecting this diversity and the difficulty of providing actual and verifiable measurements. Reflecting a diversity of conditions requires far more data, which increases measurement and verification costs. For the time being, a single emissions factor for urine deposits has been applied across the country. As a result, nitrous oxide emissions may be overestimated in some places and underestimated in others.
Further research is being conducted to look at this variability, e.g. emissions of nitrous oxide from hill country. As new information emerges, it is fed into refinements of the inventory, with a constant effort to improving the inventory to better reflect this diversity and changes in farm systems over time. National greenhouse gas inventories are subject to annual international expert review through the UNFCCC. New research, published in peer reviewed journals, adds more detail to the overall picture each year and is necessary to justify any changes made to emission factors. The inventory seeks to reflect the real world as closely as possible, so it should be revised as scientists discover more. Revisions must apply all the way back to the start of the accounting period (1990) so that reported long-term trends reflect as well as possible genuine changes in emissions, rather than changes in how emissions are calculated. Future changes in the inventory are inevitable. These could arise
from ongoing improvements in measurement techniques, and growth in the number and types of measurements taken (which will pick up more variability, for instance, between sites and farms), other changes in activity data, emission factors and methodology, or when new emission sources are identified.
How reliable is the inventory?
Scientists have a good understanding of how emissions increase in line with changes in animal productivity, which affects feed intake (especially important for methane) and the amount of nitrogen excreted (which drives nitrous oxide emissions). This, combined with good quality data on agricultural productivity and animal population changes, means New Zealand’s inventory provides clear, reliable evidence of
trends in agricultural greenhouse gas emissions. The inventory is an essential and robust tool to show changes in absolute emissions, and in emissions intensity (i.e. emissions per unit of product). While emissions from agriculture have increased overall, mainly because of increased total production, emissions per litre of milk or kilogram of meat produced have declined, because of the increased productivity of animals
and efficiency of farm systems.
While we can be reasonably certain about whether emissions are increasing or decreasing, it is much more difficult to put an accurate number on the absolute amount of emissions from the country as a whole. It is possible to measure emissions of individual animals or at paddockscale,
but it is hard to be certain that these measurements are truly representative of the national herd. When we multiply estimated emissions from a limited number of individual animals to a population of millions of animals, any inaccuracy or lack of representativeness in the original measurements will be reflected in the national total. The absolute level of uncertainty in the national agricultural methane emissions inventory is estimated to be 16%, a value higher than for energy (about 6%), but significantly lower than that for forestry (at 54%).
New Zealand’s total gross greenhouse gas emissions by sector and agricultural sub-sector (graph)
New Zealand’s total greenhouse gas emissions by agricultural sub-sector including nitrogen fertiliser use from 1990 (graph)
New Zealand’s emissions intensity, by agricultural sub-sectorincluding nitrogen fertiliser use from 1990 (graph)
About greenhouse gas metrics
About New Zealand's Greenhouse Gas Inventory (exernal website)
About inventory methodology and calculations (external website)
Factsheet on how New Zealand measures emissions
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