2.2 REVIEW OF AGRICULTURAL SOURCES AND SINKS OF CO2
2.2.1 Methodology
There are potential ambiguities in attributing greenhouse gas emissions to different types of activity or sectors of the economy. In particular some emissions could be attributed either to a producer or consumer. The OECD methodology for constructing national greenhouse gas emission inventories relies heavily on existing procedures for collecting national energy statistics. Thus it is most likely that the classification system to be used in meeting obligations under the FCCC will also be based on these existing procedures. In particular the OECD approach is to use the International Standard Industrial Classification of Economic Activities (ISIC) as a basis for categorising activities. The effect of this is very largely to attribute emissions to producers rather than consumers. Thus CO2 emissions from electricity generation are attributed to an "Energy Transformation" sector and not distributed across end users. Similarly CO2 emissions from the manufacture of lime are attributed to a "Chemical Mining and Quarrying" sector rather than to agriculture.
Thus the "agricultural" component of the New Zealand greenhouse gas emission inventory will include only emissions directly related to farming activity. This should include use of tractors, top-dressing aircraft etc, but is unlikely to include general use of petrol in the rural community.
It may be relevant to note that the ISIC category Agriculture also includes forestry and hunting unless it is broken down into separate sub-categories. At present Ministry for the Environment do not have emissions data separated into Agriculture and Forestry. However, Ministry of Commerce do have more detailed energy consumption statistics from which such a separation could be made in future. It is not yet clear what level of detail will be used when New Zealand submits estimates of its greenhouse gas emissions under the FCCC rules.
Estimates of gas emissions are derived from other more easily measurable indicators of economic activity. Thus emissions by vehicles are calculated from the amount of liquid fuel consumed using "emission factors" determined to be representative of the vehicle fleet in respect of age, standard of maintenance, etc. Emissions of methane by livestock are similarly based on reported stock units using emission factors for methane emitted per unit which are calculated to be representative in terms of feed quality, etc.
The uptake of CO2 by forestry or increasing stocks of plant and soil carbon is to be reported along with emissions. Uptake for forestry will be calculated using reported forest areas, tree age distributions, planting and harvesting information. Detailed studies have been carried out for Pinus Radiata giving average carbon accumulation rates which can be used to relate forest size and age statistics to CO2 uptake.
2.2.2 Agricultural Sources of CO2
As discussed above the agricultural sector is not regarded as a major source of C02, and its sources have not been studied to the same level of detail as say transport. However, the dominant sources of C02 attributable to the agricultural sector are
- on farm use of fossil fuels, essentially diesel and gasoline, with some use of aviation fuel and LPG, for transport, traction, power and heating;
- similar use of fossil fuels for road and possibly rail transport between the farm and factory;
- on-farm burning of forest, scrub or waste materials;
- changes in carbon storage associated with land use change, e.g forest/ scrub to pasture transition, or drainage of swamp land.
The Ministry of Commerce have some 1991 statistics ("Energy Data Files") indicating that the agricultural sector is responsible for about 4.8% of total liquid fuel consumption. Thus, about 0.6 Mt CO2/annum would be attributed this way.
For a more detailed analysis of the emissions due to farm fuel use, one would use emission factors for different types of fuel. There are minor variations in CO2 emissions per unit of fuel consumed depending on vehicle type and condition. Standard figures have been derived from the OECD greenhouse gas emissions methodology (OECD, 1991) and are given in Table 2.3.
Table 2.3 Emission factors for liquid fuel use
Fuel type |
tonne CO2 per tonne fuel |
| Diesel oil | 3.2 |
| Gasoline | 3.1 |
| Aviation fuel | 3.1 |
| LPG | 3.0 |
| CNG | 2.8 |
(Note that combustion of fuel increases the weight of the products as it incorporates oxygen from the atmosphere in the C02 produced.)
It is not known whether agriculture as a whole is a net C02 emitter or net sink, possibly agriculture is CO2 neutral.
Burning is not a major feature of agriculture in New Zealand as it is in tropical countries. Figures developed by Lassey et al (1992), from estimates of the amount of forested land burned, suggest that around 0.15 Mt CO2 per annum to be emitted this way. Farm burning is likely to be small in comparison.
Near surface soil carbon can store from 70 to 400t CO2 equivalent /ha depending on the land use. Pasture land, tends to be in the middle to upper half of this range, whereas crop lands, particularly with high tillage, are at the low end. Tate (in Williams, 1992) discusses possible changes in New Zealand soil carbon, and concludes that changes such as scrub to plantation forestry, and pasture to scrub are unlikely to cause significant changes in carbon sequestration. However, any extensive transition from pasture to crop land could be expected to result in nett CO2 emissions and data cited by Tate suggest such emissions could be around 10t CO2/ha/annum.
At present there is no specific OECD methodology for calculating CO2 emissions due to changes in soil carbon, but it is assumed that nations will make their own estimates where these emissions are significant. Internationally the main concern is with the effect of deforestation on soils in tropical countries.
From a scientific perspective increasing temperatures tends to reduce soil carbon across a wide range of land types and so may be more important in estimating total global emissions. However, this climate impact on soil carbon reservoirs will presumably not be attributed to "agriculture?' as this is not a consequence of farmers' actions.
2.2.3 Agricultural CO2 Sinks
The dominant sinks of CO2 attributable to agriculture are: plantation forestry in agroforestry contexts; planting trees for soil conservation, windbreaks, etc; and reversion of pasture to scrub and secondary forest. As covered above, reversion of pasture to scrub is not expected to cause a significant uptake of C02, and planting for soil conservation and windbreaks is likely to be far more limited than planting for direct economic production. Thus the most important options to consider are those of agro-forestry. Should pasture revert eventually to climactic native forest (such as mature beech forest) then the rate of sequestration appears to be around half that of pine trees. The eventual total carbon stored in a mature beech forest may be more than for pine.
There is no specific OECD methodology for determining the uptake of CO2 due to forestry. However, the FCCC recognizes the need to take into account greenhouse gas sinks and these are considered as part of national emission inventories. New Zealand scientists have developed some of the most detailed methods for assessing carbon storage in production forestry, and it is possible to make reasonably reliable estimates of this sink.
Rates of uptake Of C02 by forest trees are dependent on the tree species, climate, and forest management practices. Internationally a wide range of values has been quoted in the literature ranging from 1 to 30 tonne CO2/ha/yr, varying somewhat with age. Faster growing trees will generally maintain their uptake rates for 60-100 years. Slower growing species have lower uptake rates (eg. <10 tonne CO2/ha/yr) but can continue this for 150-200 years and give greater long term sequestration of carbon per hectare. Rates of uptake as high as 50 tonne CO2/ha/yr are anticipated for genetically adapted species.
Hollinger et al, 1992, calculate that on average 447t CO2/ha are sequestered on a long term basis by managed Pinus radiata forest with a 28 year rotation period (chosen on the basis of current economic considerations). This long-term sequestration increases by 22t CO2/ha for every year increase in rotation length. However, the CO2 uptake rate is low immediately following planting and for the first 4-5 years, thus the average uptake rate over the economic rotation period is about 16t CO2/ha/yr.
Once land used for production forestry reaches maturity and is recycled the implications for CO2 emissions depends to a large extent on the use made of the harvested timber, and whether the land is replanted. Short rotation forestry generally supplies short lived products and we adopt here the assumptions that all carbon in the harvested timber is returned to the atmosphere as CO2, and that the land is immediately replanted. Thus our consideration of the production forestry option assumes a commitment to land use change for the very long term (100 years or longer).
The implications of increasing forested land can be considered from either a short or long term perspective. In the short term each additional forested hectare has little or no effect for about five years and then takes up CO2at a rate of about 20t CO2/ha/yr. When the forest is harvested the carbon stored in the forest is released, the land is replanted and proceeds to re-absorb the equivalent amount of CO2. Thus the first cycle acts to reduce CO2 emissions whereas subsequent growth and production are in equilibrium. In order to maintain a long-term net CO2 sink it is necessary to continually increase the forested area.
From the long term perspective each hectare of land committed to production forestry can be considered part of a pool managed on a sustainable basis with a mix of trees of all ages. This pool of forested land holds the equivalent of 447 t CO2/ha. In order to maintain a net CO2 sink the size of the pool has to be increased each year.
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