There is a widespread perception that accounting for carbon in the biosphere is inherently more difficult than accounting for carbon emitted by the burning of fossil fuels. This perception is only partly true. Determining the amount of carbon held in vegetation and soils does not require measurement of all of the carbon in all areas. Well-established statistical sampling techniques such as stratification and random sampling can be used to determine the carbon stocks of the biosphere as precisely and accurately as would be possible with complete measurement.
Neither accuracy (the absence of bias) nor precision (how well a measurement can be repeated) is compromised when appropriate statistical methods are used. Nevertheless, in statistical sampling there is a tradeoff between precision and effort (cost). A greater number of samples reduces error (i.e., increases precision) in the estimate but also requires more effort. In fact, each successive increment in precision requires a proportionately greater increase in effort. The decision about whether or how precisely to measure a carbon sink will depend on the effort required for that precision and the magnitude of the expected sink. Small changes in carbon or changes that require a large effort for measurement may not be worth the effort. Table 2-7 gives the costs of different methods in US$ per hectare for project-level measurements. Costs per ton of carbon are not given because the costs vary from country to country not only with the desired precision but with the types of land-use change and the magnitude of the change in carbon per unit area. Thus, costs per ton of carbon at the national level must be determined for each country individually. Similarly, the combination of methods most appropriate for national-level estimates of change will vary for each country; we do not attempt to suggest a single appropriate combination.
There are at least two significant generic problems with the estimation of change in terrestrial biospheric carbon stocks. The first problem relates to resolution (i.e., the smallest detectable change). Because the rate of change of most biospheric pools is slow, particularly in relation to the size of the pool, resolvable changes in stock are typically not easily obtained for the larger pools.
The second problem is practical. Most countries do not have the established infrastructure required for regular measurement of biospheric carbon (although all Annex I countries have a regular forest inventory in place). Where no infrastructure exists, measurement of carbon to the required degree of precision and accuracy is an expensive and logistically complex exercise. Most of the developed countries, as well as some less developed countries, have at least part of the required infrastructure already in place: certifiable analytical laboratories equipped to measure the carbon content of soils and biomass; a national forest and soil inventory system; accurate soil and vegetation maps on which to base the sample stratification; trained field, analytical, and statistical staff; and a physical infrastructure that allows access to remote sites. Even where this capacity exists, the incremental cost of performing a national-scale carbon inventory may be substantial. Australia, for example, is investing an additional $5 million annually in anticipation of upgrading its carbon accounting system for the Kyoto Protocol. The costs may be greater in countries in which the inventory infrastructure is less well developed. The use of models and stratified and multi-objective sample programs may reduce these costs, however.
The cost of conducting biospheric carbon inventories depends on the size of the area inventoried-but more on the range of ecological conditions within it because the spatial scale over which soil and biomass carbon varies is quite small: a few tens to hundreds of meters. The sample size needed to achieve the desired precision may be similar for a small country and a distinct region within a large country. Both need approximately the same analytical equipment and statistical treatment. Thus, the cost to a country will depend more on the range of different bio-geophysical regions that exist within its borders than on its actual size.
The total inventory costs for a single project will be substantially less than the inventory costs for an entire country, but the cumulative cost of inventorying several projects soon reaches the level of a national inventory. This is because the costs of forest inventory vary considerably depending on the desired accuracy, the accessibility of the forest, the availability of pre-information, the degree of automation employed, the availability of allometric relations, and so forth. At a typical project scale of tens of thousands of hectares, the costs are on the order of US$11-18 ha-1 (Nabuurs et al., 1999). The typical cost of a national forest inventory, on the other hand, is on the order of US$0.05-0.6 ha-1. Costs are declining with the increased use of automated data collection and analysis (see also Table 2-7).
The methods for determining changes in carbon storage are described briefly in the following sections. These methods are divided into methods for the measurement of stocks, the measurement of flux, and the measurement of area, as well as models. Table 2-7 summarizes the characteristics of different methods.
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