When a plant grows, carbon is extracted from the air as CO2 is absorbed in photosynthesis, so becoming 'locked into' carbohydrate material both above and below ground. Significant amounts of CO2 are released in plant metabolism, but the net carbon flow is into the plant. Carbon concentrations in the soil may also increase 'indirectly' from organic matter formed from plant detritus in fallen leaves and branches. Such removal of the greenhouse gas CO2 from the atmosphere is called a 'carbon sink'. Consequently a dedicated programme to increase plant growth will offset temporarily an increase in atmospheric CO2 from burning fossil fuels. However, all plants die and the vast majority of all such direct and indirect carbon eventually returns to the atmosphere, so joining a natural cycle which neither depletes nor increases atmospheric CO2 concentrations. Only if the plant material is burnt to replace (abate) specific use of fossil fuel will there be a long-term benefit, by preventing that fossil carbon from otherwise reaching the atmosphere. It follows that such abated fossil carbon should always stay beneath the ground and never be extracted.
The Kyoto Protocol is an international treaty which aims to mitigate climate change induced by greenhouse gas emissions; amongst other things it encourages countries to plant new forests. However, as explained above, such carbon sinks are only temporary because when the forest is harvested all its above-ground carbon will be returned relatively quickly to the atmosphere: within months if used for paper, years if used for construction. Thus only a new and thereafter continuing forest can be a carbon sink, albeit 'once only'. However, a replanted forest cannot be so considered, unless the biomass of the previous forest was used to abate fossil fuel. Figure 11.8 illustrates such effects. Consequently, the effectiveness of forest plantations to act as carbon sinks is limited by the availability of 'unused' land, which, from a global perspective, is itself limited by the increasing need for land for food.
Biomass can be both a carbon sink and a substitute for fossil fuels, in the many ways described in this chapter. Used as bioenergy, biomass has an additional value as a carbon offset because the fossil fuel which it displaces remains underground. Its value in this way has not been fully recognised in the Kyoto Protocol. The carbon offset of CO2 emissions by using bioenergy is sustainable (at least in principle), since each annual crop replaces the same amount of would-be fossil fuel emissions, regardless of previous or subsequent harvests - unlike the sink effect, which is negated by each successive harvest (see Figure 11.8). As indicated in Figure 11.8, coppicing can give a similar effect.
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