In equation (3), these options reduce emissions from FLULUC and promote CO2 uptake in the land component of FSq.
Reforestation, Afforestation, and Land Restoration
The potential global carbon sequestration due to increased forest extension and other land uses is in the order of 1 PgC y-1 by 2010 (Watson et al. 2000b). In the past few decades major plantation has occurred in many countries, often where previous deforestation took place. During the 1990s alone, the global area of plantations increased by
3.1 Mha y-1 (Braatz 2001). Regionally, reforestation and afforestation in China during the past two decades is contributing 80 percent of the total current Chinese carbon sink (Fang et al. 2001), and the return of abandoned farmland to native vegetation is estimated to be responsible for 98 percent of the current sink in the eastern United States (Caspersen et al. 2000).
If done with proper regard for the ecology and history of human land use in a region, expansion of forest area usually brings a number of collateral benefits: preservation of biodiversity, decreased soil erosion and siltation, decreased salinization, and watershed protection with associated water supply and flooding regulation. These benefits will most likely come with little adverse effect on agricultural production, employment, economic well-being, and cultural and aesthetic values in countries with agricultural surpluses such as Europe and the United States. In developing countries, however, which are usually dependent on exports of raw materials and confronted with high levels of social segregation, diverting land for reforestation may have negative consequences if it is not adapted to the local environment, land use patterns, and institutional settings. In these regions, land for afforestation is usually created by logging primary forest (Schulze et al. 2003). This practice affects vulnerable communities such as small-holding farmers (Silva 1997). There are also some environmental concerns: reduction of runoff during dry periods (due to increased soil infiltration and transpiration) may have impacts downstream, for instance in farmlands and wetlands. In boreal regions, changes in albedo due to darker forest canopies may lead to positive climate forcing and regional warming (Betts 2000; Betts et al. 2000). Taking into account a set of institutional and socioeconomic constraints only, Cannell (2003) estimated that only 10—25 percent of the potential C sequestration could realistically be achieved by 2100.
Managing wood products is an important part of a terrestrial sequestration strategy. It involves recycling paper, making use of long-lasting wood products that can substitute for high fossil-carbon-content materials, and other steps to increase the residence time of carbon sequestered as utilized wood.
Reduction of Net Deforestation and Emissions from Land Use
Deforestation over the past 200 years has contributed 30 percent of the present anthropogenic increase of atmospheric CO2. Current estimates of emissions from deforestation are 1.6 PgC y-1, or about 20—25 percent of total anthropogenic emissions (Houghton et al. 2001). Therefore, reduction of deforestation has large carbon mitigation potential: a 3 to 10 percent reduction of deforestation in non-Annex I (developing) countries by 2010 would result in 0.053-0.177 PgC y-1 carbon mitigation, equivalent to 1 to 3.5 percent of Annex I base-year emissions (Watson et al. 2000b). The environmental benefits of slowing down deforestation are numerous, including most of the benefits mentioned for reforestation, with maximum value for biodiversity conser vation in pristine forests, particularly in the tropics. Although ending forest deforestation is a laudable goal, it has, however, proved difficult or impossible to implement in many regions unless there are tangible socioeconomic incentives and the other drivers of deforestation (such as markets and policy climate) are specifically addressed. This is especially so in developing countries, where most deforestation occurs (Lambin et al. 2001). Another unintended outcome of rewarding the preservation of carbon stocks in forests could be a more relaxed emphasis on reduction of energy emissions, the most important pathway for long-term CO2 stabilization (Figure 6.1).
Changes in forest management could increase carbon stocks with an estimated global mitigation of 0.175 PgC y-1 (Watson et al. 2000b). One management activity being suggested is fire suppression. Fire is a natural factor for large forest areas of the globe, and therefore an important component of the global carbon cycle. Fire is a major short-term source of atmospheric carbon, but it adds to a small longer-term sink (<0.1 PgC y-1) through forest regrowth and the transfer of carbon from fast to inert pools including charcoal (Watson et al. 2000b; Czimczik et al. 2003). For instance, in the immense extent of tropical savanna and woodland (2.45 gigahectares [Gha], Schlesinger 1997), a 20 percent fire suppression would result in carbon storage of 1.4 tC ha-1 y-1 with associated mitigation of 0.7 PgC y-1. Tilman et al. (2000) estimated that additional fire suppression in Siberian boreal forest and tropical savanna and woodland might conceivably decrease the rate of accumulation of atmospheric CO2 by 1.3 PgC y-1, or about 40 percent. The biggest problem with fire suppression, however, is that these estimated rates of carbon storage are not sustainable in the long term and that potential catastrophic fires in high biomass density stands could negate the entire mitigation project apart from some sequestration in charcoal.
Changes in agricultural management can restore large quantities of soil carbon lost since the onset of agriculture. The global potential for this strategy is estimated at 40—90 PgC. Estimates of potential soil C sequestration vary between 0.3—0.5 PgC y-1 (Smith, Chapter 28, this volume) and 0.9 PgC y-1 (Lal 2003), which in the best case would restore most of the lost carbon within 50 to 100 years. A large number of "stock-enhancing" practices bring environmental benefits: reduced erosion and pollution of underground and surface water, maintenance of biodiversity, and increased soil fertility (Smith, Chapter 28). However, the frequently proposed "win-win" hypothesis, according to which strategies aimed at increasing carbon accumulation will always bring other environmental benefits, needs to be verified on a case-by-case basis and with reference to sustainable development goals assigned to a region. For instance, increasing soil C stocks also leads to increasing soil organic nitrogen, which provides a source of mineralizable N, and therefore potential for increased N2O emissions, further enhanced by the use of nitrogen fertilizer (Robertson, Chapter 29, this volume). Also, activities such as the use of fertilizer and irrigation have an associated carbon cost that often exceeds the carbon benefit (Schlesinger 1999). Other management options such as zero or reduced tillage may reduce the carbon cost of production (owing to less on-farm diesel use, despite a carbon cost associated with the extra herbicide required), but the extra use of herbicide and pesticides may create further negative environmental impacts. Taking into account a number of environmental and socioeconomic constraints, Freibauer et al. (2003) estimated that only 10 percent of the potential mitigation in the agricultural sector in Europe by 2020 is realistically achievable.
Non-CO Mitigation from Land Biosphere
In the livestock sector, methane and N2O emissions can be reduced by better housing and manure management. Methane emissions from enteric fermentation can be reduced by engineering ruminant gut flora or by use of hormones, but in some countries there are societal and legislative constraints on these technologies. N2O emissions from agriculture can be lessened by reducing N fertilization. This decline could be achieved without loss of productivity by better timing, spatial placement (precision farming), and selection of fertilizers (Smith et al. 1996). The total anthropogenic flux of N2O is 8.1 teragrams (Tg) N2O-N y-1, equivalent to 1.0 PgCequivy-1. More than 80 percent of this amount is from agriculture; most of the rest is from the industrial production of adipic and nitric acids and can be abated with available technology.
Genetically modified organisms (GMOs) were initially developed to increase food production, but also have possibilities for increasing biomass production through disease-and pest-resistant genes that promote higher productivity. This potential has possible implications for increasing both carbon sequestration and biofuel production. GMOs have still largely unknown ecological consequences, however. Major known hazards are increased weediness of GM plants, genetic drift of new genes to surrounding vegetation, development of pest resistance, and development of new viruses (Barrett 1997). Given the large uncertainty surrounding the ecological consequences of GMOs, this technology is banned in many countries.
A comment applicable to all of the options described involving terrestrial biological sequestration and disturbance reduction is that there are timescales associated with both biological sequestration and disturbance. In general, carbon losses to the atmosphere by land disturbance occur partly through delayed emissions long after the disturbance event (one to two decades). Gains from reforestation are quite slow, with similar time frames needed to rebuild carbon stocks. It follows that even if -FLULUC were to stop, emissions from disturbed ecosystems would still continue for some time. On the other hand, if reforestation programs are to make a major contribution to closing the carbon gap, they must be implemented early enough to see their effect on stabilizing CO2.
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