Economically constrained Potential - estimated sustain ably achievable potential

Figure 28.1. Relationships between reported carbon sequestration potentials depending on the number and type of constraints considered land, biomass cropland, grassland, rangeland, and forest; the protection and creation of wetlands and urban forest/grassland; the manipulation of deserts and degraded lands; and the protection of sediments, aquatic systems, tundra, and taiga. Other estimates are as high as 5.65—8.71 PgC y-1, around two times the annual increase in atmospheric carbon levels (Metting et al. 1999).

What Is Meant by Carbon Sequestration Potential?

Estimates of carbon sequestration potentials are often confused by the choice of constraints. Some authors quote biological potentials (Metting et al. 1999), others quote potentials as limited by available land or resources (Smith et al. 2000a), and others also consider economic and social constraints (Cannell 2003; Freibauer et al. 2004; Figure 28.1). An analysis of the estimates presented in Freibauer et al. (2004) and the assumptions used by Cannell (2003) suggest that the sustainable (or conservative) achievable potential of carbon sequestration (taking into account limitations in land use, resources, economics, and social and political factors) may be about 10 percent of the biological potential. Though this value is derived predominantly from expert judgment, it may be useful in assessing how different estimates of carbon sequestration potential can be compared and how they might realistically contribute to CO2 stabilization.

Methods to Enhance Terrestrial Carbon Sinks

Various management practices have been suggested to manipulate the terrestrial carbon sink (Box 28.1). All of the management options in Box 28.1 have the potential to increase sink strength. Some of these options, however, also have an associated carbon

Box 2S.1. Management Options to Enhance Terrestrial Carbon Sinks


• Afforestation/reforestation (planting trees to increase the per area NPP of a unit of land)

• Forest management (fire suppression, thinning, fertilization, other techniques to improve per area NPP of a tree-covered unit of land)

• Revegetation (increasing vegetation carbon stocks using plants other than trees)

• Cropland and grazing land management (increasing the aboveground standing stock of carbon in agricultural systems)

• Increase in soil carbon stocks though afforestation, reforestation, improved forest management, or revegetation

• Zero or reduced tillage

• Set-aside or conservation reserve program

• Conversion to permanent crops

• Conversion to deep-rooting crops

• Improved efficiency of animal manure use

• Improved efficiency of crop residue use

• Agricultural use of sewage sludge

• Composting

• Improved rotations

• Fertilization

• Irrigation

• Bioenergy crops

• Extensification or deintensification of farming

• Organic farming

• Conversion of cropland to grassland

• Improved management to reduce wind and water erosion


(continued on next page)

Grazing Land

• Conversion to deep-rooting species

• Improved efficiency of animal manure use

• Improved efficiency of crop residue use

• Improved livestock management to reduce soil disturbance

• Improved livestock management to maximize manure C returns

• Agricultural use of sewage sludge

• Composting

• Fertilization

• Irrigation

• Extensification or deintensification of farming

• Improved management to reduce wind and water erosion


• Increase of soil carbon stocks by planting vegetation other than trees with higher carbon returns to soil or with litter more resistant to decomposition

Other Potential Sinks

• Protection and creation of wetlands

• Protection and creation of urban forest and grassland

• Improved management of deserts and degraded lands

• Protection of sediments and aquatic systems

• Protection of tundra and taiga

Compiled from Lal et al. (1998), Metting et al. (1999), Follett et al. (2000), IPCC (2000), Smith et al. (2000a), Freibauer et al. (2004), and Smith (2004).

cost. Schlesinger (1999) has suggested that a number of these potential sink-enhancing activities (e.g., mineral fertilization, irrigation) may be carbon-negative in that more carbon is consumed in producing the resources and implementing the management than is gained in the sink. Others have argued that at least some of these options (e.g., animal manure) are simply a redistribution of the resource produced as a by-product of other activity (Smith and Powlson 2000) and as such have no additional associated carbon cost. In any case, it is important to consider the full C impact of a C sequestration practice (Schlesinger 1999). In fact, the full impact on global warming potential (GWP; including the impact on non-CO2 greenhouse gases) should be assessed (Robertson et al. 2000) because some sequestration options can be severely reduced or negated by increases in non-CO2 greenhouse gas emissions (Smith et al. 2001). Some sequestration practices, such as conversion of cropland to set-aside land, actually derive most of their climate mitigation potential from reductions in non-CO2 greenhouse gases, even though they were originally implemented as carbon sequestration practices (G. P. Robertson, pers. comm.).

Various studies have been conducted to examine the relative potential of each of the measures listed in Box 28.1. Estimates for the sequestration potential of these activities range from about 30 to 80 g C m-2 y-1, with some estimates lower and others higher (Nabuurs et al. 1999; Watson et al. 2000; Follett et al. 2000; Smith et al. 2000a; West and Post 2002; Lal 2004). Estimates of the total sequestration (or mitigation) potential of these and other mitigation scenarios are discussed in Caldeira et al. (Chapter 5, this volume).

Time Course of Sink Development and Permanence

The time course of sink development is sink specific. Soil carbon sinks increase most rapidly soon after a carbon-enhancing land management change has been implemented. Vegetation carbon sinks may be most rapid a few years after a land use change, as it may take some time for the new trees and vegetation to become established. Sink strength (i.e., the rate at which C is removed from the atmosphere) in both soil and vegetation becomes smaller as the ecosystems approach new equilibria. At equilibrium, the sink has saturated: the carbon stock may have increased, but the sink strength has decreased to zero (Figure 28.2).

The detailed time course for each sink is, of course, highly variable, as is the time for sink saturation (i.e., new equilibrium) to occur. Soils in a temperate location reach a new equilibrium around 100 years after a land use change (Jenkinson 1988; Smith et al. 1996), but tropical soils may reach equilibrium more quickly. Soils in boreal regions may take centuries to approach a new equilibrium. As a compromise, current IPCC good practice guidelines for greenhouse gas inventories use a figure of 20 years for soil carbon to approach a new equilibrium (IPCC 1997; Paustian et al. 1997). Temperate tree species may reach maturity within 100 years but slower-growing species and those in boreal latitudes may take longer.

Terrestrial sinks are not permanent. Because sink strength decreases as ecosystems approach a new equilibrium (Figure 28.2), the net amount of C removed from the atmosphere decreases. If a land management or land use change is reversed, the carbon accumulated will be lost, usually more rapidly than it was accumulated (Smith et al. 1996). For the greatest potential of terrestrial C sinks to be realized, new C sinks, once established, need to be preserved in perpetuity. Within the Kyoto Protocol (discussed in the next section), suggested mechanisms provide disincentives for sink reversal (i.e., when land is entered into the Kyoto process it must remain in the accounting, and any sink reversal will result in a loss of carbon credits).

Management charge Time since management change Figure 28.2. Illustrative graph showing the possible time course of sink development

In the soil, factors leading to carbon stabilization include chemical, physical, and biological processes, with a varying balance. More research is required to elucidate fully the balance of these processes in determining soil C stabilization under different conditions.

Sinks in the Political Arena

In the political arena, sinks are perhaps more hotly debated than they are in the purely scientific arena. Terrestrial sinks have received close political scrutiny since their inclusion in the Kyoto Protocol at the 4th Conference of Parties (COP4) to the United Nations Framework Convention on Climate Change (UNFCCC). Under Articles 3.3 and 3.4 of the Kyoto Protocol, biosphere sinks (and sources) of carbon can be included by parties in meeting greenhouse gas emission reduction targets by comparing emissions in the commitment period (the first CP is 2008-2012) with baseline (1990) emissions. The exact nature of the sinks to be allowed and the modalities and methods that could be used were addressed in subsequent meetings of the COP, culminating in agreement at COP7 in 2001, leading to the publication of the Marrakesh Accords.

Under the Marrakesh Accords, Kyoto Article 3.3 activities limited to afforestation, reforestation, and deforestation are included but are subject to a complex series of rules. These rules attempt to factor out any impacts that are not directly human induced. Kyoto Article 3.4 activities can be included under forest management, cropland management, grazing land management, and revegetation. The details of the rules and modalities governing how sinks can be used are too complex to describe in detail but include safeguards to avoid double counting of sinks, to avoid carbon credits being claimed and the land subsequently reverting to low carbon stock levels (e.g., by deforestation), and to ensure that accounting is transparent and verifiable.

An interesting outcome of the Marrakesh Accords is that crop- and grazing land management and revegetation will be reported on a net-net basis, that is, net emissions during the commitment period will be compared with net emissions during the baseline year. Under this form of accounting, a carbon credit will arise if a party reduces a source, even though a sink (i.e., a net removal of C from the atmosphere) has not been created. In this respect, decreasing a source is equivalent to creating a sink.

Scientific issues related to the use of biosphere sinks in the Kyoto Protocol were addressed in the IPCC special report Land Use, Land-Use Change, and Forestry (Watson et al. 2000). Though many of these issues no longer apply (because the political negotiations have moved on), this text is still a useful source of information on the potential of biosphere sinks for climate mitigation.

Biological sinks on land will remain in the political spotlight at least until the end of the first commitment period in 2012. With further commitment periods set for negotiation in the near future, sinks are likely to remain politically important for the foreseeable future.

Vulnerability of Current C Sinks

A number of the biosphere stocks are vulnerable both to future climatic change and to current and future human activity (Gruber et al., Chapter 3, this volume). These impacts will reduce the strength of biosphere C sinks and will militate against activities to increase sink strength in the future. A few examples of potentially threatened C stocks are given here. Highly organic boreal soils contain huge stocks of carbon. Many are in low temperature environments or even under permafrost. Climate change is projected to warm these areas, which will potentially release large amounts of carbon to the atmosphere and dramatically constrain the sink potential in these regions. Another example of a threatened C stock is carbon in tropical and subtropical soils threatened by desertification. Increasing temperatures and more frequent droughts (caused by increased climate variability) threaten to enhance desertification, while an increasing human population and changing dietary habits further exacerbate the problem by overexploitation of these vulnerable soils. Desertification in these regions will also threaten aboveground C stocks. Because degraded soils account for about half of the soil C sequestration potential (Lal 2004), further degradation and desertification will not only increase emissions but will also militate against enhancing the soil C sink through C sequestration.

Among vegetation C stocks, deforestation poses the greatest current threat. The same factors that lead to the overexploitation of vulnerable soils (e.g., population growth and changing dietary habits) also lead to forest clearance. Deforestation reduces the size of the biosphere stock and adds extra C to the atmosphere. It also reduces future sink strength, because over the course of a year, most crops remove less atmospheric C than do trees and cropland loses more soil carbon than forest does. Increases in climate variability also pose a threat to vegetation C stocks. Increasingly frequent severe storms will fell trees, whereas increased frequency of drought may increase the incidence of fire.

In short, increasing pressure on limited land resource, from increasing population and changing dietary habits, is likely to cause conflict between the need to maintain and enhance C sink strength and other competing demands upon the land. Further, a changing climate, including increasing climate variability, is likely to reduce the current terrestrial C sink strength and will militate against efforts to increase future sink strength.

Costs of Enhancing, Measuring, Monitoring, and Verifying Sinks

The costs of methods to enhance C sinks are likely to vary over a large range. Some technological solutions may be costly (e.g., genetic engineering of new plant varieties), whereas others (e.g., changing animal manure use) could be implemented at very low or no cost. The fact that the parties to the UNFCCC have negotiated for the inclusion of sinks in the Kyoto Protocol and intend to use them to meet greenhouse gas emission reduction targets suggests that at least some methods to enhance C sinks are deemed to be cost-effective. A number of joint implementation (JI) and clean development mechanism (CDM) projects, mainly based on forestry programs (Brown et al. 2000), already exist and have shown that C sink enhancement can be cost-effective.

In addition to the cost of implementing a sink-enhancing measure, other costs are associated with measuring, monitoring, and verifying the enhancement of the sink. These costs are not trivial (Smith 2004). Costs are highest where the change in carbon stock is small, relative to the background C content (as in soils). Smith (2004) suggests that in many cases the value of the carbon sequestered will be less than the cost of demonstrating the increase in the carbon sink. Costs can be reduced in integrated national programs, including benchmark sites and modeling. He also suggests that a low level of verifiability may be achievable by most UNFCCC parties at a small cost but that an intermediate level will be available to only a few countries that have made significant investments in national carbon accounting or inventory systems. Australia, for example, is investing an additional US$5 million annually to upgrade its carbon accounting system (Watson et al. 2000).

In terms of cost-effectiveness, the best options for enhancing sinks occur where there are co-benefits associated with a sink-enhancing activity. For example, a project to create urban woodlands will enhance leisure and amenity opportunities in addition to carbon stocks. Similarly, the creation of woodlands or wetlands for wildlife will not only improve wildlife value and enhance biodiversity, but also enhance carbon stocks. Increasing the organic matter of a soil provides a number of agronomic benefits, includ ing improved water-holding capacity, nutrient status, and workability, while also increasing the soil C sink. It is these "win-win" options that will likely be the most cost-effective mechanisms for increasing carbon sequestration. In agriculture, many low-cost management options could be implemented quickly, would be beneficial agronomically, and would enhance terrestrial C sinks. Such "no regrets" policies could enhance sink strength now and increase the resilience of the sink in the future. Smith and Powlson (2003) developed these ideas for soil sustainability, but they apply equally well to terrestrial C sink enhancement.

Social Dimensions

Engineered sinks are, by definition, entirely under human influence. As a consequence, the human dimension to terrestrial sink enhancement is strong. Because there will be increasing competition for limited land resources in the coming century, terrestrial sink enhancement cannot be viewed in isolation from other environmental and social needs (Raupach et al., Chapter 6, this volume). Terrestrial C sink enhancement needs to go hand in hand with other aspects of sustainable development. In any scenario, there will be winners and losers. The key to enhancing sinks, as part of wider programs to enhance sustainability, is to maximize the number of winners and minimize the number of losers. Another possibility for improving the social and cultural acceptability of measures to enhance C sinks is to include compensation costs (for losers) when assessing the costs of strategies for implementing C sequestration.

In agriculture and forestry nearly all of the techniques to enhance carbon sinks are well known. Because the implementation of known technology is currently hampered by ignorance at a regional scale (Sanchez 2000), the education of farmers, foresters, land managers, and regional planners would help to enhance soil carbon sinks. Economics, poor infrastructure, and distribution bottlenecks also hamper implementation.

Terrestrial C sink enhancement needs to be tackled hand in hand with other related problems. Global, regional, and local environmental issues such as climate change, loss of biodiversity, desertification, stratospheric ozone depletion, regional acid deposition, and local air quality are inextricably linked (Houghton et al. 2001). Terrestrial C sink enhancement clearly belongs on this list. Recognizing the linkages among environmental issues and their relationship to meeting human needs provides an opportunity to address global environmental issues at the local, national, and regional level in an integrated manner that is cost-effective and meets sustainable development objectives (Houghton et al. 2001). The importance of integrated approaches to sustainable environmental management is becoming ever clearer.

Attempts to solve a raft of environmental problems in an integrated way must also address social and economic problems in the same package. All of the scientific and technical measures outlined in this chapter have the potential to enhance C sinks, but the extent to which these are sustainable also needs to be considered.

Threats to the soil C stock and to large portions of the tropical vegetation forest stock would be reduced by controlling the size of the human population because this would ease the pressure on land for food production. Poverty remains the main driver for landscape degradation (e.g., by slash-and-burn agriculture) and C loss in the poorest parts of the world, where some of the most vulnerable C stocks occur (Barbier 2000). At the global scale, relief of poverty in these regions would probably do more to protect existing C stocks than could be achieved by any of the scientific or technical measures practiced in the developed world.

The political and economic landscape of the future will determine the feasibility of many strategies to enhance C sinks, but there are a number of management practices available that could be implemented to protect and enhance existing C sinks now and in the future (a no-regrets policy). Because these practices are consistent with and may even be encouraged by many current international agreements and conventions, their rapid adoption should be promoted as widely as possible.

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