Terrestrial ecosystems offer significant potential to capture and hold carbon at modest social costs. The Intergovernmental Panel on Climate Change (IPCC) Second Assessment Report estimated that about 60 to 87GtC could be conserved or sequestered in forests by the year 2050 and another 23 to 44 GtC could be sequestered in agricultural soils. In this chapter, we describe and assess biological mitigation measures in terrestrial ecosystems, focusing on the physical mitigation potential, ecological and environmental constraints, economics, and social considerations. Also the so-called geo-engineering options are discussed.

The mitigation costs through forestry can be quite modest, US$0.1–US$20/tC in some tropical developing countries, and somewhat higher (US$20–US$100/tC) in developed countries. The costs of biological mitigation, therefore, are low compared to those of many other alternative measures. The costs would be expected to rise, however, if large areas of land were taken from alternative uses. The technologies for preserving existing terrestrial C and enhancing C pools, while using biomass in a sustainable way, already exist and can be further improved.

Increased carbon pools from management of terrestrial ecosystems can only partially offset fossil fuel emissions. Moreover, larger C stocks may pose a risk for higher carbon dioxide (CO₂) emissions in the future, if the C-conserving practices are discontinued. For example, abandoning fire control in forests or reverting to intensive tillage in agriculture may result in rapid loss of at least part of the C accumulated during previous years. However, using biomass as a fuel or wood to displace more energy-intensive materials in products can provide permanent carbon mitigation benefits. It is useful to evaluate terrestrial sequestration opportunities alongside emission reduction strategies as both approaches will likely be required to control atmospheric CO₂ levels.

Carbon reservoirs in most ecosystems eventually approach some maximum level. Thus, an ecosystem depleted of carbon by past events may have a high potential rate of carbon accumulation, while one with a large carbon pool tends to have a low rate of carbon sequestration. As ecosystems eventually approach their maximum carbon pool, the sink (i.e., the rate of change of the pool) will diminish. Although both the sequestration rate and pool of carbon may be relatively high at some stages, they cannot be maximized simultaneously. Thus, management strategies for an ecosystem may depend on whether the goal is to enhance short-term accumulation or to maintain the carbon reservoirs through time. The ecologically achievable balance between the two goals is constrained by disturbance history, site productivity, and target time frame. For example, options to maximize sequestration by 2010 may not maximize sequestration by 2020 or 2050; in some cases, maximizing sequestration by 2010 may lead to higher emissions in later years.

The effectiveness of C mitigation strategies, and the security of expanded C pools, will be affected by future global changes, but the impacts of these changes will vary by geographic region, ecosystem type, and local abilities to adapt. For example, increases in atmospheric CO₂, changes in climate, modified nutrient cycles, and altered disturbance regimes can each have negative or positive effects on C pools in terrestrial ecosystems.

In the past, land management has often resulted in reduced C pools, but in many regions like Western Europe, C pools have now stabilized and are recovering. In most countries in temperate and boreal regions forests are expanding, although current C pools are still smaller than those in pre-industrial or pre-historic times. While complete recovery of pre-historic C pools is unlikely, there is potential for substantial increases in carbon stocks. The Food and Agriculture Organization (FAO) and the UN Economic Commission for Europe (ECE)’s statistics suggest that the average net annual increment has exceeded timber fellings in managed boreal and temperate forests in the early 1990s. For example, C stocks in the live tree biomass has increased by 0.17billion tonnes (gigatonnes = Gt) C/yr in the USA and 0.11GtC/yr in Western Europe, absorbing about 10% of global fossil CO₂ emissions for that time period. Though these estimates do not include changes in litter and soils, they illustrate that land surfaces play a significant and changing role in the atmospheric carbon budget and, hence, provide potentially powerful opportunities for climate mitigation.

In some tropical countries, however, the average net loss of forest carbon stocks continues, though rates of deforestation may have declined slightly in the last decade. In agricultural lands, options are now available to recover partially the C lost during the conversion from forest or grasslands.

Land is a precious and limited resource used for many purposes in every country. The relationship of climate mitigation strategies with other land uses may be competitive, neutral, or symbiotic. An analysis of the literature suggests that C mitigation strategies can be pursued as one element of more comprehensive strategies aimed at sustainable development, where increasing C stocks is but one of many objectives. Often, measures can be adopted within forestry, agriculture, and other land uses to provide C mitigation and, at the same time, also advance other social, economic, and environmental goals. Carbon mitigation can provide additional value and income to land management and rural development. Local solutions and targets can be adapted to priorities of sustainable development at national, regional, and global levels.

A key to making C mitigation activities effective and sustainable is to balance C mitigation with other ecological and/or environmental, economic, and social goals of land use. Many biological mitigation strategies may be neutral or favourable for all three goals and become accepted as “no regrets” or “win–win” solutions. In other cases, compromises may be needed. Important potential environmental impacts include effects on biodiversity, effects on amount and quality of water resources (particularly where they are already scarce), and long-term impacts on ecosystem productivity. Cumulative environmental, economic, and social impacts could be assessed within individual projects and also from broader, national and international perspectives. An important issue is “leakage” – an expanded or conserved C pool in one area leading to increased emissions elsewhere. Social acceptance at the local, national, and global scale may also influence how effectively mitigation policies are implemented.

In tropical regions, there are large opportunities for C mitigation, though they cannot be considered in isolation from broader policies in forestry, agriculture, and other sectors. Additionally, options vary by social and economic conditions: in some regions, slowing or halting deforestation is the major mitigation opportunity; in others, where deforestation rates have declined to marginal levels, improved natural forest management practices and, afforestation and reforestation of degraded forests and wastelands are the most attractive opportunities.

Non-tropical countries also have opportunities to preserve existing C pools, enhance C pools, or use biomass to offset fossil fuel use. Examples of strategies include fire or insect control, forest conservation, establishing fast-growing stands, changing silvicultural practices, planting trees in urban areas, ameliorating waste management practices, managing agricultural lands to store more C in soils, improving management of grazing lands, and re-planting grasses or trees on cultivated lands.

Wood and other biological products play several important roles in carbon mitigation: they act as a carbon reservoir; they can replace construction materials that require more fossil fuel input; and they can be burned in place of fossil fuels for renewable energy. Wood products already contribute somewhat to climate mitigation, but if infrastructures and incentives can be developed, wood and agricultural products may become vital elements of a sustainable economy: they are among the few renewable resources available on a large scale.
A comprehensive analysis of carbon mitigation measures would consider:

• potential contributions to C pools over time;
• sustainability, security, resilience, permanence, and robustness of the C pool maintained or created;
• compatibility with other land-use objectives;
• leakage and additionality issues;
• economic costs;
• environmental impacts other than climate mitigation;
• social, cultural, and cross-cutting issues as well as issues of equity; and
• the system-wide effects on C flows in the energy and materials sector.

Activities undertaken for other reasons may enhance mitigation. An obvious example is reduced rates of tropical deforestation. Furthermore, because wealthy countries generally have a stable forest estate, it could be argued that economic development is associated with activities that build up forest carbon reservoirs in the long run.

Marine ecosystems may also offer possibilities for removing CO₂ from the atmosphere. The standing stock of C in the marine biosphere is very small, however, and efforts could focus not only on increasing biological C stocks, but also on using biospheric processes to remove C from the atmosphere and transport it to the deep ocean. Some initial experiments have been performed, but fundamental questions remain about the permanence and stability of C removals, and about possible unintended consequences of the large-scale manipulations required to have significant impact on the atmosphere. In addition, the economics of such approaches have not yet been determined.

Geo-engineering involves efforts to stabilize the climate system by directly managing the energy balance of the earth, thereby overcoming the enhanced greenhouse effect. Although there appear to be possibilities for engineering the terrestrial energy balance, human understanding of the system is still rudimentary. The likelihood of unanticipated consequences is large, and it may not even be possible to engineer the regional distribution of temperature, precipitation, etc. Geo-engineering raises scientific and technical questions as well as many ethical, legal, and equity issues. And yet, some basic inquiry does seem appropriate.

In practice, by the year 2010 mitigation in land use, land-use change, and forestry activities can lead to significant mitigation of CO₂ emissions. Many of these activities are compatible with, or complement, other objectives in managing land. The overall effects of altering marine ecosystems to act as carbon sinks or of applying geo-engineering technology in climate change mitigation remain unresolved and are not, therefore, ready for near-term application.

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