Background

Forests, through growth of trees and an increase in soil carbon, contain a large part of the carbon stored on land. Forests present a significant global carbon stock. Global forest vegetation stores 283 Gt of carbon in its biomass, 38 Gt in dead wood and 317 Gt in soils (top 30 cm) and litter. The total carbon content of forest ecosystems has been estimated at 638 Gt for 2005, which is more than the amount of carbon in the entire atmosphere. This standing carbon is combined with a gross terrestrial uptake of carbon, which was estimated at 2.4 Gt a year, a good deal of which is sequestration by forests. Approximately half of the total carbon in forest ecosystems is found in forest biomass and dead wood.

Other terrestrial systems also play an important role. Most of the carbon stocks of croplands and grasslands are found in the below-ground plant organic matter and soil.

Human activities, through land use, land-use change and forestry (LULUCF) activities, affect changes in carbon stocks between the carbon pools of the terrestrial ecosystem and between the terrestrial ecosystem and the atmosphere.

Management and/or conversion of land uses (e.g. forests, croplands and grazing lands) affects sources and sinks of CO2, CH4 and N2O. According to the IPCC WGIII (2007), during the decade of the 1990s, deforestation in the tropics and forest re-growth in temperate and boreal zones remained the major factors contributing to emissions and removals of greenhouse gases (GHG) respectively. The IPCC WG1 (2007) reported that estimated CO2 emissions associated with land-use change, averaged over the 1990s, were 0.5 to 2.7 GtC yr–1, with a central estimate of 1.6 GtCyr-1.

The role of LULUCF activities in the mitigation of climate change has long been recognized. Mitigation achieved through activities in the LULUCF sector, either by increasing the removals of GHGs from the atmosphere or by reducing emissions by sources, can be relatively cost-effective.

 

Agriculture	Forestry
		Estimated sectoral economic potential (Gt CO2-eq /yr) for global mitigation for forestry and agriculture as a function of carbon price (USD) in 2030 from bottom-up studies (Source: SPM IPCC 4AR, 2007)

General mitigation options could include forest-related activities such as reducing emissions from deforestation and degradation, enhancing the sequestration rate in new or existing forests, and using wood fuels and wood products as substitutes for fossil fuels and more energy-intensive materials. A variety of options for mitigation of GHG emissions also exists in other land systems. The most prominent example is agriculture, where options include improved crop and grazing land management (e.g., improved agronomic practices, nutrient use, tillage and residue management), restoration of organic soils that are drained for crop production, and restoration of degraded lands.

However, the main drawback of LULUCF activities is their potential reversibility and non-permanence of carbon stocks as a result of human activities, (with the release of GHG into the atmosphere), disturbances (e.g. forest fires or disease), or environmental change, including climate change.