The scientific community essentially agrees on the phenomenon of global change (IPCC, 2001). The main cause of climate change is the anthropogenic increase in greenhouse gas concentrations in the earth's atmosphere.
Carbon dioxide (CO2) is the principal greenhouse gas. Its concentration in the atmosphere is the result of a cycle between different carbon pools: CO2is the product of the oxidation of carbon from these pools. The carbon cycle at the earth level is presented in the following diagram.
Diagram 1: A simplified diagram indicating carbon pools and CO2fluxes between the earth and the atmosphere
Source:Edinburgh Centre for Carbon Management(http://www.eccm.uk.com/climate.htm)
CO2concentration in the atmosphere has increased by 31% since the beginning of the industrial era, from 280 to 360 ppm (IPCC, 2001). Anthropogenic emissions of CO2originate primarily from the burning of fossil fuels and deforestation in tropical regions. Some of these emissions (on the order of 6 GtC/year) are reabsorbed by the terrestrial and oceanic ecosystems. The net atmospheric increase (on the order of 3 GtC/year) is small compared to the size of the carbon pools. However, this flow, that began more than a century ago with the Industrial Revolution, continues to grow, and is sufficient to explain global warming and the resulting imbalance in the climate system.
Carbon pool:A reservoir of carbon. A system which has the capacity to
accumulate or release carbon.
Forests are important carbon pools which continuously exchange CO2with the atmosphere, due to both natural processes and human action. Understanding forests' participation in the greenhouse effect requires a better understanding of the carbon cycle at the forest level.
Organic matter contains carbon susceptible to be oxidized and returned to the atmosphere in the form of CO2. Carbon is found in several pools in the forest:
· the vegetation: living plant biomass consisting of
wood and non-wood materials. Although the exposed part of the plant is the
most visible, the below-ground biomass (the root system) must also be
considered. The amount of carbon in the biomass varies from between 35 to 65
percent of the dry weight (50 percent is often taken as a default value).
· dead wood and litter: dead plant biomass, made up of plant debris. Litter in particular is an important source of nutrients for plant growth.
· soil1 organic matter, the humus. Humus originates from litter decomposition. Organic soil carbon represents an extremely important pool.
At the global level, 19 percent of the carbon in the earth's biosphere is stored in plants, and 81 percent in the soil. In all forests, tropical, temperate and boreal together, approximately 31 percent of the carbon is stored in the biomass and 69 percent in the soil. In tropical forests, approximately 50 percent of the carbon is stored in the biomass and 50 percent in the soil (IPCC, 2000).
· Wood products derived from harvested timber are also significant carbon pools. Their longevity depends upon their use: lifetimes may range from less than one year for fuelwood, to several decades or centuries for lumber.
The oxidation of carbon found in organic matter and the subsequent emissions of CO2result from the following processes:
· respiration of living biomass,
· decomposition of organic matter by other living organisms (also called heterotrophic respiration),
· combustion (fires).
The process of photosynthesis2 explains why forests function as CO2sinks, removing CO2from the atmosphere. Atmospheric CO2is fixed in the plant's chlorophyll parts and the carbon is integrated to complex organic molecules which are then used by the whole plant.
Diagram 2: The carbon cycle in the forest
The participation of forests in climate change is thus three-fold:
• they are carbon pools
• they become sources of CO2when they burn, or, in general, when they are disturbed by natural or human action
• they are CO2sinks when they grow biomass or extend their area.
The earth's biosphere constitutes a carbon sink that absorbs approximately 2.3 GtC annually. This represents nearly 30 percent of all fossil fuel emissions (totaling from 6.3 to 6.5 GtC/year) and is comparable to the CO2emissions resulting from deforestation (1.6 and 2 GtC/year).
"Current scientific evidence suggests that managed and even old growth forests (of the temperate and boreal zone) sequester carbon at rates of up to 6 ton ha. These results question the paradigm that old growth forests are in equilibrium with a net carbon balance. On the other hand infrequent disturbances (fires, pest outbreaks, storms.) are triggering a sporadic, but massive return of carbon to the atmosphere"(Valentiniet al.,2000). A soil specialist has emphasized that "there is a potential for reversing some of these processes and sequestering carbon in soils in terrestrial ecosystems. The magnitude of the potential is estimated to be up to 50 to 75 percent of the historic carbon loss. Theoretically, the annual increase in atmospheric CO2can be nullified by restoration of 2 billion ha of degraded lands, which would increase their average carbon content by 1.5 ton / ha in soil and vegetation."(Lal, 2000)
The carbon cycle (photosynthesis, plant respiration and the degradation of organic matter)in a given forest is influenced by climatic conditions and atmospheric concentrations of CO2. The distinction between natural and human factors influencing plant growth is thus sometimes very difficult to make.
The increase of CO2in the atmosphere has a "fertilizing effect" on photosynthesis and thus, plant growth. There are varying estimates of this effect: + 33 percent, + 25 percent, and + 60 percent for trees, + 14% for pastures and crops (IPCC, 2001). This explains present regional tendencies of enhanced forest growth and causes an increase in carbon absorption by plants. This also influences the potential size of the forests carbon pool.
There are still questions regarding the long-term future of the biospheric carbon pool. Several bio-climatic models indicate that the ecosystems' absorption capacity is approaching its upper limit and should diminish in the future, possibly even reversing direction within 50 to 150 years, with forests becoming a net source of CO2. Indeed, global warming could cause an increase in heterotrophic respiration and the decomposition of organic matter, and a simultaneous decrease of the sink effectiveness, thereby transforming the forestry ecosystems into a net source of CO2(Scholes, 1999).
In 2000,Naturepublished the results of a simulation made by theHadley Center. It analyzed the possible effects of global warming and of the increase of atmospheric CO2concentration on plant life and the oceans, and the subsequent emissions by these pools during the course of the 21stcentury. They tested three hypotheses:
· A 5.5 percent (4_ globally) increase of the average ground temperature. The model predicted the decline of a large part of the Amazonian forest, due to the increase in drought. The decomposition of the soil's organic matter would accelerate and the result would be an emission of 60 GtC by the earth's ecosystem.
· An increase in CO2concentration to 700 PPM, with no rise of the global temperature: the earth biosphere would globally absorb 750 GtC.
· A combination of the increase of CO2emissions and the temperature rise, with dramatic results: the CO2concentration in the atmosphere reaches 980 PPM, the average increase in ground temperature reaches 8_ (5.5_ globally), and the earth's biosphere emits 170 GtC (Coxet al.,2000).
Coxet al.,2000. Acceleration of global warming due to carbon-cycles feedback in a coupled climate model.Nature, 408.
The Hadley Center's simulation result is somewhat questionable, since it depends upon an uncertain direct link between an increase in earth temperature and respiration. The capacity of the vegetation to adapt to an increase in temperature is also largely unknown. An article written by 18 climate specialists published inScience(2000), gives a different opinion: "recent results from long-term soil warming in a boreal forest contradict the idea that the projected rise in temperature is likely to lead to forests that are now carbon sinks becoming carbon sources in a foreseeable future".This article postulates that the strength of the sink should increase in the future (by 10 to 20 percent) due to CO2fertilization, and then decline, followed by long-term saturation, due to the respiration increase caused by the rise in average temperatures (Falkowski P., Scholes R.J.et al.,2000). These forecasts refer to ecosystems that are not used for production, and are not managed or reforested.
Carbon sinks and measures for reducing industrial emissions: complementary or opposing actions?
Opposing the inclusion of carbon sinks in the Climate Convention negotiation process is often seen as an attempt to avoid more stringent emission reduction measures in the energy sector. However, it would be mistaken not to use the forestry potential simulateneously, since climate change is not a linear phenomenon, and there undoubtedly exist critical threshold levels beyond which the climate system would change unpredictably and timing of reduction measures counts (Pederson, 2000).
TheEdinburgh Centre for Carbon Management(ECCM) created two simulations involving CO2concentrations in the atmosphere.
The first compared a pessimistic forestry situation (constant deforestation, an inversion of the Amazon sink" to a source, and no large-scale reforestation), with an improved forestry situation (reduced deforestation and significant reforestation programs). In both cases, the atmospheric concentration exceeded 500 ppm, which the ECCM considers a critical threshold level for climate change, with a difference of ten years: about 2050 in the first variation, and 2060 in the second. This proved that forestry measures alone will not solve the problem of climate change.
The other scenario involved a large reduction of greenhouse gas emissions in the energy sector, with the same variations in the forestry sector. With a pessimistic forestry situation, the critical threshold level was reached in 2070. With the improved forestry situation however, the threshold was never reached. Instead, the curve of CO2concentration in the atmosphere started to decrease in 2050 slowly until 2100.
The ECCM concluded that the only way to fight climate change was to combine vigorous fossil fuel emission reductions with a voluntary program for improving forestry management, forest conservation and reforestation.
Several actions can be taken in the forestry sector in order to mitigate climate change.
Planting new forests, rehabilitating degraded forests and enriching existing forests contribute to mitigating climate change as these actions increase the rate and quantity of carbon sequestration in biomass. This potential has certain physical limitations such as plant growth and available area. Agro-forestry and the planting of multiple- use trees (fruit trees, rubber wood, etc.) also contribute to this objective.
Tree planting projects are doubly interesting from the point of view of CO2sequestration, inasmuch as carbon storage in durable products such as boards, plywood, or furniture complements the permanent stock in standing trees. Even if the life span of products is limited, an average life span of several dozen years is still significant, since it allows to "gain time" while waiting for cleaner technologies in the energy and transportation sector to develop, and it can also help avoid concentration peaks of CO2in the planet's atmosphere. If a part of the annual harvest replenishes and increases the pool of wood products, the forestry sector's storage capacity can increase considerably without occupying more space in the landscape.
The carbon reservoir in the forest biomass and soils is very large, highlighting the importance of conserving natural forest, and eliminating agricultural practices which contribute to the deterioration of these reservoirs.
One aspect of the debate about carbon sinks is whether conservation activities should be accounted or not. These activities aim to protect a forest area threatened by human-induced deforestation, particularly from farming. Climate specialists consider this conservation option to be the "best strategy for sink maintenance" (Valentiniet al.,2000) to the extent that it contributes more effectively to carbon storage and preserves the biodiversity associated with old-growth forests.
Numerous forestry activities emit greenhouse gases; these emissions can be curtailed by applying appropriate techniques.
·Forest harvesting can cause serious damage to the soil and the forest stand when carried out inappropriately. Reduced impact logging in the context of forest management and harvest plans involves using a set of techniques, such as pre-planning skidding trails; optimizing landings; directional felling; employing appropriate skidders, which reduce damage to soils, harvested trees and the remaining stand; these damages would heighten mortality and release carbon unnecessarily.
·Timber processing also generates a considerable quantity of waste wood, which could either be reduced, or used as a raw material for production or as fuel. Improving the forest industry's efficiency helps limiting the amount of wood waste created by the production process. This could be achieved by increasing product yield, reducing residues, or adding production lines which utilize them as parquet, moulding, etc. Using wood wastes in combined heat and power generation, thereby simultaneously generating heat for kiln-drying of wood, energy for running the machines, and electric power for the outside would reduce emissions and valorize these residues, which can substitute for fossil fuels3. Moreover, charcoal production also is a process of widely varying efficiency, depending on the method and techniques used, which could be improved.
Using lumber instead of materials requiring large amounts of energy during production helps fight the greenhouse effect, e.g. in replacing concrete or steel constructions by wood as frames, beams, etc. Using 1m3of lumber in buildings sequesters 1 ton of CO2for an average period of 20 years, and reduces net emissions by 0.3 t of CO2if concrete is replaced, and 1.2 t of CO2if steel is substituted.
Producing wood for energy purposes mitigates climate change by combining sink action with emissions reduction. Substituting fossil fuels, such as coal, natural gas, or oil by fuelwood for domestic use, electricity production, or industrial use, e.g. in iron smelters, reduces CO2emissions because wood is renewable. The expected sequestration of carbon through the growth of trees after sustainable harvest compensates for the CO2emitted by combustion.
However, this assumes that fuelwood production does not cause irreversible deforestation, i.e. that wood stocks are managed in a sustainable manner. Good management may even increase the productivity of forests and hence their sequestration capacity both in above-ground and below-ground biomass.
Different actions related to fuelwood can be taken:
· Increasing fuelwood supply by creating new plantations or enhancing productivity of existing forests through forest management. The contribution to climate change mitigation depends on the size and permanence of the carbon pool, and on the fuelwood increment.
· Increasing the energy efficiency of fuelwood use and derived products. Charcoal will often replace fuelwood in households. Improving and adapting stoves is necessary in order to raise energy efficiency and avoid the over-exploitation of certain species which have low wood density and burn rapidly. Charcoal contains two to five times more energy than wood by weight. Its use may also improve the distribution of fuelwood resources by reducing transportation costs from distant forest areas4.
· Increasing the efficiency of charcoal production. In Africa, productivity ratios can be as low as 10 to 15 percent, which corresponds to energy ratios of 20 to 40 percent. There are techniques which can obtain conversion ratios of 25 to 30 percent, or energy ratios of 65 to 80 percent (Girard and Bertrand, 2000). These techniques are particularly important for Africa, where urbanization has caused households to rapidly shift from wood to charcoal5.
The following table summarizes forestry activities that mitigate the greenhouse effect.
Creation and managementof carbon sinks and pools
Reduction of greenhouse gas emissionsby sources
Biomass and soil organic matter in forests
Emissions resulting from forestry activities or products
Introduction of trees on non-forest or
degraded forest lands:
Conservation of threatened forests
Combat against pests and fires
Reduced impact logging
Substitution: avoided emissions
products with long
Fossil-fuel substitution by
Table 1: Forestry activities that mitigate the greenhouse effect
In addition to helping protect the environment, forestry activities that mitigate climate change can provide global, regional and local benefits, as long as they are adapted to the local context.
·They can offer potential income to rural populations in forest areas. Industrial plantations can generate employment in nursery operations, harvesting, tending operations. Community plantation projects may involve direct payments to villagers by an investment fund.
·Timber plantation projects, particularly if undertaken in combination with efforts to increase forest industry efficiency, raise competitiveness by adding value to production and processing. They also help supply construction materials adapted to both urban and rural populations. In countries with large wood industries, such as Nigeria, Ivory Coast, Ghana, Cameroon, this could reduce the pressure on their natural forests.
·Reduced-impact logging techniques contribute to maintaining sustainable timber production by curbing forest degradation through destructive harvesting.
· Multiple-use plantations can contribute to the combat against desertification and erosion in vulnerable areas. Tunisia and several Sahelian countries believe that they can also produce carbon sequestration, provide income and supply fuelwood to rural populations.
· Conserving forests is a means of adapting to climate change. It helps provide protection against surface erosion, regulates water flows and limits landslides and rock falls. Forests at the coastline provide protection against wind and water erosion as well as water and sand intrusion.
· Improving the management of natural forest ecosystems as a source of fuelwood or charcoal contributes to energy supply at a moderate cost, reducing the country's dependence on fossil fuel imports. Biomass energy development permits decentralized electricity production in areas inadequately served by the national electricity grids. This can be of particular interest to dry areas, especially in the Sahel.
soil also contains mineral carbon from geological processes.
2Sugar synthesis from atmospheric CO2and water in the plants' chlorophyll parts.
3Waste-wood is the ultimate by-product of timber conversion. Using it for energy results in a net saving of fossil fuels and, therefore, a reduction of CO2emissions.
4The degradation of forest resources in Sahelian countries is primarily linked to improper selection of harvesting sites: forest stands close to cities are over-exploited, while more remote sites are underused. Alarming predictions of the 1970s concerning a fuelwood crisis have been partially refuted and the resource turned out to be more abundant and resilient than predicted, with possible exceptions, e.g. Mauritania.
5The transition to gas or oil is still impeded by low incomes, but the changeover is nonetheless inevitable.