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You are here: Home  Publications  Project reports  Climate change and forests  FAQ - Role of forests in mitigating climate change


FAQ - Role of forests in mitigating climate change

Some frequently asked questions about the role of forests in mitigating climate change

1) How do forests store carbon dioxide ?
2) How much carbon is stored globally ?
3) How does deforestation result in the release of carbon dioxide ?
4) What forestry measures can be implemented to significantly mitigate climate change ?
5) What is the Kyoto Protocol ?
6) What contribution can forests make towards meeting Irelands Kyoto commitments ?
7) How much carbon dioxide is stored in Irish forests ?
8) What influences the rate at which forest store carbon ?
9) How may climate change affect the sequestration potential of forests ?


1) How do forests store carbon dioxide?

As globally important storehouses of carbon, forests play a critical role in influencing the Earth's climate. Forest plants and soils drive the global carbon cycle by sequestering (storing) carbon dioxide through photosynthesis and releasing it through respiration. When the uptake of carbon dioxide (photosynthesis) is greater than losses via respiration, harvest and management then forests store carbon (C sinks).

In an undisturbed forest ~ 74 % of the carbon dioxide (stored as carbon (C)) is stored in live stems and branches, 16 % is stored in roots and 10 % in soils. However, when forests are clear felled or deforested 32 % of the stored C is lost due to decomposition processes. The remaining C is initially retained either on site or in harvested wood products, but this is slowly released over time. Most of the C stored on site will be lost if land is converted to agriculture or settlements. The loss of C due to harvest can be minimised if land is replanted immediately after harvest.

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2) How much carbon is stored globally?

The global sink in forest vegetation and soils is estimated to be 1,200 Gt of carbon (1Gt = 1000,000,000 tonnes). This increases at a rate of 1-3 Gt annually. Forest and land-use measures have the potential to reduce net carbon emissions by the equivalent of 10-20% of projected fossil fuel emissions through 2050.

The net terrestrial sink of northern temperate and boreal forests appears to have increased on average from the 1980s to the 1990s. However, the magnitude of these sink is still highly uncertain. In the tropics, the net carbon flux is close to zero, that is, tropical land areas are in balance with respect to carbon exchange. This suggests that the carbon sink there is large enough to offset carbon emissions associated with deforestation. Due to sparse atmospheric and ecological data for the tropics, however, the uncertainty around this result is significant. Recent information suggest that the tropical forest represent a small sink of ~0.2 Gt C per year.

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3) How does deforestation result in the release of carbon dioxide?

In many parts of the world, forests are being rapidly cleared for agriculture or pasture, destructively logged and mined, and degraded by human-set fires. When forests are degraded or cleared, their stored carbon is released back to the atmosphere during harvest and through respiration, thus these forests are net contributors of carbon to the atmosphere. Tropical deforestation is responsible for approximately 20% of total human-caused carbon dioxide emissions each year, and is a primary driver of extinction of forest species.

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4) What forestry measures can be implemented to significantly mitigate climate change?

Forestry-based measures can be an effective complement to abatement options focused on fossil fuel emissions. Forest-based mitigation of global warming include:
- Increasing forest carbon absorption (sequestration) capacity - either by planting trees on un-forested land (i.e. afforestation), facilitating the natural regeneration of forests on marginal land and by managing forests to increase biomass accumulation.
- Conservation of existing forests - to avoid emissions associated with tropical deforestation, forest degradation or clearing.
- Substitution of sustainably produced forestry products - substituting wood products for materials requiring energy-intensive production, such as aluminium or concrete, and substituting woody biomass for fossil fuels as an energy source.

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5) What is the Kyoto Protocol?

The United Nations Framework Convention on Climate Change (UNFCCC), agreed at the Earth Summit in Rio de Janeiro in 1992, was the first major attempt to mitigate global climate change. Whilst it did not set targets for the reduction of greenhouse gas (GHG) emissions, member parties (including Ireland) are required to develop, publish, update and make available national inventories of GHG emission by sources and removals by sinks.
In 1997 the Kyoto Protocol (KP) was signed and ratified in 2004. Its main features are as follows:
- 1990 is the base year against which all emission reductions are calculated.
- Developed countries committed agreed to reduce annual GHG emissions to 5.2% below 1990 levels by the first commitment period of 2008-2012.
- The European Union committed itself to a reduction on 8%. This burden is shared between member states and under this agreement Ireland is committed to limiting its GHG emissions to 13% above 1990 levels by 2008-2012.
- The Protocol entered into force when it was ratified by at least 55% of Annex I countries, which cumulatively represent at least 55% of global GHG emissions.
- The Protocol made provision for the use of C sequestration by land use, land use change and forestry (LULUCF) as a means to offset GHG emissions. The principal articles of the KP that refer to forestry are 3.3 and 3.4. Article 3.3 refers to net changes in greenhouse gas emissions by sources and removals by sinks resulting from direct human-induced afforestation, reforestation and deforestation which has taken place since 1990; Article 3.4 refers to additional human-induced activities in the agriculture, land-use change and forestry sectors. The rules for the implementation of the KP were agreed at the Seventh Conference of the Parties at Marrakesh in November 2001. Under this agreement, there is no limit to the amount of credits a Party may accrue from Article 3.3 while limits have been placed on the amount of credits which can be obtained from forest management under Article 3.4; for Ireland this limit is set at 50,000 t C yr-1 during the first commitment period i.e. 2008-2012.

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6) What contribution can forests make towards meeting Irelands Kyoto commitments?

Under the agreed terms of the Kyoto protocol, Ireland is committed to reduce green house gas (GHG) emissions by 13 % above the 1990-base year level. This posses a tremendous challenge, given the rapid growth of the Irish economy in the past decade, since current green house gas (GHG) emission levels are 23 % above the 1990 level (EPA 2004 National Inventory). Assuming a business as usual scenario, it is estimated that the contribution of national forests, under Article 3.3, may offset ca. 16 % of the required GHG emissions for the first commitment period (2008 to 2012). However, estimation of the extent to which forests sequester carbon in the mid to long-term is hindered is by a high degree uncertainty due to spatial heterogeneity and temporal variability. These estimates are continuously being updated as new research information and national inventory data becomes available (see CARBWARE).

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7) How much carbon dioxide is stored in Irish forests?

Current estimates based the national C reporting model (CARBWARE) suggest that the 625,800 ha of forest land in Ireland has accumulated ~ 80 Mt CO2 increasing at a rate of 3.3 to 5 Mt CO2 per year before harvest*. Harvest removed each year amounts to 2 to 3 Mt CO2.
* Under the terms of agreement under the Kyoto protocol timber removed at harvest is deemed to represent an immediate source (i.e. The KP does not include storage of harvested wood products).

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8) What influences the rate at which forest store carbon?

The sequestration potential of forests is influences by many factors:
- Afforestation rate: Afforested in Ireland amounted to 233,890 ha between 1990 and 2005. It is crucial that this afforestation rate is sustained to maintain the National sequestration rate.
- Species: The faster growing species generally sequester C at a faster rate. However, long term storage would be increased in wood products derived from slower growing species.
- Climate: Warmer wetter stands are generally more productive and sequester more C annually.
- Stand age: The sequestration of forest will initially increase up to canopy closure followed by a decline as photosynthesis declines, primarily due to a decrease in leaf area and light absorption, and decomposition of harvest residues increase.
- Silviculture, cultivation and harvest management: Species mixture, planting density and site preparation activities such as cultivation may influence the rate at which forests sequester C and how quickly replanted or drained peat lands change form a net source to a net sink. Thinning and harvest result in a loss on C from the system. Therefore the intensity and timing of thinning would influence the sink capacity of forests
- Soil type: Some soils, such as peat soils, are a source of CO2 following drainage and afforestation. This is due to the oxidation of previously non-decomposable organic matter following the creation of aerobic conditions. Alternatively, most wet mineral soils are considered to be a C sink because of the accumulation of organic matter in these soils. Regardless of soil type, most Irish forests will represent a net sink over an entire rotation, but the size of the sink depends on soil type.

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9) How may climate change affect the sequestration potential of forests?

How long the current carbon sink capacity of the terrestrial biosphere is likely to be maintained into the future is a matter of conjecture; several hypotheses proposed as the basis for quantitative explanation are discussed here.

Increases in atmospheric CO2 levels and temperature affect plant growth processes such as photosynthesis (sunlight activated uptake of CO2 and release of oxygen) and respiration (shade or darkness activated release of CO2). Increased atmospheric CO2 levels increase carbon sequestration due to increased photosynthesis however increases in temperature may increase the rate of respiration both in plants and soil. Some climate models indicate that this process may turn forests into a carbon source, although work carried out in boreal forests suggests that plant processes 'acclimatise' to elevated temperatures.

Amazon rainforests currently act as a small carbon sink, in part through continued uptake of CO2 by mature trees. However if CO2 and temperature continue to rise, climatic models predict that this may result in a 'drying out' of Amazonian rainforests. This drying out of tropical rainforest may result in the creation of a large semi-arid region, thus turning a large carbon sink into a major carbon source.

Research has demonstrated that in the short term, increased photosynthesis resulting from the rise in atmospheric CO2 concentration diminishes at higher CO2 concentrations, whereas respiration and decomposition increase exponentially with increasing temperature. Thus, Scholes et al. (1998) have hypothesized that as atmospheric CO2 concentration and temperature rise, the overall capacity to take up additional carbon from the atmosphere will progressively diminish so that at some point respiration will exceed photosynthesis and the carbon sink will become a source. Based on these two short-term physiological response functions, Scholes (1999) evaluated the global terrestrial carbon sink under various assumptions and concluded that the global sink would be likely to decline from its current level of approximately 2 Gt C yr-1 and become a source within a few decades. This approach assumes that acclimation to higher CO2 concentrations and temperatures does not occur, that respiration is independent of photosynthesis, and that there are no feedbacks involving nutrition between the processes. A similar, parallel, hypothesis is that the present sink might be a temporary consequence of CO2 fertilization that eventually will be overtaken by respiratory losses of carbon as temperature rises and respiration "catches up." It is supposed that while photosynthesis increases in response to the increased availability of CO2, respiration will initially lag behind but will eventually catch up as the supply of substrate increases or the temperature lag resulting from the thermal inertia of the oceans declines.

The creation of new forests in boreal regions may affect the earth's albedo. Albedo is a measure of the amount of solar radiation (heat absorption) reflected by the land surface back into the atmosphere. Albedo changes with land use: the bright white cover of snowy fields and snow covered land has a high reflectivity while the canopies of boreal forests, which retain less snow cover and are therefore darker and absorb more radiation. The increased heat absorption due to boreal forest expansion may cancel out the increase in carbon sequestration by trees, particularly in high latitudes and may contribute to global warming.

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