Climate change mitigation may be defined as any attempt to reduce the rate as which greenhouse gasses are accumulating in the atmosphere. The two options that can be considered are:
- Reducing emissions
- Locking up the main culprit, carbon dioxide, this is known as carbon sequestration.
Due to natural inertia within the system, a delay or lag time exists between the accumulation of greenhouse gasses in the atmosphere and increasing temperatures (global warming). Thus, the changes in climate that we are experiencing now are as a result of past emissions in addition the carbon dioxide that is already in the atmosphere dictates that it is inevitable that temperatures will continue to rise for the next 50-100 years. The successful stabilisation of greenhouse gas emissions is therefore vital if we are to reverse this trend for future generations. Carbon stocks and fluxes associated with trees, forests and wood products are further explored in the Climate change mitigation programme pages.
Trees and carbon
Like all green plants, trees play a key role in the carbon cycle. Through the process of photosynthesis they are able to absorb carbon dioxide from the atmosphere, however much of the carbon that is initially captured is released through respiration.
Trees partition the carbon that they capture into different products such as leaf, root, seed, wood and branch, these different fractions are referred to as biomass. Carbon that is stored in biomass with a relatively short life span such as leaves is soon returned to the atmosphere through decomposition. Only the carbon that is stored in woody biomass such as roots, stems and branch material is locked away for the longer term.
Forest soils are also an important pool of carbon. They too are a vital part of the carbon cycle and can act as both sources and sinks.
At global scale the carbon that is contained in the worlds forests, if released would be enough to raise the carbon dioxide concentration in the atmosphere to over 1000ppm and with it would follow a potentially catastrophic rise in temperature of 5-8oC. At a global scale maintaining the world’s woodlands and forests is therefore an essential element of any measure to mitigate climate change.
The UK role of forestry in climate change mitigation
In the UK, a maximum carbon stock of about 250 tC ha-1 can be achieved in biomass in woodland. This value would represent ‘old-growth’ forest, and woodland conventionally managed for timber production only achieves about half this value. When woodland is mature, losses of carbon through respiration and decay balance uptake through photosynthesis. At this stage, the woodland is no longer a carbon sink.
Bioenergy production
Wood can be grown directly for energy production – often as short rotation coppice poplar or willow. Alternatively, thinnings or forest residues can be used as fuel in bioenergy plants.
- Product substitution
Harvested wood products represent a carbon stock in their own right. This pool is thought to amount to about 150 MtC, with an additional 100 MtC in harvested wood products in landfill. The harvested wood products pool is difficult to expand, and thus its role in climate change mitigation is limited. However, wood can also substitute for other materials which have high energy requirements associated with their production. These materials include concrete and steel, and it is this role that represents the largest long-term and ongoing contribution that woodland can make to climate change mitigation in the UK.
To place the potential role of woodland in the UK in context, the total carbon stock associated with above ground woody biomass (~130 MtC) is less than the total of fossil fuel emissions in a single year. However, the net uptake of carbon in UK woodland (~2.5 tC yr-1) is significant in comparison with national emissions reductions commitments made under the Kyoto Protocol (20 MtC yr-1). Therefore, although small in absolute terms, woodland can play a significant role in emissions reductions in the short term, and ‘buy time’ to allow new, low carbon technologies to be implemented. However, the conclusion has to be that climate change mitigation is unlikely to be effective over the short to medium term, and significant climate change is unavoidable.
