The UK is one of the least forested countries in Europe. Although the amount of woodland cover has increased substantially since its nadir after the First World War, growth has slackened in recent years. The growing maturity of UK woodlands means that carbon sequestration is falling rapidly. An independent assessment commissioned by the Forestry Commission has proposed one way forward: a million new hectares devoted to woodland, generating a reduction of up to 15% of the UK emissions in 2050. The UK's woodland was depleted by the needs of industry, urbanization and agriculture and fell to little more than 6% of national land area in the early 1920s. Wood was virtually absent from many lowland areas in England. A recovery in the area given over to woodland means that about 12% of the UK is now forested but this number is only rising very slowly. Net new forestation is now well below 10,000 hectares (100 sq km) a year, much of which is in Scotland.
The UK is significantly behind other countries in Europe.
Percentage of land area under forest and woodland
Source: Combating Climate Change: A Role for UK Forests (Edinburgh: The Stationery Office, 2009), p. 1.
As trees grow, they extract CO2 from the atmosphere by photosynthesis. Young trees don't capture much as their absolute growth is slow. Old trees have largely ceased to grow and also don’t extract much carbon dioxide. The UK's newer woods, mostly planted thirty to fifty years ago, are now just past their peak at sequestering carbon. The 2005 figure was about 16m tonnes CO2. In 2010, the figure will fall to about 10m tonnes, and by 2020 the figure could be as low as 5m tonnes (less than 1% of national emissions).
Combating Climate Change, a report commissioned by the Forestry Commission makes a powerful case for a sharp increase in the rate of new planting. It suggests that 1m new hectares, about 4% of total UK land area, should be given over to forest cover by 2050, increasing the planting to almost 25,000 hectares a year, triple today's rate. This would, says the report, reduce UK emissions by about 15m tonnes of CO2 a year by mid-century. Parliament has legislated to cut UK emissions to about 150m tonnes of CO2 by this date. New forestry could therefore reduce the national CO2 total by about 10% below its expected level.
Is a million new woodland hectares possible? Easily. About 4m hectares are given over to rough pastureland in England alone. I haven’t got the exact figures for Wales, Scotland, and Northern Ireland, but these countries probably have another 4m hectares. So transferring a million hectares into woodland is perfectly feasible.
What about the cost? The report suggests that it strongly depends on what sort of forestry we use. 'Energy forestry' using, for example, coppiced hazel and willow for fuels may well have a net cost below zero per tonne of CO2 saved. (That is, the wood fuel costs less than the fossil energy it replaces.) At the other extreme, the creation of new broadleaf woodlands, managed for biodiversity, is estimated to cost about £41 per tonne of carbon dioxide. The Climate Change Committee says that any proposal costing less than £100 per tonne is potentially cost-effective. So although £41 per tonne is almost certainly greater than the cost of, for example, carbon capture at coal power stations by 2050, it is in line with other projects for reducing CO2.
The cheapest form of reforestation – giving over large plantations to single species for frequent harvesting of wood for heating and electricity generation – is broadly unpopular in the UK. Even still, it probably needs to be considered carefully. Using biomass to generate electricity is a very good way of providing 'dispatchable' electric power, electricity that can provided exactly when needed. The last few weeks of cold, still weather in the UK should remind us that we need huge amounts of biomass as a reliable source of renewable power as a backup for wind.
 D. J. Read and others, Combating Climate Change: A Role for UK Forests: An assessment of the potential of the UK's trees and woodland to mitigate and adapt to climate change: The Synthesis Report (Edinburgh: The Stationery Office, 2009). Available here as a free PDF from the Forestry Commission website.