Climate change and forests are intrinsically linked. On the one hand, changes in global climate are already stressing forests through higher mean annual temperatures, altered precipitation patterns and more frequent and extreme weather events. At the same time, forests and the wood they produce trap and store carbon dioxide, playing a major role in mitigating climate change. And on the flip side of the coin, when destroyed or over-harvested and burned, forests can become sources of the greenhouse gas, carbon dioxide.

FAO has warned that action should be taken now to manage these complex relationships in a more holistic manner.

“We need to stop deforestation and expand the land area covered by forests, certainly,” says Wulf Killmann, who chairs FAO's interdepartmental climate change working group. "But we also need to substitute fossil fuels with biofuels,-- like wood fuels from responsibly managed forests -- in order to reduce carbon emissions, and we should use more wood in long-lasting products to keep trapped carbon out of the atmosphere for longer periods of time."

How forests trap one trillion tons of carbon

When fossil fuels are burned they release carbon dioxide into the atmosphere, contributing to an atmospheric carbon dioxide increase that, in turn, contributes to global warming and climate change.

Trees and forests help alleviate these changes by removing carbon dioxide from the atmosphere and converting it during photosynthesis to carbon, which they then "store" in the form of wood and vegetation, a process referred to as "carbon sequestration."

Trees are generally about 20 percent carbon by weight and, in addition to the trees themselves, the overall biomass of forests also acts as a "carbon sink." For instance, the organic matter in forest soils – such as the humus produced by the decomposition of dead plant material -- also acts as a carbon store.

As a result, forests store enormous amounts of carbon: in total, the world's forests and forest soils currently store more than one trillion tons of carbon -- twice the amount found floating free in the atmosphere -- according to FAO studies.

Destruction of forests, on the other hand, adds almost six billion tons of carbon dioxide into the atmosphere each year, and preventing this stored carbon from escaping is important for the carbon balance and vital in conserving the environment, the UN agency says.

Forests could be better used in combating climate change

This can be achieved not just by preventing forests from being cut down, but through afforestation (new plantings) and reforestation (replanting of deforested areas) of non-forested lands.

Particularly in the tropics, where vegetation grows rapidly and therefore removes carbon from the atmosphere more quickly, planting trees can remove large amounts of carbon from the air within a relatively short time. Here, forests can store as much as 15 tonnes of carbon per hectare per year in their biomass and wood.

FAO and other experts have estimated that global carbon retention resulting from reduced deforestation, increased forest regrowth and more agro-forestry and plantations could make up for about 15 percent of carbon emissions from fossil fuels over the next 50 years.

Harvested wood is also a carbon sink -- wood used in construction or for furniture effectively stores carbon for centuries. High-energy construction materials used in place of wood, such as plastics, aluminum or cement, typically require large amounts of fossil fuels during manufacturing. Replacing them with wood therefore has additional benefits in terms of reducing carbon emissions.

Similarly, the use of wood fuel instead of oil, coal and natural gas, can actually mitigate climate change. Although burning wood and biomass does release carbon dioxide into the air, if those fuels come from a sustainably-managed forest, those carbon releases can be offset by replanting. Indeed, if managed properly, forests can supply bioenergy virtually without contributing any greenhouse gas to the atmosphere.