Forests

3. How can forests affect climate change?

Figure 2.12: Total carbon stock in forests by region

Forests influence climate change largely by affecting the amount of carbon dioxide in the atmosphere. When forests grow, carbon is removed from the atmosphere and absorbed in wood, leaves and soil. Because forests (and oceans) can absorb and store carbon over an extended period of time, they are considered “carbon sinks”. This carbon remains stored in the forest ecosystem, but can be released into the atmosphere when forests are burned. Quantifying the substantial roles of forests in absorbing, storing, and releasing carbon is key to understanding the global carbon cycle and hence climate change.

Growing stock is a measure of the volume of stemwood in a given area of forest or wooded land, usually measured in solid cubic metres (m³). It provides information on existing wood resources but also a basis for estimating the amount of carbon contained. The world’s total growing stock is estimated at 434 billion m³, of which some 30% are found in South America. Overall, growing stock has been decreasing slightly with some regional differences: Africa, Asia, and South America show a slight decrease, while Europe as well as North and Central America show a slight increase.

Carbon stock refers to the amount of carbon stored in the world’s forest ecosystem, mainly in living biomass (44%) and soil (46%), but to a lesser extent also in dead wood (6%) and litter (4%). The amount of carbon stored in a hectare of forest and the relative contribution of the different parts of the ecosystem to the total carbon stock vary from region to region (Table 2.8).

Overall, the world’s forest ecosystems are estimated to store some 638 Gt (638 billion tonnes) of carbon, which is more than the amount of carbon in the entire atmosphere. Because of large data gaps for soil carbon in major boreal forests, this figure probably underestimates the total amount of carbon stored in forest ecosystems.

From 1990 to 2005, the overall amount of carbon stored in living biomass decreased, mainly as a result of decreases in South and Southeast Asia Western and Central Africa, and South America. The amount of carbon stored in living biomass remained approximately constant in Oceania and increased in Europe and in North and Central America (see Table 2.10 for regional breakdown). More...