Half of the CO2 emissions caused by human activity are absorbed by the sea and vegetation on land. Changes in these natural sources and consumers of CO2 may therefore be extremely significant for climate changes in the future. Absorption and emission of greenhouse gases on land occurs as a natural cycle between the earth and the atmosphere (see separate fact sheet on ocean consumption of CO2). Growing plants consume CO2 from the air through photosynthesis, and the carbon is temporarily stored in biomass, topsoil, peat and organic humus. Through respiration of both plants and animals, and through breakdown, burning or removal of organic material, CO2 is again released into the atmosphere. Deforestation disturbs the natural balance between absorption and emission and contributes to a rise in CO2 content in the atmosphere.
The rainforest ecosystems
Most of the consumption of carbon on land occurs in the forest ecosystems, which cover about 30 per cent of the total land area on the planet. In Europe, about one-third of the land area is forested, while in Norway, abut 40 per cent of the land is covered with forests. The forests absorb about 20 per cent of the total global carbon emissions. Forests that are left untouched eventually go into a phase of equilibrium in which the consumption and emission of carbon are approximately equal.
In modern forestry, forests are frequently cut down before this equilibrium is reached. When the forest is cut, the soil absorbs the carbon from the deforestation debris, meaning branches, twigs, needles, leaves and dead roots. This eventually entails that much of the CO2 is released and returns into the atmosphere. The increase in CO2 emissions from a deforested land area is temporary, and in Norwegian forests this emission is estimated to last for about 10–20 years. After a forest is harvested, the soil is exposed to more light, more precipitation and greater changes in temperature. This affects the breakdown of organic material stored in the soil.
Because deforestation increases the breakdown of organic material in the soil, it will also release more carbon from the soil. This in turn affects the concentration of greenhouse gases in the atmosphere and thereby intensifies the greenhouse effect. About one-sixth of the annual greenhouse gas emissions are due to deforestation and forest deterioration.
How important are rainforests?
Tropical forests represent the largest storehouse of carbon on land. In these forests, the annual absorption is about 1.3 billion tons (gigatons) of carbon, which corresponds to about 15 per cent of the total carbon emissions due to human activities.
According to the most recent general report from the UN's panel on climate, changes in the use of land areas and release of carbon from deforestation account for about 15 per cent of all global greenhouse gas emissions. A significant portion of the necessary cuts in greenhouse gas emissions can be made through reduced deforestation, more environmentally friendly agriculture and preservation of moors and turf land areas. Cultivation of moors results in significant transformation of organic materials and loss of CO2 into the atmosphere. The loss of carbon is estimated to be 500 kg carbon per year per quarter-acre of cultivated moorland. Carbon loss/emission is greatest immediately following cultivation. The figures correspond to the emission from a tropical rainforest in a 20-year perspective.
According to calculations of the Norwegian Environment Agency, the consumption of CO2 in Norwegian forests has increased from 14 million tons in 1990 to 32 million tons in 2011. The net consumption likely reached its highest level during the period 2003–2007. Because of the age distribution of the forests, the annual net growth will decline in the future and attain a level of about 19 million tons of CO2 in 2020. The main reason for the large net consumption of CO2 in Norwegian forests is due to the relatively large areas with young forests in which CO2 consumption is greatest. This situation owes to the fact that we have harvested forested areas previously and have thereby released large amounts of CO2, which are now being absorbed by the young and rapidly-growing forests of today.
Research at the Norwegian Forest and Landscape Institute shows that 10–30 year-old forest binds more carbon in trees, ground vegetation, straw and soil than what is emitted in the form of CO2 from decomposed organic material in the soil. Accordingly, young forests in healthy growth will absorb a lot of CO2. Forests ready for harvesting absorb to a lesser extent, whereas forests overripe for harvesting can yield increased CO2 emissions.
Does it help to plant forests?
Although the main consumption of carbon occurs in trees, the latter is not the greatest storehouse of carbon. Trees (including trunk, branches and sprigs) account for about 10 per cent, and the timber itself accounts for no more than 5–6 per cent of the forest's carbon storage. More than 60 per cent of the carbon storage in the forest's ecosystem is found in the soil of the forest floor, while storage in the moors accounts for about 35 per cent. Although the figures are highly debatable, it is important to take into account the ways that various forestry measures affect the great carbon storehouse that is found in forest soil and moors. This storehouse should be left intact to the greatest extent possible.
If we are to create new storehouses for carbon in the forests, we need to plant trees where forests did not naturally exist previously, or plant other types of forests that store more carbon than does the natural vegetation. Such measures can be very harmful to the indigenous natural environment, however, and for biological diversity.
Accompanying forestry activities involving harvesting, road construction and maintenance of ditches, there is also the risk that carbon stored in the soil and moors will be released, and that the climate balance sheet will be in the red. This is the case whether one harvests a forest in the usual manner or uses it for bioenergy.
Another alternative that may be a positive climate measure is to permit the forest to remain pristine and grow old. This will contribute to long-term and secure storage of vast amounts of carbon in the forest floor. In this respect, preservation of old forests would actually be a highly rational climate measure. The potential for increased carbon capture and storage in boreal forests – which exist throughout Canada, Russia, Alaska and Scandinavia – is estimated to be low. However, they represent the second largest storehouse of carbon, and this asset can be lost into the atmosphere through forest fires, drainage of moors, deforestation and mining.
Norsk Klimapolitikk [Norwegian Climate Policy]. Report to the Storting, No. 21 (2011-2012)).
Pan et al. 2011. A Large and Persistent Carbon Sink in the World’s Forests Science, 333, 988–993.