Changes in land use also absorb carbon. New forests are being planted on a huge scale, for example, and the productivity of farmland is being improved. These changes remove carbon from the air. The overall effect is to remove about 2.09 billion tons (1.9 billion tonnes) of carbon a year.
Although we are emitting more carbon dioxide every year, the rate at which it accumulates in the atmosphere is not increasing. About 3.52 billion tons (3.2 billion tonnes) of carbon are added to the air each year and the rate has remained unchanged since the 1980s. No one knows why this is so, but the most likely explanation is that plants grow more vigorously when there is more carbon available to them for photosynthesis. This is called the carbon dioxide fertilization effect.
The figure of 2.09 billion tons (1.9 billion tonnes) for the amount of carbon that is being moved out of the atmosphere as a result of changes in land use is an estimate introduced in order to balance the carbon budget. When the amount of carbon being emitted is compared with the amount that is being absorbed by the oceans, this is the amount of carbon that remains unaccounted for. It is sometimes called the "missing carbon sink." Obviously the carbon must go somewhere, and the most likely explanation is that most of it is being removed from the air by the expansion of temperate forests throughout the Northern Hemisphere.
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