Mires contain a large amount of carbon

In Finland, peat bogs contain even more carbon than the forests. For this reason, drainage carried out as a forestry measure also has an impact on climate change.

A natural mire has a twofold effect on the volume of greenhouse gases. On the one hand the peat and the plants bind carbon, on the other hand the mire releases methane, which is much worse than carbon dioxide as a greenhouse gas. All in all, however, the natural mires in Finland are estimated to bind about a million tons of carbon per year.

After drainage, the topmost peat layer of the mire is compacted and the decomposition of organic matter in the peat layer is accelerated, which generally leads to a sinking of its level. This means that not only carbon, but also methane contained in the peat is released into the atmosphere. The greenhouse emissions will be the larger, the more watery the mire was to begin with, but after a while the methane emissions will clearly decrease. The vegetation of drained mires will increase, which will bind some of the released carbon. For instance, the tree stock of drained mires has doubled from the start of the drainage to about 20 million cubic metres.

Overall, about one half of the Finnish mires, or 4.8 million hectares, have been drained. Currently the carbon sink formed by drained mires binds 1.9–4.5 million tons of carbon per year.

To a great extent, first-time drainage has been abandoned in Finland with the advent of forest certification. Many areas previously drained are slowly returning towards their natural state, which will mean that increasing amounts of carbon are bound into the mires. Mires are also being rehabilitated, that is, returned to their natural state by such means as filling in the ditches, if the drainage has not proved beneficial for forestry. Nevertheless, the restoration may take decades. By the end of 2002, 7,200 hectares of mires had been rehabilitated, the majority of them in western Finland. Rehabilitation is most urgently needed in southern Finland, where three quarters of the mires have been drained. In particular, the range of species living on eutrophic mires has declined due to drainage.

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