wise use of mires and peatlands - Peatland Ecology Research Group

74 VALUES AND FUCTIONS OF MIRES AND PEATLANDSThe role 177 of pristine mires: As statedabove, mires sequester carbon dioxide fromthe atmosphere and transform it into plantbiomass that is eventually stored as peat.Peat accumulation in mires is the result ofvarious processes including carbonsequestration by plant photosynthesis(primary production), direct carbon lossesduring litter decomposition, decompositionin the acrotelm, and decomposition losses inthe catotelm. Only about 10% of the primarilyassimilated carbon is sequestered in the peatin the long term. Annual long-term carbonaccumulation of the world’s mires isapproximately 1% of the carbon emitted byglobal fossil fuel consumption in 1990, or 10%of the carbon emitted by USA electricityutilities in 1998.In the long run, mires withdraw enormousamounts of carbon dioxide from theatmosphere and store it as peat deposits. Atpresent approximately the same amount ofcarbon is stored in the world’s peatlands asin the whole atmosphere. The decreasingatmospheric concentrations of carbondioxide during interglacial periods as a resultof peat formation, and the consequentsteadily reducing greenhouse effect, is seenby some scientists as a major cause of theorigin of ice ages.Pristine mires affect the global climate bothby the sequestration of carbon dioxide fromthe atmosphere and by the emission of othergases, especially methane and nitrous oxide.Methane is the second most importantgreenhouse gas after carbon dioxide and isexpected to contribute 18% of the totalforeseen global warming over the next 50years, as opposed to 50% attributable tocarbon dioxide. Furthermore methaneparticipates in tropospheric ozone formation.Global methane production is dominated bynatural wetlands, rice paddies, and animallivestock. Methane emissions in mires arehighly variable, but are generally higher inpristine fens than in pristine bogs.Nitrous oxide is a greenhouse gas and alsocauses destruction of stratospheric ozone.Nitrous oxide emissions from pristine miresare low. Occasionally such mires may evenconsume nitrous oxide due to the reductionof nitrous oxide to dinitrogen (N 2) underconditions of severe oxygen deficiency.Because all gases have a different lifetime inthe atmosphere and a different “globalwarming potential”, the combined effects ofall three gases together depend on the timehorizon chosen. On a 100-year horizon, forexample, Finnish undisturbed mires increasethe greenhouse effect, whereas on a 500-yearhorizon they decrease it. This is due to thechanging impact of methane emissions (cf.Table 3/14).Recent general overviews indicate that overa short time-scale (cf. Table 3/15) pristinemires contribute to the greenhouse effectChemical Atmospheric Global warming potential (mass basis) (time)species lifetime (years) 20-year horizon 100-year horizon 500- year horizonCO 2variable 1 1 1CH 412 ± 3 56 21 6.5N 2O 120 280 310 170Table 3/14: The atmospheric lifetime and the IPCC (1996) accepted global warmingpotentials over different time horizons of radiatively important gases 178 .