wise use of mires and peatlands - Peatland Ecology Research Group

VALUES AND FUCTIONS OF MIRES AND PEATLANDS73Like a window pane in a greenhouse, a numberof gases in the atmosphere allow solarradiation (visible light) to pass to the surfaceof the earth while trapping infrared (heat)radiation that is re-emitted by the surface ofthe earth. This trapping of heat radiation,that would otherwise escape to space, isreferred to as the greenhouse effect. Gasesthat influence the radiation balance are calledradiatively active or greenhouse gases(GHG) 175 .Greenhouse gases fall into three categories:■ radiatively active gases such as watervapour (H 2O), carbon dioxide (CO 2), ozone(O 3), methane (CH 4), nitrous oxide (N 2O),and the chlorofluorocarbons (CFCs) whichexert direct climatic effects,■ chemically/photochemically active gasessuch as carbon monoxide (CO), nitrogenoxides (NO x), and sulphur dioxide (SO 2)which exert indirect climatic effects throughtheir influence on the atmosphericconcentrations of hydroxyl radicals (OH),CH 4and O 3, and■ aerosols: 10-6- 10 -2 mm large fluid or solidparticles dispersed in the air.Even without human interference the naturalgreenhouse effect keeps the Earth’s surfacesome 30 0 C warmer than it would be if all solarradiation were transferred back to space.Water vapour, carbon dioxide and cloudscontribute roughly 90 percent to the naturalgreenhouse effect; and naturally occurringozone, methane and other gases account forthe remainder. The emission of greenhousegases resulting from human activities causesa change in the radiation balance of the Earth(radiative forcing).Carbon exchange: A major characteristic ofmires is that they sequester, or capture,carbon dioxide from the atmosphere andtransform it into plant biomass and eventuallypeat. Mires and peatlands also emitgreenhouse gases. The type of gases thatmires and peatlands thus exchange with theatmosphere is not always the same. Differentmire types emit different amounts andproportions of gases. In the course of theirlong-term development, some mire typesbecome spontaneously wetter and theproportion of emitted methane consequentlyincreases. Peatland drainage generallyincreases carbon dioxide emissions anddecreases those of methane, and peatlandagriculture additionally increases emissionsof nitrous oxide. As all these gases have adifferent radiative forcing, their effect on theradiation balance of the atmosphere differswith the type of mire or peatland and the typeof exploitation 176 .Carbon stores: The other important aspectof mires and peatlands is their function asstores of carbon. This is carbon that isexcluded from short-term (e.g. annual) carboncycling. Stores are only important when theirvolumes change. The increase ofatmospheric carbon dioxide in the recent pasthas been caused principally by burning longtermcarbon stores (fossil fuels such as coal,lignite, gas, and oil). The felling and burningof tropical rainforest increases carbon dioxideconcentrations in the atmosphere because ofthe mobilisation of the carbon stored in forestbiomass, not because plant productivitydecreases.The carbon store in peatlands can besubdivided into three components:■ the carbon store in the biomass,■ the carbon store in the litter, and■ the carbon store in the peat.Each of these components may behavedifferently under different managementoptions (such as agriculture, forestry,extraction, in fires, and under re-wetting).To understand the integrated effects ofpeatlands on climate, and the consequencesfor climate of human impact, it is thereforenecessary to consider both■ the types, volumes, and proportions ofgreenhouse gases exchanged, and■ the carbon stores in peatlands.