Feedback of peatlands on the greenhouse effect

Climatic change has an impact on many natural ecosystems, e.g. peat land ecosystems. The greenhouse gases have an impact on the climate and this is called the feedback effect of peat lands on the enhanced greenhouse effect. This report addresses the question (in a qualitative manner) whether the interposition of peatlands will reduce or enhance the greenhouse effect. Around 96% of the mondial carbon dioxide flux to the atmosphere is natural, of which only 0.1% is released by peatlands. Natural ecosystems determine presently only 40% of the methane fluxes, of which 65% are peatland emissions. The amount of the CH4 emission from peatlands is not precisely known. lt is often assumed to be 110 Mt methane per year. This is equal to 21% of all identified CH4 sources. Globally the CO2 fixation rate of virgin peatlands may neutralize the greenhouse enhancing effect of the CH4 emissions, as well as that they may be neutral or reduce the greenhouse effect. However, anthropogenic use of peatlands changes the flux rates. Peatlands are also important in the global carbon cycle since the world's peatlands represent a carbon pool equal to 60% of all carbon stored in the atmosphere. A small change in oxidation rate can cause large carbon emissions to the atmosphere. Presently, peatlands accumulate about 76 to 200 Mt C per year, while peat mining is estimated to cause an emission of about 55 to 73 Mt C/yr. Drainage of peatlands for agriculture and forestry adds an emission of about 200 Mt C/yr. The large uncertainty about the emissions allows both the conclusion that peatlands globally enhance the greenhouse effect and that they are globally neutral or even reduce the greenhouse effect (the present anthropogenic intervention is excluded). However, the greenhouse effect itself may change this position. Though about 60% of the peatland area in the European Union is drained for agriculture and forestry and only 3% is used for peat mining, the latter causes relatively the largest carbon source of both. The carbon emission from peat mining in the EU is estimated to be about 6.1 Mt C/yr, while EU-peatlands are estimated to accumulate 3.7 Mt C annually. When the peatlands of entire Europe are considered, an emission of 12-17 Mt C/yr and a fixation rate of about 16.5 Mt C/yr is estimated (excluding CH4 emissions). There are indications that global warming will have a significant impact on trace gas emissions of peatlands, but the global effect of climatic warming on peatlands is still unclear. In this report the major effects are discussed and (qualitatively) compared. Firstly, climate change is expected to include a temperature increase. The effect on peatlands is that net less carbon can be accumulated since the oxidation rate of peat increases. Because the production of methane is more temperature dependent (increases with raising temperatures) than the degradation of methane, the CH4 emissions will also increase. An additional effect is that large areas of perma-frost-peatlands are expected to melt and release methane gas. However, melting is estimated to add maximally 5% to the mondial methane flux and will be quite insignificant during at least the coming thirty years. An other effect of climatic change is that the water level in peatlands is expected to decrease (with about 18 cm). This causes increased oxidation of peat (by the extension of the aerated layer), resulting in more CO2 emissions. This effect is only partly compensated by the reduction of methane emissions, so that the total effect is that more carbon dioxide equivalents are released to the atmosphere. A lower water table also causes N2O fluxes to increase (which enhances the greenhouse effect), but this is estimated to be insignificant compared to the above mentioned effects on CO2 and CH4. The third discussed topic in this report, is the effect of the elevated CO2 concentration in the atmosphere, though it is generally believed to be of minor influence on the greenhouse gas balance of peatlands compared to the effect of temperature and water table level. It will enhance both carbon fixation and methanogenesis, of which the first effect - a negative feedback to the enhanced greenhouse effect - dominates. However, the latter negative feedback is much smaller than the positive feedback resulting from both the lowered water table and the increased temperature. Therefore it seems a tentative, though reasonable, supposition that the overall greenhouse gas feedback of peatlands is positive. 174 refs