Department of Forest Ecology and Management
Swedish University of Agricultural Sciences

Mires are wetlands that interact with the atmosphere by accumulating carbon over millennia in the form of peat, removing CO2 in the process, and emitting methane. Although Boreal mires cover only 3% of the earth’s land area, they contain a quarter to a third of the global pool of soil organic carbon, mostly in peat that has accumulated since the last deglaciation. Global estimates of long-term apparent carbon accumulation rates in mires are in the range of between 15-30 g C m-2 y-1.

Figure 1. The mire carbon exchange is dominated by carbon fixation through photosynthesis and losses through carbon dioxide respiration, methane emission and water runoff carbon export

Methane emissions from wetlands account for about 90% of total emissions from natural sources, and a third of total global emissions, while emissions from high latitude mires account for about a third of the emissions from natural sources, and 10% of total emissions. The contemporary perturbations to the atmosphere due to northern mires are a decrease of ~35 ppmv CO2 and an increase of 100 ppbv CH4. The net radiative forcing impact of northern mires currently amounts to about -0.2 to -0.5 Wm-2 (a cooling). It is likely that mires initially caused a net warming of up to 0.1 Wm-2, but have had an increasingly net cooling effect for the past 8000–11 000 years.

A major concern is that the long-term contributions of mires to the global carbon cycle may be about to change. According to theories on mire development, peat accumulation ceases eventually as mires age, because as peat accumulates over time the total amount of organic material available for decomposition increases, so at a certain point the total amount of carbon released from the peat will equal the amount taken up through photosynthesis at the surface. In addition to these assumed natural processes climatic changes are also expected to affect the accumulation of peat and methane emission. High latitude ecosystems, including most mires, are predicted to be especially vulnerable to climate change. The awareness of potential changes in the role of mires in global carbon cycles has prompted intensive research into mire carbon biogeochemistry.

Figure 2. Examples of carbon accumulation rates in two boreal mires in northern Sweden: a) mesotrophic tall sedge fen; b) swampy sedge fen. The accumulation of peat has varied with time since the last glaciation period. In ten boreal minerogenic mires (fens) in northern Sweden the rate of peat accumulation were lower during the last 500 – 2000 years, i.e. before any large-scale perturbation of mire ecosystems, compared to the long-term average. Contemporary carbon accumulation rates in the region are not significantly different from the historical carbon accumulation rates

Does the current rate of peat carbon accumulation deviate from the natural, “pre-industrial”, carbon accumulation rate? To answer this, data on both contemporary mire carbon exchange rates and Holocene peat accumulation rates are needed. The fluxes that significantly contribute to mire carbon budgets are land-atmosphere exchanges of carbon dioxide and methane, together with runoff carbon exports, which mainly consist of dissolved organic carbon (DOC) and inorganic carbon (Fig. 1). The establishment of Eddy-Covariance measurement systems has facilitated attempts to estimate complete mire carbon budgets. However, mire-specific estimates of all significant fluxes covering entire years have been obtained from very few mires to date, although data spanning eight years are available from one ombrotrophic mire (bog) in Canada, Mer Bleu, and four years from a nutrient poor, minerogenic, mire (fen) in northern Sweden, Degerö Stormyr. The results indicate that both mires still constitute significant sinks, with accumulation rates of similar magnitude to those that occurred during the Holocene (the period following the last glaciation).

Data from northern Sweden on past peat accumulation rates in some of the most common types of mires indicate the rate of peat accumulation decreased prior to any significant disturbance by human activities (Fig 2). The contemporary rate of carbon sequestration in Degerö Stormyr, in the same region, is at (or possibly slightly higher than), the rate in mires during the late Holocene.

Making peat more favourable as an energy resource

Another important research goal is to assess the impact of using peat as an energy source in terms of greenhouse gas emissions, since although its use for energy production on a global scale is limited, it is still used in significant quantities at local or national levels in some parts of the world. More than 90% of all peat used for energy production is consumed by Belarus, Finland, Ireland, Russia and Sweden. The global warming potential (GWP) of methane is 21, and the vast majority of mires are therefore net contributors to GWP. Draining and extracting peat leads to the termination of methane emissions, but also cessation of the current accumulation of CO2, and burning peat also releases CO2 that has been stored in it for a long time. Nevertheless, most attempts to assess the impact of using peat as an energy source have concluded that it is similar to using fossil fuels. The way to make peat most favourable as an energy source, in terms of GWP, is to utilise earlier drained peat areas. The most recent assessment by the IPCC classifies peat in a separate category, simply as peat.

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