Peatlands and climate change
Peatlands at Ceide Fields neolithic archaeological site, Ireland. @ C.Michael Hogan
Peatlands and climate change represent an important aspect of climate science, since peat is a significant storage reservoir for carbon. In the last several millennia humans have extracted peat for domestic heating and cooking, but in the most recent two centuries the rate of exploitation of peatlands have expanded, not only for end use of the peat, but also due to urbanization in reponse to the rapidly growing human population. In addition to interest in peatlands' role in carbon storage, the layers of peat built up over time provide a veritable laboratory for delving into the human history, climate changes and Plant successions of ancient time. An important example of such a locale is Ceide Fields in Ireland, where the largest Neolithic village in Europe has been well preserved under five millennia of peat buildup.
Peatlands, some of the most carbon rich soils found on earth, can store carbon for thousands of years, and therefore, are valuable as carbon sequestration in their natural form, and can be utilized as long-term terrestrial carbon sinks.
Contents
Introduction
Soil are the largest terrestrial reservoirs of carbon. In total, soils contain approximately 1500 gigatonnes (1 Gt = 1 billion tonnes) of carbon compared to 540-610 Gt held in terrestrial vegetation and 750 Gt in the atmosphere (UNEP-GRID, 2008). Carbon found in soils and sediments is in either one of three forms: elemental (coal, graphite, soot), inorganic (calcite or dolomite), and organic (decomposition of organic matter) (Schumacher, 2002).
With regards to climate change, organic carbon in soils is the most important because organic carbon is converted to CO2 primarily through respiration (Bellamy et al., 2008). While globally the majority of carbon is concentrated in soils, there is a dramatic difference among soil types in their ability to store carbon (ESF, 2009). Peatlands, for example, are some of the most carbon rich soils found on Earth (Parish et al., 2008). Peatlands are characterized by a thick layer of organic matter, which is permanently saturated with water (Erwin, 2009; Wetlands International, 2007).
The term peatland covers a wide array of ecosystem types across the globe such as bogs, fens, fen-meadows, moors, peat-swamp forests and permafrost tundra. The wide ranging distribution of peatlands results in considerable temporal and spatial variability. Peatlands are disproportionately implicated in atmospheric carbon flux, since methane is a significant component of the gas transfer between peaty soils and the atmosphere, methane having a greenhouse gas potency of approximately 23 times that of carbon dioxide.
Essentially, peatlands are either ombrotrophic, meaning they receive water solely from precipitation or minerotrophic, receiving water and nutrients from nearby mineral soils, rocks surface waters or groundwater as well as from precipitation (Saarnio et al., 2008).
For example, moorlands are upland areas covered with peaty soils and tend to be ombrotrophic (Peak District National Park Authority, 2001). Mires and bogs are also ombrotrophic. Oftentimes mire and bog are used synonymously, however, some use mire to indicate a peatland that is in the process of forming or accumulating peat (Northern Ireland Environment Agency, 2004). Mires or bogs, like moorlands have acidic peaty soils (Northern Ireland Environment Agency, 2004). Fens are minerotrophic, and tend to have neutral or alkaline peaty soils as they are typically fed by groundwater high in calcium and Magnesium.
In total, it is estimated that peatlands cover only three percent (approximately 400 million hectares) of total land area yet contain 30% of terrestrial carbon or approximately 550 Gt of carbon (Parish et al. 2008; Hooijer et al. 2006).
Figure 1. The global carbon cycle shows the carbon reservoirs in GtC (gigatonne= one thousand million tonnes) and fluxes in GtC/year. The indicated figures are annual averages over the period 1980 to 1989. The component cycles are simplified and the figures present average values. The riverine flux, particularly the anthropogenic portion, is currently very poorly quantified and is not shown here. Evidence is accumulating that many of the fluxes can fluctuate significantly from year to year. In contrast to the static view conveyed in figures like this one, the carbon system is dynamic and coupled to the climate system on seasonal, interannual and decadal timescales. (Source: UNEP-GRID 2008)
Peatlands and greenhouse gases
The potential for carbon offsetting via natural terrestrial sinks (e.g. biomass, necromass, and soils) is significant (Moura-Costa and Stuart, 1999). The Intergovernmental Panel on Climate Change (IPCC) in the 2001 Third Assessment Report estimates that biological sinks have the potential to capture up to 20 percent of carbon dioxide emissions generated from fossil fuels from the time of writing of the report (2001) through 2050.
Figure 2. Role of peat in the carbon cycle (Source: Holden, 2005)
When thinking about ecosystems and climate change, the main area of focus in the latter twentieth century was deforestation, particularly rainforests. This is important, since forests cover approximately 30% of total land surface and can potentially store about 45% of terrestrial carbon (Bonan, 2008). Recent estimates state that tropical rainforests alone account for approximately 25% of terrestrial carbon (Bonan, 2008). Furthermore, drylands can contribute up to 40 percent of the soil carbon terrestrial cycling. (Fan et al., 2002) For perspective in temporal change in the cumulative net carbon flux from the period 1850 to 1990 AD, there was a release of 150 Pg of carbon terrestrially. (Houghton, 1995)
However, the majority of carbon in these forests and drylands ecosystems is contained in biomass above the ground, where as with peatlands, the majority of carbon is concentrated in the soils (Bonan, 2008, Erwin, 2009; ESF, 2009). Consequently, efforts directed towards conserving ecosystems with large quantities of soil carbon could lead to significant reductions in GHG emissions. Furthermore, the high methane fluxes inherent in peatland fluxes guarantee the disproportionate role of peatlands in any climate change analysis.
Peatlands are extremely carbon rich because the rate of decomposition of organic matter—mostly plants and leaf litter—is slow (Holden, 2005). The slow rate of decomposition coupled with inundation, which further delays decomposition, leads to the long-term accumulation of carbon (Holden, 2005). As carbon-rich material builds up, conditions become anaerobic, which induces methane (CH4) production (Saarnio et al. 2008). CH4 is produced by methanogens—microorganisms that use carbon as their source of energy as they breakdown the organic matter and release CH4 as waste (Saarnio et al. 2008; Schütz et al., 1990).
CO2 is released to the atmosphere mainly through mineralization of organic carbon found in the soils and plant respiration (Holden, 2005). CH4 is released to the atmosphere (Earth's atmosphere) either through diffusion, the release of gas bubbles from saturated peat (referred to as ebullition), or through root tissues (Holden, 2005).
Furthermore, the potential quantity of methane stored in peat soils should increase incentive for reducing deforestation, soil erosion, and/or degradation in such ecosystems as CH4 has 23 times the global warming potential of CO2, thus even small releases of CH4 can have drastic implications for global climate change. While two-thirds of CH4 released is still anthropogenic, natural sources also play an important role, particularly reductive, anoxic environments such as peatlands (Saarnio et al. 2008). One report by the International Union for the Conservation of Nature (IUCN) estimates that wetlands, including peatlands, and rice paddies contribute up to 40% of global methane emissions (Bergkamp and Orlando, 1999).
Variability
Moreover, the ability of peat soils (in the absence of anthropogenic activities) to emit or sequester GHGs varies temporally and spatially as a consequence of climatic or environmental factors such as extent of inundation (regulated by precipitation, groundwater, and nearby lakes and streams) and temperature. These variables can change not only from one location to the next, but also at different depths of the same peatland. Finally, barometric pressure can also influence how these gases migrate through the soils or between the soil and atmosphere.
Overall, tropical peatlands are believed to contain, and therefore potentially emit, significantly more carbon than peatlands located in temperate regions. To highlight the potential extent of variability, It has been estimated tropical wetlands account for 60% of total CH4 emissions from wetlands while northern wetlands account for 34% and temperate wetlands account for the remaining 6% of global CH4 emissions (Hanson and Hanson, 1996).
The fact that tropical peatlands can potentially contain significantly more carbon than temperate peatlands is an important reason for focusing on peatland degradation in South East Asia for palm oil and timber plantations (Hooijer et al. 2006). Peatlands cover an estimated 27.1 million hectares in South East Asia (Indonesia, Malaysia, Brunei and Papua New Guinea) and contain an estimated 42,000 metric tons (Mt) of carbon (Hooijer et al. 2006). The conversion of these peatlands to palm oil or timber plantations is releasing an estimated 2000 Mt of CO2 (8% of total global emissions from fossil fuels) emissions per year, with emissions only expected to increase (Hooijer et al. 2006).
Inundation
The extent of inundation is influenced by precipitation, groundwater or surface water influxes, or a combination of the three depending on the location of the peatland. Essentially, peatlands areeither ombrotrophic meaning they receive water and nutrients from precipitation and other atmospheric inputs or minerotrophic meaning such peatlands receive water and nutrients fromnearby mineral soils, rocks, surface waters or groundwater as well as from precipitation (Saarnio et al., 2008).
The extent of inundation influences soil moisture, which in turn influences respiration of CO2 fromthe soil to the atmosphere. Respiration is reduced when conditions are dry as a result of microbial and root activity (Luo and Zhou, 2006). Respiration increases as moisture increases butonly until a certain point because when the soil becomes too saturated, gases cannot diffuse as readily through the soils, which can lead to anaerobic conditions (Luo and Zhou, 2006). As soils dry up, diffusion of gases that were once restrained by super saturated soils are now more mobile (Luoand Zhou, 2006).Therefore as soils dry out, more of the gases that were once sequestered are now being released into the atmosphere.
Inundation has a strong effect on CH4 fluxes. Saturated soils have higher CH4 fluxes then drier soils because as the water table declines and soils begin drying out, the rate of oxidation increases thus reducing CH4 fluxes (Hanson and Hanson, 1996; Moore and Knowles, 1989). As soils dry out, they stop producing CH4, and in some cases, may actually absorb atmospheric CH4 (Hanson and Hanson, 1996).
The hydrology of peatlands, which goes beyond simply looking at the water table, is often poorly understood because focus tends to be on the balance of water and not on how the water is movingthrough the system (Holden, 2005). Studies that have looked at the water balance of peatlandshave mainly relied on a two-layered system, the acrotelm-catotelm model (Holden, 2005). The upper layer or the acrotelm layer is considered the active layer where the water table fluctuates and water is actively moving through the peat and the lower layer or the catotlem layer, is the inertlayer that is considered to be permanently saturated (Holden, 2005). This approach ignores a whole system of macropores and natural pipes that transport water and nutrients between the shallow and deep layers of peatlands (Holden, 2005). The hydrology of peatlands has strong influences on the rate of gas diffusion, nutrient availability and cycling, redox status (reduction-oxidation), peat accumulation or decay and ultimately carbon storage or release (Holden, 2005).
Temperature
Temperature also has a strong influence on soil respiration. There is a positive correlation between soil respiration and temperature—as the temperature increases the rate of respiration increases exponentially (Lloyd and Taylor, 1994). This increase in the rate of respiration or an increase in activity is caused by increased microbial activity and rates of mineralization as a result of warmer temperatures (Lloyd and Taylor, 1994). This exponential relationship is expressed by the Q10 equation. Q10, the temperature quotient, is the difference in the rate of respiration or activity as temperature increases by 10°C intervals:
Q10= (R1/R2)^10/(T2-T1)
Where R is the rate of respiration and where R1 and R2 are rates at different temperatures (T1 andT2) (Lloyd and Taylor, 1994). As an example, an assumed doubling of respiration for every 10°Cincrease in temperature would mean a Q10 value equal to 2 (Luo et al., 2001).
One study that looked at the effects of temperature on CH4 production determined Q10 values thatranged from 2.9 to 3.6 for different temperature ranges (Hulzen et al., 1999). Basically, temperature influences CH4 production by affecting the rate of anaerobic carbon mineralization, the availability ofelectron acceptors (reduction) as well as the level of methanogenic activity (Hulzen et al., 1999). Ultimately, the study showed that although CH4 production occurs in a range of temperatures from the northern tundra to tropical peatlands, the rate of production tends to be higher in warmer soils (Hulzen et al., 1999).
There also appears to be a difference in sensitivity to changes in temperature depending on the initial temperature of the soils (Lloyd and Taylor, 1994). Soils found in lower temperatures are moresensitive to a slight increase in temperature than soils found in warmer temperatures (Lloyd andTaylor, 1994). However, a threshold to temperature sensitivity has also been observed. Soil respiration will increase with warming, yet there is point when the rate of respiration can actually decline in response to warming (Luo et al., 2001). This decline is mainly thought to be the cause ofdrier soils and consequently reduced root and microbial activity (Luo et al., 2001).
Moreover, temperature also affects the rate of decomposition. As decomposition of soil organic matter (SOM) is mainly dependent on microbial activity, the rate of decomposition will be more rapid in warmer, humid areas than cooler, temperate areas (Carter, 2001).
Overall, warmer temperatures as a result of climate change could result in faster decay rates ofpeat and consequently increased CH4 and CO2 emissions (Parish et al., 2008). For example the melting of permafrost will release CH4 form the thick layers of decaying matter of the tundra (Parish et al., 2008).
Pressure
While climatic conditions can determine how much CO2 or CH4 is being produced, it is possible thatpressure differentials between barometric and subsurface can determine how these gases migratethrough the soil. Gas migration and pressure relationships are widely understood with regards to hazardous ground gas risk assessment, such as with CH4 and Landfills.
The difference between below ground pressure and barometric pressure will determine how gasesmigrate through the substrate/soil (Young, 1992). When barometric pressure is higher than pressure below the surface, underground gases will tend to move horizontally—the atmosphere ispressing downwards, preventing upward migration (Young, 1992). However, when barometric pressure decreases, then underground gases will rise, migrating towards the surface in order to reach a balance (Young, 1992).
Yet, there appears to be a gap in research with respect to determining relationships between pressure and ground gas migration in peat soils. This is a void that needs to be filled as it is highly likely that similar responses to differences in pressure are influencing how CH4 and CO2 migratethrough the soil and consequently into the atmosphere (Atmosphere layers).
Feedback with climate change
Peatlands are long-term carbon stores—they have been storing carbon for thousands of years (Parish et al., 2008; Holden, 2005). That is why the sustainable management of global peatlands is imperative to keep these ecosystems functioning as terrestrial carbon sinks. However, this may bemore difficult than simply abstaining or reducing anthropogenic activities that directly lead to the loss of peatlands if there is in fact a positive feedback with soil erosion and climate change. Simplyput, climate change has been linked with increasing rates of soil erosion, the erosion of soils increases release flux of carbon into the atmosphere and also reduces the land’s ability to store carbon, thereby increasing GHG emissions, which further exacerbates climate change and further erosion of soils (Nearing et al., 2004). However there is evidence from some climate change models that higher atmospheric temperatures will increase soil and vegetative evapotranspiration at northern latitudes between 40 and 70 degrees, thus inducing a reduction of groundwater levels in these northern peatlands; as a result the peatlands should become a much more robust carbon sink (for a modeled two degree Celsius temperature increase), with increases of 50 to 80 percent in carbon influx into these peaty soils. (Lai, 1995)
Increased rates of soil erosion are caused by changes in rainfall patterns, temperature as well as changes in vegetation and land-use patterns. More intense, stronger rain periods can increase theamount of soil that is washed off (IPCC, 2007; Nearing et al., 2004). Longer drier and hotter periods can cause soils to dry up directly by increasing rates of evapotranspiration or by inducing droughts, which can lead to vegetation dying off and therefore leaving soil exposed and vulnerableto increased surface runoff (Surface runoff) and erosion (Nearing et al., 2004). As the climate is changing, the distribution of species is likely to change; however, vegetation may not be able to adapt or migrate fast enough, which could lead toa decline in biomass cover in some parts of the world (Nearing et al., 2004). Finally, as a consequence of climate change coupled with irresponsible land-use policies and increasingdemands for food and other goods from booming populations, agricultural practices will have toexpand or move to new areas that are more suitable to farming as previous areas dry up, soilconditions become too poor, or simply to expand production (Nearing et al., 2004).
Peatlands under Kyoto and beyond
Peatlands can store carbon for thousands of years, and therefore, can be utilized as long-term terrestrial carbon sinks (Parish et al., 2008; Holden, 2005).
At the 2007 United Nations Framework Convention on Climate Change Bali Conference, Wetlands International, a non-governmental organization, advocated that any post-Kyoto plans include actions for avoiding emissions from peatlands and include avoided emissions from peatlands as part of avoided deforestation (Wetlands International, 2007).
Natural carbon sinks have been under utilized mainly because at the time of writing of the Kyoto Protocol (1997), which has been criticized for its high costs required to achieve targets and lack of flexibility in compliance measures, GHG sinks were regarded as an area with high uncertainty and this has therefore led to their restrictive use (Amano and Sedjo, 2006; Lanchbery, 1998). However, as governments and the scientific community try to improve the existing climate change platform (i.e. improving options for meeting targets and improving the current methods for verification define verification) there is a huge potential to increase the role of natural sinks in climate change mitigation due to the technological feasibility for implementation and verification as well as the relatively low costs associated with such projects (reference)
The technology to carry out forest sequestration projects, either by means of afforestation, reforestation, or avoided deforestation already exists—it simply requires the planting of tress or designating an area for conservation (Amano and Sedjo, 2006). Furthermore, the costs of utilizing such projects to offset or reduce GHG emissions is much less compared to other carbon-reducing projects (e.g. wind farm, dams, carbon capture and storage, etc.) therefore reducing a government’s total expenditure needed to address climate change (Bodansky et al. 2004). A study by Stanford University’s Energy Modeling Forum in 2006 concluded that using natural sinks could reduce the costs of climate change mitigation by trillions of dollars (Amano and Sedjo, 2006).
A climate change platform that legitimizes the use of existing natural carbon sinks like peatlands will not only provide a potentially cost-effective way of mitigating climate change, but will also preserve other vital ecosystem services that society depends on.
Epistemic communities that understand the important role peatlands play in climate change are urging any agreements made at United Nations Framework Convention on Climate Change in Copenhagen in December 2009 to include financial incentives that will deter the draining of peatlands or the re-saturation of already drained peatlands (The Economist, 2009). Even though the probability of this actually happening is uncertain, there is still the likelihood that if politicians fail in this realm, private firms that are dealing in carbon offsets may turn to peatlands (The Economist, 2009).
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