What is Peat?
Peat, a precursor to coal, is the surface or near-surface layer of a soil consisting of partially decomposed plant material, which has built up in a water-saturated, low-oxygen environment, marked by high acidity (low pH) and nutrient deficiency. 1
Wherever peat is found, its environment is water-saturated and the underlying soil is rich in organic content. But the specific details of peat’s composition vary from region to region.
For instance, in temperate, boreal and tundra biomes, where low temperatures (sub-zero for long periods in winter) slow down the rate of decomposition, peat is formed mostly from mosses, shrubs, herbs, and small trees.
But in the humid tropics and sub-tropics, peat forms from rainforest trees (roots, trunks, branches and leaves) under consistently high temperatures. It also derives from species of plants that are able to grow in swampy, conditions where waterlogged soils prevent dead leaves and wood from decomposing. Coastal mangrove forests are a typical site for this type of peat.
How Does Peat Form?
Fossil fuels like peat are made up of organic matter derived mostly from the partially decomposed remains of plants, like trees, shrubs and grasses. This organic material is mostly made up of hydrogen and carbon compounds, which is why fossil fuels are also called hydrocarbons.
Plants get all their energy from the sun, via photosynthesis, and this energy passes along the food web to other life forms (like animals). Usually, when plants or animals die, their remains are broken down by decomposers and the solar energy is released. But sometimes these organic remains become buried or waterlogged, thus preventing further decomposition, and if they remain undisturbed over millions of years they eventually turn into coal, oil or natural gas.
Peat itself is a precursor to coal. It typically forms over a period of several thousand years, from the accumulation of organic matter in waterlogged places such as swamps, bogs or fens. On a micro-scale, nature takes about 100 years to create peat moss of 5 cms (2 inches) in depth.
So how does peat turn into coal? Well, if left undisturbed for millions of years, the peat gradually becomes more and more compacted by the weight of the overlying sediment, and all water is squeezed out of the peat layer. This compression process reduces the thickness of the peat by 90 percent or more, eventually converting it into a seam of carbon-rich coal.
In other words, the difference between peat and coal is that peat is younger and still water-saturated, while coal is older, compacted and dry.
Historically, the formation of peat and coal has been going on ever since plants appeared on land, during the Cambrian Period around 500 million years ago. However, most of the peat harvested today in the northern hemisphere was formed mostly over the last 10,000 years, following the retreat of the glaciers that once covered most of the area.
The Peat Ecosystem
As mentioned, the habitat requirements for the formation of peat are similar everywhere – waterlogged soil, low pH, low oxygen supply, and so on. This is because all these factors help to slow down the rate of decomposition. For example, the activity of bacteria – the organisms most responsible for breaking down organic matter – slows down significantly once the pH falls below 6.0. Also, in water-saturated conditions, the water fills the air spaces in the soil, depriving the microbes of oxygen.
Peatlands (mires) are constantly evolving. For example, peat may start to materialize in standing water – such as lakes or the waterlogged margins of slow flowing rivers. But over time, this watery ecosystem often undergoes a step-by-step process of chemical, biological and physical changes (known as a hydrosere), which results in the conversion of a water body and its community into a land community.
Generally speaking, this sequence of changes typically begins in an area of fresh water, such as in oxbow lakes and kettle lakes, and proceeds through the fen stage (regulated by nutrient-rich ground water and rainfall), to bog (that receives nutrients and water only from rainfall) and finally to woodland and even grassland.
This ecological metamorphosis, may also involve the peat habitat evolving into other types of wetland such as marshes (nutrient-rich wetlands that host a range of reeds and grasses); or swamps (able to support woody plants and trees); or sedge meadows (wetland communities with saturated soils and marsh edges, characterized by dense groups of tussock-forming sedges).
Precisely how the habitat develops, depends on its physical and chemical characteristics, including its climate, topography, water depth and flow, as well as the availability of plant species.
What’s the difference between peatland and a mire?
The term ‘peatland’ is used for any terrain containing peat to a minimum depth of 30 cm (12 in), even if it has been completely drained of water. A ‘mire’ however must be waterlogged and actively forming peat. That said, the terms are used interchangeably in this article.
Is Peat a Fossil Fuel?
Answer: Yes and No.
Yes, because it shares certain common characteristics with other fossil fuels: it’s a hydrocarbon material that spends thousands of years in sediments, and its emission factor is similar to fossil fuels.
If this sounds odd, remember that in April 2018, the U.S. Environmental Protection Agency (EPA) announced that wood was henceforth deemed to be a “carbon-neutral” fuel. This despite evidence that chimney emissions from burning wood for heat can be 30 percent higher than those of coal, and 2.5 times greater than those of natural gas, per unit of generated energy. 2
What’s more, when burning wood for electricity, smokestack emissions can be 1.5 times greater than those of coal (per MWh) and more than three times higher than those of natural gas.
How Does Peat Compare as an Energy Source?
Peat has been employed as a fuel for at least 2,000 years. It was used in temperate and boreal regions of Europe as an alternative to firewood for both cooking and heating. Historically, it was especially popular in Ireland, Scotland, Finland, the Netherlands, Sweden, Germany, Poland and parts of Russia.
But with the introduction of natural gas and oil during the 20th century, the use of peat for domestic purposes declined in all areas, although growing demand for electricity led to the development of peat-fired power plants, notably in Ireland, Finland and Russia.
The use of peat for energy purposes – as a cheaper alternative to oil and natural gas – became even more attractive following the oil crisis in 1973.
Today, however, as our climate crisis intensifies, peat consumption has collapsed due to its relatively high greenhouse gas emissions. The problem is, when hydrocarbons like coal, oil and peat are burned in the presence of oxygen they emit carbon dioxide (CO2) into the atmosphere. Carbon dioxide is a powerful, heat-trapping greenhouse gas and is the leading cause of global warming.
Peat has a relatively high carbon content and, when burned, releases relatively large amounts of CO2. Worse still, the preparation of bogs for peat production produces significant extra emissions. This is because, when boglands are industrially drained and stripped to prepare them for peat harvesting, they become exposed to oxygen which re-starts the decomposition of the stored organic material in the soil, and releases carbon dioxide into the atmosphere.
According to one Irish study, each hectare of prepared peatland emits 2.1 tons of carbon per year — the same emissions that come from driving a car for 30,000 kilometers. And this is before a single kilo of harvested peat is burned. 3
Comparative Calorific Value of Peat
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Despite its comparatively high level of CO2 emissions, peat has a relatively low calorific value (heat content) and generates less energy than coal when burned. As a result, it is more damaging to our climate than coal. 5
In Ireland, for example, where coal and peat continue to be used for power generation, they account for 40 percent of all carbon emissions from electricity generation (2018), but produce only 14 percent of the electricity. As a result, the carbon intensity of Irish electricity is one of the highest in the European Union. 6
What Types of Peat are Used as Fuel?
Four types of commercial peat are commonly distinguished:
This is a granulated form of peat, large quantities of which are used in power plants. It has an average moisture content of about 40-50 percent. Milled peat is commonly produced by large scale mechanized peat extractions.
Peat pellets are made from dried, powdered peat. They are used mainly in heating or power plants, either as a co-fired material, or as a replacement for coal. One ton of peat pellets is reported to have the same heat content as 500 liters of diesel, or 4 tons of wood.
This consists of slabs of peat that are cut direct from the peat bog and dried for domestic heating and cooking. Air-dried sod peat typically has a moisture content of about 30-40 percent. It is typically produced on a smaller scale by manual, semi-mechanical or mechanical methods, either in dry or in wet conditions.
These are small blocks of highly compacted and dried peat with a moisture content of 10-20 percent, used mostly in homes, or sometimes in older electricity power plants as a replacement for coal. Similar to sod peat, briquettes are produced on a smaller scale by manual, semi-mechanical or mechanical means.
Peatlands or mires are basically wetlands whose soils are comprised almost entirely of organic material derived from the remains of dead and decaying plant matter. But exactly how much peat is there in the world? And where is it located? There are still no precise answers to these questions.
To begin with, there is no universal definition of what constitutes peat. Some experts say that peat is an organic soil which must contain at least 20 percent organic matter, increasing to 30 percent if 60 percent of the mineral matter is clay. Other experts say that peat should have at least 30 percent organic content and a thickness of at least 30 cm.
To confuse things more, the average thickness of an area’s peat layer is difficult to assess precisely, due to a lack of detailed data for most regions. Therefore, it is not possible to determine accurately the overall volume of peat in the ground or the amount of carbon it contains.
The largest area of peatland – around 72 percent of the global total, is found in the tundra, boreal/taiga, and temperate zones of the Northern Hemisphere. For example, very large expanses of Canada, Scandinavia and Siberia are home to a wide variety of boreal mires.
Peatlands are also widespread in the tropics and the wettest areas of the sub-tropics. Tropical mires – mostly underlying tropical rainforests and coastal mangals – account for around 11 percent of global peatland. These are divided largely between the Amazon Rainforest and the Pantanal wetlands in Brazil, the Congo Rainforest in Africa, and Southeast Asia.
Overall, peatlands have declined globally due to drainage and peat harvesting, and because of agricultural activities. For example, more than half of European mires, an area of more than 300,000 square kilometers, has been lost. 7 The greatest losses have been in Russia, Belarus, Finland, Poland, the Netherlands and Scotland.
But new discoveries of active mires continue to be made, notably in the tropics. For example, in 2017, researchers identified and mapped the largest peat habitat in the tropics, located in the Cuvette Centrale swamp forest in the Congo Basin. The peatland covers 145,000 square kilometers and is believed to store around 30 billion tonnes of carbon. 8
Region Peatland (sq km)
Estimates of the total carbon stored in peat mires around the globe vary between 6,000 and 13,800 billion cubic meters, or 300 to 695 billion tonnes of carbon. Others, like Strack (2008), claim that the global carbon reservoir stored in peatlands is around 500 billion tonnes. But no ground level measurements of global peat reserves have yet been taken.
Greenhouse Gas Emissions from Peat
Peatlands affect global warming in two ways. First, like all areas of plant vegetation, they absorb CO2 during photosynthesis, and release CO2 when they respire. In general, plants absorb significantly more CO2 than they release. This net uptake of CO2 exerts a cooling effect on climate.
Second, peatlands – being waterlogged areas whose soils are rich in partly decomposed plant matter (mostly hydrocarbons) – act as huge reservoirs of carbon, that would otherwise find its way into the atmosphere and heat up the planet. By locking up all this carbon, peat mires play a vitally important role in climate change mitigation.
It looks like wetlands have a hugely beneficial effect on our climate. But it’s not quite as simple as it sounds. Although waterlogged soils lead to slow decay because there is insufficient oxygen available for the regular bacteria that decompose organic matter (giving off CO2 in the process), these low oxygen environments attract what’s called “methanogenic” decomposers that don’t need oxygen.
The trouble is, these methanogenic bacteria give off methane (CH4) as they work. And methane is a much more powerful greenhouse gas than CO2. Despite the mitigating effect of methanotrophic bacteria (a third type of decomposer that eats methane), scientists estimate that wetlands are responsible for 31 percent of all methane emissions, annually, of which peat mires account for around one third. 11
Other factors complicate the situation further.
Yes, peatlands release methane, and Yes, it’s more powerful than CO2. But methane’s lifetime in the atmosphere is much shorter than for CO2. So, over a long time period, most peatlands have a net cooling effect on the atmosphere. Thus, for instance, over the last 10,000-14,000 years, the northern peatlands have had a net cooling effect on global climate. 12
Peatlands can also be a source of nitrous oxide (N2O), which is another potent greenhouse gas. Usually, so long as they are left undisturbed, peatlands emit only small amounts of N2O. But, drainage and fertilization of peatlands, or any type of preparation for agricultural use, can greatly increase N2O emissions. 13 As we have seen, disturbing peat bogs can also lead to increased CO2 emissions.
To summarize the situation so far: generally speaking, peatlands have a net cooling effect on global climate, subject to two caveats. The first is that the level of methane emissions are hard to pin down. It could be more than we think. Secondly, the amount of CO2 and N2O released can increase significantly if the peatland is disturbed or allowed to dry.
Effect of Global Warming on Peatlands
Climate change is already affecting the ability of peatlands to store carbon. This is because when soils become warmer, the rate of organic decomposition increases which leads to the release of more carbon. Any drying of the peat habitat will also boost emissions.
Peatlands that are particularly vulnerable to global warming include those in the vast circumpolar areas of permafrost in Russia, Alaska, Canada and Greenland, where temperatures are rising faster than anywhere else in the world, as shown by the recent Arctic fires in Siberia.
As permafrost thaws, so too does the plant material as well as the microbes inside it, who can now begin decomposing the material. This decomposition process emits carbon dioxide (if oxygen is present during decomposition), or methane (if no oxygen is present).
In its annual survey of the state of the Arctic, the U.S. National Oceanic and Atmospheric Administration (NOAA) warns that thawing permafrost throughout the Arctic could be releasing an estimated 1.1 billion to 2.2. billion tons of carbon dioxide annually into the atmosphere. These emissions are likely to grow “two to three times bigger by the end of the century” said a NOAA spokesperson. 14
According to one recent survey, estimates of the carbon emitted through thawing, and from losses of peat into rivers and streams, are 30-50 percent higher than previously thought. 15
According to researchers, the northern peatlands contain about 415 billion tonnes of carbon. Which is roughly equivalent to 46 years of current global CO2 emissions.
- International Peatland Society
- “Range and uncertainties in estimating delays in greenhouse gas mitigation potential of forest bioenergy sourced from Canadian forests.” Jerome Laganiere, et al; 2015.
- “Carbon emissions and removals from Irish peatlands: present trends and future mitigation measures.” David Wilson, et al. Irish Geography. Sep 2013.
- “Energy Use of Peat.” fao.org
- “Why peat is most damaging fuel in terms of global warming, even worse than coal.”
- “Electricity.” Sustainable Energy Authority of Ireland.
- Joosten, H.; Clarke, D. (2002)
- “Age, extent and carbon storage of the central Congo Basin peatland complex.” Dargie, G., Lewis, S., Lawson, I. et al. Nature 542, 86–90 (2017).
- “An expert system model for mapping tropical wetlands and peatlands reveals South America as the largest contributor.” Thomas Gumbricht et al. Global Change Biology. (2017)
- “PEATMAP: Refining estimates of global peatland distribution based on a meta-analysis.” Jiren Xu et al. (2018). Catena. 160. 134-140.
- Source: Global Methane Budget 2000-2017 (Supplemental Data) Global Carbon Project 2019.
- “Holocene radiative forcing impact of northern peatland carbon accumulation and methane emissions.” Steve Frolking, Nigel T. Roulet. Global Change Biology. 2007.
- “Nitrogen-rich organic soils under warm well-drained conditions are global nitrous oxide emission hotspots.” Parn, J. et al. Nat Commun 9, 1135 (2018).
- “Arctic Report Card: Update for 2019.” NOAA.
- “Large stocks of peatland carbon and nitrogen are vulnerable to permafrost thaw.” Gustaf Hugelius, et al. PNAS August 25, 2020 117 (34) 20438-20446; first published August 10, 2020;
- “Introduction. The boreal forest and global change.” K.E Ruckstuhl et al. Philos Trans R Soc Lond B Biol Sci. 2008 Jul 12; 363(1501): 2245–2249.