First broadcast on 22 April 2021
In this episode, Tom Heap and Dr Tamsin Edwards consider carbon-storing peatlands and how we can return drained bogs to life.
There's a lot of carbon locked up in the peatlands of the UK and Ireland. But historically many of them were drained for agriculture and to use for fuel, resulting in a massive loss of carbon to the atmosphere. However, rewetting the bogs can not only stop that leaching of carbon but also potentially return them to sequestering carbon. Could these once forboding 'creepy' habitats be something of an underrated super solution?
Tom Heap speaks to expert Florence Renou-Wilson and takes a virtual tour of a new carbon farm - hoping to harvest carbon back from the atmosphere. Dr Tamsin Edwards from Kings College London assesses the potential of this solution and the considerations.
Listen now on BBC Radio 4
We asked Society Fellows, Professor Christopher Evans from the UK Centre for Ecology & Hydrology and Mike Peacock from the Swedish University of Agricultural Sciences to offer some observations on the potential impact of rewetting peat on reducing carbon emissions. Their points take some of the themes of the programme a step further.
Global peatland area is 420-460 million hectares, of which over 10% has been drained. It has been calculated that these areas emit the equivalent of 1.2 - 1.9 gigatonnes (Gt) of carbon dioxide (CO2) which is 2.3 - 3.8% of all anthropogenic greenhouse gas (GHG) emissions. This can be calculated by combining maps of drained peatland with emission factors from the IPCC. Thus, fully rewetting peatlands could lead to a reduction of 1.9 Gt carbon dioxide equivalent (CO2e) per year. This rewetting will result in increased methane emissions, but it has been shown that rewetting still has a a long-term cooling effect on the climate because the emitted methane only stays in the atmosphere for 12 to 15 years, whereas the CO2 emissions without rewetting would have warmed the atmosphere for centuries.
Current emissions: There are relatively firm estimates for the amount of GHG emissions from peat under agriculture - in the order of 1 Gt CO2e/yr (gigatonnes carbon dioxide equivalent per year), mostly from Europe and SE Asia. Peat fires (mainly in Asia) are responsible for another 0.2-0.4 Gt CO2e/yr. There is some additional emission from nitrous oxide (N2O) and non-agricultural degraded peatlands. In the UK, peatland emissions have recently been fully incorporated in national emissions inventory reporting and have been estimated 18.8 metric tons of carbon dioxide equivalent per year (Mt CO2e/yr), which is around 3.5% of the UK’s total GHG emissions.
Emissions reduction potential
The UK Centre for Ecology & Hydrology recently led a global analysis under review that suggests that agricultural peat greenhouse gas (GHG) emissions could be reduced by about 65% by halving average drainage depths in cropland and grassland. That equates to about 1.3% of global GHG emissions.
According to the analysis above, complete rewetting could turn the same agricultural peatland area into a marginal net sink for GHGs - the limiting factor is that at very high water levels, the higher methane emission starts to cancel out the higher CO2 sequestration. At optimal water levels (and based on the standard IPCC Global Warming Potential for methane (CH4) we estimated that peatlands would be approximately climate-neutral on a 100-year timescale (becoming strongly net cooling over longer periods). Carbon sequestration potential has been estimated at about 2000 Mt CO2 in England alone, similar to the CO2 storage capacity of gas wells under the North Sea, which form a key component of Carbon Capture and Storage (CCS) schemes aimed at removing CO2 from the atmosphere.
Degraded blanket bog, Grey Mares Tail, Moffat, Scotland (Image: Emma Goodyer/Flickr, CC BY-NC-SA 2.0)
Carbon sequestration potential has been estimated at about 2000 Mt CO2 in England alone, similar to the CO2 storage capacity of gas wells under the North Sea, which form a key component of Carbon Capture and Storage (CCS) schemes aimed at removing CO2 from the atmosphere.
Peatlands are important areas for food production. The National Farmers’ Union (UK) say “although it covers less than 4% of England’s farmed area, the Fens produces more than 7% of England’s total agricultural production, worth a staggering £1.23 billion”. In the UK uplands there has been some concern by farmers that rewetting would flood the land and reduce grazing quality for sheep. However, a study from Wales found no evidence for this.
Beyond the UK, drainage for agriculture has affected peatlands all over the world. In a new paper, we show that raising the water table in managed peatlands would also generate large greenhouse gas emission savings: halving water table depth in all agricultural peatlands could reduce potential GHG emissions by 390 – 650Mt CO2e per year, in the region of 1% of all anthropogenic CO2 emissions. It could offer the best means of reducing GHG emissions in the short term without displacing large populations from the land they farm. In the longer-term, new forms of wetland-based peatland management are likely to be needed to fully protect the carbon stored in global peatlands.
Some peatlands damaged beyond repair
In some cases, it may be impossible to restart the peatland carbon sink after rewetting, especially if peat soils have wasted/been lost. However, rewetting will still lead to 'avoided losses' of GHGs.
Funding for restoration work
This is a decreasing limitation, at least in the UK where the Westminster and devolved governments have invested £100ms into peat restoration, mainly in the uplands. There is also huge economic potential from individual and corporate carbon offsetting but this does require that the climate benefits can be demonstrated and verified which are largely in the form of avoided emissions.
Peatlands switched to food production
It is important to engage farmers in discussions around food production options with high water levels, reducing use of fertilisers and protection from soil erosion. The concept of ‘paludiculture’ (agricultural management of wet peat) has also gained traction in recent years, although further development is needed before this can provide a viable economic option for many farmers.
Time taken for peatland to start sequestering carbon again
Halting emissions may occur relatively quickly following rewetting but re-establishing the natural role of peatlands as carbon sinks is likely to take longer, as peat-forming wetland vegetation such as Sphagnum mosses takes time to re-establish.
Wet peatlands emit CH4, so there is a risk that rewetting could have a net warming impact in the short-term (i.e. decadal timescale of Paris Agreement), even if it will undoubtedly have a net cooling effect in the long term. Raising water levels in cropland would reduce CO2 emissions without making the system wet enough to emit methane, but is unlikely to completely halt carbon loss, and certainly will not lead to peat formation. There is evidence that some types of peatland vegetation, such as Sphagnum, can suppress CH4 emissions, so it may be possible to manage rewetted peatlands in ways that maximise their CO2 uptake while minimising their CH4 emissions, but this remains an important and not fully resolved challenge.
Sphagnum moss (Image: Madlily58/Flickr, CC BY-NC-SA 2.0)
Peatland rewetting in the UK uplands has been shown to be beneficial for crane flies, and therefore beneficial for upland birds (golden plover, dunlin) that feed on crane flies. Rewetting has also been shown to reduce suspended sediment concentrations in streams, with positive effects on stream invertebrates.
Peatland rewetting also reduces concentrations and exports of 'dissolved organic carbon' (DOC). Large amounts of drinking water in the UK comes from upland peatlands, and this DOC has to be removed during water treatment. Additionally, the treatment of this DOC produces trihalomethanes as disinfectant by-products. There are adverse health effects associated with trihalomethanes, and so water treatment plants must remove them and DOC. These procedures can be expensive.
About 10% of the UK is covered by peat, most of it upland blanket bog which drape the landscapes of Western Britain and Ireland. Blanket bogs are globally rare habitats, which can only form in temperate maritime climates. Over many areas, blanket bogs have been damaged by drainage, grazing, burning and air pollution, and rewetting them is therefore helping to restore one of the UK’s most important habitats, as well as reducing GHG emissions. In Southeast Asia, peat swamp forests are vital habitats for threatened species such as orangutans, Sumatran tigers and clouded leopards, so protecting their carbon stores will also help to save these species from extinction. While the biodiversity benefits of peat restoration are clear, actively managing peatlands to remove CO2 from the atmosphere (‘carbon farming’) may involve some trade-offs, for example it is possible that the highest rates of CO2 removal could be achieved through intensive ‘monoculture’ management of individual wetland species, rather than by simply restoring the natural ecosystem.
There is good evidence that restoring natural wetland vegetation and blocking erosion gullies on degraded blanket bogs can slow rates of water flow across the peatland, which may help to reduce the height of flood peaks in urban areas downstream if peat restoration is implemented at scale. However, in the lowlands, the ditch networks in agriculturally drained peatlands are actually used as flood water storage, which would be lost following rewetting, but there is potential to redesign landscapes so that restored peatland areas can provide this flood storage instead. The Ouse and Nene Washes of the Fens have been providing flood storage since the land was first drained in the 17th century.
In tropical peatlands keeping water levels high greatly reduces the severity of fires. When fires burn down into dry peat, this causes huge CO2 emissions and the severe ‘haze’ events that can affect the whole region and have major health impacts. In the UK there is an active debate about whether burn-managed grouse moors need regular controlled burning to reduce the risk of woody biomass accumulation leading to more damaging wildfires. However, a natural peatland does not need burn-management, because the rapid growth of Sphagnum mosses restricts the growth of the heather, so if sites can be fully restored, the wildfire risk should diminish.
Reduced land subsidence
Drainage leads to the compaction and oxidation of peatlands, which in turn results in rates of land subsidence in the region of 1-2cm per year in temperate regions, and 3-5cm per year in the tropics. Historically, over 3m of peat has been lost from parts of Eastern England, as well as other areas such as the Netherlands, resulting in large areas of land that are now below sea-level and require pumped drainage to stop them from flooding. Continuing subsidence causes damage to roads, buildings, pipelines and overhead cables. In the lowland peatlands of Southeast Asia, peat subsidence may ultimately make it impossible to sustain agriculture over large areas as they become increasingly prone to river and coastal flooding. Rewetting peatlands will effectively halt subsidence,
Department for Business, Energy & Industrial Strategy. 2021. Planned methodology changes for UK greenhouse gas emissions statistics 1990 – 2019. Crown copyright 2021
Günther, A., Barthelmes, A., Huth, V. et al. Prompt rewetting of drained peatlands reduces climate warming despite methane emissions. Nat Commun 11, 1644 (2020).
Leifeld, J., Menichetti, L. The underappreciated potential of peatlands in global climate change mitigation strategies. Nat Commun 9, 1071 (2018).
Leifeld, J., Wüst-Galley, C. & Page, S. Intact and managed peatland soils as a source and sink of GHGs from 1850 to 2100. Nat. Clim. Chang. 9, 945–947 (2019).
Lowland Peat (2021) Lowland Peat 2 Project
39 ways to save the planet is a new radio series by BBC Radio 4 developed in partnership with the Society and broadcast in 2021. It showcases 39 ideas to relieve the stress that climate change is placing on the Earth. In each 15 minute episode Tom Heap and Dr Tamsin Edwards meet the people behind a fresh and fascinating idea to cut the carbon.
Over the course of 2021, the Society will be producing events and digital content to accompany the series.
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