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Nature-based solutions: rhetoric or reality? - The potential contribution of nature-based solutions to net zero in the UK Contents

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Chapter 2: Nature-based solutions in the UK

The state of the UK’s natural environment

9.The UK’s natural environment is degraded, and its biodiversity has declined over many decades. An index from the Royal Society for the Protection of Birds and the Natural History Museum ranked the UK as the 12th worst country in the world, and the worst in the G7, in terms of the amount of its biodiversity it has destroyed.6 The State of Nature Report has tracked biodiversity in the UK since the 1970s. The 2019 report found that 41% of the wild species monitored in that period had declined, 15% were at risk of extinction from the UK and 2% had already disappeared.7 The Government legislated in the Environment Act 2021 to halt species decline by 2030 in the UK.8

10.There are many reasons for the decline in biodiversity, but the State of Nature Report identified land use as the single biggest driver. The majority, 72% (17.3m ha), of the UK’s land is primarily managed for agriculture. Peatland covers 2.6 million hectares; 10% of the UK’s land area. But the International Union for the Conservation of Nature estimates that only 20% of UK peatland is in a near-natural state. A recent reassessment found that UK peatland is so degraded that, overall, it is emitting more CO2 than it is sequestering.9 Around 13% of the UK’s land area is forest, but just 44% of the forested area is sustainably managed.10 Only half of the UK’s fish stocks are sustainably managed and 57% of the seafloor in UK waters was disturbed by bottom contact fishing gear between 2010 and 2015.11

Key environments in the UK

11.There is potential for nature-based solutions to be deployed across UK habitats. We go into more detail below, but Natural England recently estimated carbon stores and potential sequestration rates for a range of different habitats.

Figure 1: Carbon storage by habitat

Source: Natural England, Natural England Research Report (NERR094) Carbon storage and sequestration by habitat: a review of the evidence, second edition (20 April 2021) pp 210–221: http://publications.naturalengland.org.uk/file/6726246198411264 [accessed 12 January 2022] Colour-coding indicates the level of scientific certainty around this ecosystem, as assessed by Natural England. The grey bars illustrate the range of values measured across different sites by Natural England’s meta-analysis, while the solid bar gives a best estimate. For most land ecosystems, these figures show soil carbon to the depth of the ecosystem and carbon stored in vegetation.

*For grasslands, the data is shown for the top 15cm of soil only, and the range is across three types of semi-natural grassland—acid, calcareous, and neutral—for more information, see the Natural England report. **For the managed areas of arable and improved grasslands, only soil carbon to 1m depth is included, as the vegetation stocks are removed annually by management.

Figure 2: Greenhouse gas emission rate by habitat

Source: Natural England, Natural England Research Report (NERR094) Carbon storage and sequestration by habitat: a review of the evidence, second edition (20 April 2021) pp 210–221: http://publications.naturalengland.org.uk/file/6726246198411264 [accessed 12 January 2022] Colour-coding indicates the level of scientific certainty around this ecosystem, as assessed by Natural England. The grey bars illustrate the range of values measured across different sites by Natural England’s meta-analysis, while the solid bar gives a best estimate. These sites will vary in condition and age; the value given is an indicative estimate of the average sequestration rate for sites of this type. Negative values indicate that the habitat sequesters carbon.

*For grasslands, the data is shown for the top 15cm of soil only, and the range is across three types of semi-natural grassland—acid, calcareous, and neutral—for more information, see the Natural England report. **For the managed areas of arable and improved grasslands, only soil carbon to 1m depth is included, as the vegetation stocks are removed annually by management.

Forests

12.In the past, much of the UK was forested.12 But many trees were lost as part of the gradual conversion of land to agricultural uses, and later to the urbanisation that accompanied the industrial revolution. This decline continued into the 20th century. Forest cover in the UK has increased significantly from the low point in the 20th century after governments supported large scale tree planting. This averaged up to 40,000 hectares a year in the 1970s.13 Many of the trees planted were conifers, which are non-native species, with the exception of Scots’ pine. They account for 51% of the UK’s tree cover and 92% of timber harvested in the UK.14 Most of the remainder is broadleaved woodland, which includes species such as oak, beech and birch.15 Flexibility may be needed in the future to determine what constitutes a native species as the climate changes. Native trees like beech may struggle in the UK climate of 2050, while trees currently considered non-native will be better suited to these conditions.

13.The UK’s forests store around 1.1 billion tonnes of carbon and they sequester about 4.6% of the country’s total CO2 emissions annually.16 The Committee on Climate Change, which is the Government’s independent advisor on climate change, has calculated that, if the UK’s tree cover were to increase from 13% to 17% by 2050, and if management practices are improved, then 14 megatonnes of CO2 equivalent (MtCO2e) per year would be sequestered by 2050 with 14MtCO2e per year also stored in harvested materials.17 This is small compared with the UK’s current annual emissions of around 522MtCO2e,18 but it is a significant fraction of the residual annual emissions of about 90MtCO2e that will remain by 2050 under the rapid decarbonisation needed in order to achieve net zero emissions.19 Professor David Coomes, Director of the University of Cambridge Conservation Research Institute, told us that, with widespread afforestation, by 2030 there could be sequestration by forests of “one or 2 extra megatonnes of CO2 per year by 2030, compared with 19 megatonnes of CO2 per year at present.”20 He emphasised that the contribution trees can make to the net zero objective, relative to the UK’s overall greenhouse gas emissions, should not be overstated: “most models looking at trends to 2050, when we are meant to be achieving net zero, say that planting forests now is not going to have a huge impact in that timeframe.”21

Figure 3: The scale of the contribution of forestry to net zero by 2050

*Hard-to-mitigate emissions are calculated as the residual positive emissions in the Balanced Net Zero pathway after rapid decarbonisation in other sectors of the economy. Source: Committee on Climate Change, The Sixth Carbon Budget, The UK’s path to Net Zero (9 December 2020): https://www.theccc.org.uk/wp-content/uploads/2020/12/The-Sixth-Carbon-Budget-The-UKs-path-to-Net-Zero.pdf [accessed 17 December 2021]

Figure 4: The contribution of land-based nature-based solutions to mitigating ‘hard to mitigate emissions’ by 205022

Source: Committee on Climate Change, The Sixth Carbon Budget, The UK’s path to Net Zero (9 December 2020): https://www.theccc.org.uk/wp-content/uploads/2020/12/The-Sixth-Carbon-Budget-The-UKs-path-to-Net-Zero.pdf [accessed 17 December 2021]; Committee on Climate Change, Updated quantification of the impact of future land use scenarios to 2050 and beyond (UK Centre for Ecology and Hydrology) (9 December 2020): UK-CEH-Updated-quantification-of-the-impact-of-future-land-use-scenarios-to-2050-and-beyond.pdf (theccc.org.uk) [accessed 17 December 2021].

*Hard-to-mitigate emissions are calculated as the residual positive emissions in the Balanced Net Zero pathway after rapid decarbonisation in other sectors of the economy. ** This bar shows the sum of the nature-based solutions assessed by the Committee on Climate Change (CCC) for its Balanced Net Zero pathway. *** The contribution of peatland restoration is largely in the carbon emissions restoration prevents, rather than the active sequestration of additional carbon.

14.The Government aims to increase tree planting to 30,000 hectares a year by 2025.23 In 2019 and 2020, 13,500 hectares were planted.24 As part of its plan for the UK to meet the net zero target by 2050, the Committee on Climate Change recommends that tree cover in the UK is increased from 13% to 17% by 2050, with 460,000 new hectares of mixed woodland planted by 2035.25 This could constitute 1.5 billion new trees by 2050. The Government signed the 2014 New York Declaration on Forests, which aimed to halve international deforestation by 2020 and end it by 2030.26 The 2020 target of the New York Declaration was not met. At COP26, nations restated the target to end deforestation by 2030.

15.Planting trees removes from the atmosphere CO2 that would otherwise contribute to the greenhouse effect.27 Existing forests are carbon stores; protecting or restoring them can ensure that carbon remains locked away. Forests, as well as urban trees, have wider benefits: they support biodiversity; they mitigate flood risk; they provide space for recreation; and, crucially in a warming climate, they provide shade and help reduce the urban heat island effect.

Figure 5: Carbon sequestration over time for the main UK tree species28

Source: Written evidence from the Royal Society (NSD0050). Note that the area under the curve shows the total amount of carbon stored by the tree at a given time after planting.

16.There are different factors to consider when using trees in nature-based solutions. The amount of carbon that trees sequester varies over their lifespan and from species to species. Slow growing trees will not sequester much CO2 shortly after being planted. This means that some slow growing native species, not frequently used for commercial forestry, such as oak and Scots pine will not sequester much carbon before 2050. Fast-growing non-native species sequester carbon relatively rapidly. They are often grown commercially for timber due to their rapid growth.

17.There is a distinction between carbon sequestration (net removal of CO2 from the atmosphere) and carbon stocks (the amount of carbon stored in an ecosystem.) Newly established forests may sequester carbon at a similar or faster rate than old-growth forests as their trees grow rapidly initially. But they store much less carbon than old-growth forests, which have a greater overall biomass, and which store additional carbon in the soils. The destruction of existing forests is therefore likely to release more carbon than the growth of new forests can take up, at least within 30 years of their planting. The rates and timescales of carbon sequestration, as well as the ultimate amount of carbon storage, also depends on the species of tree and woodland type.

18.Carbon sequestration is not the only consideration. The British Ecological Society told us “native broadleaf forests provide considerably better biodiversity benefits … over conifer plantations”.29 Richard Greenhous, Director of Forest Services at the Forestry Commission, clarified that the Forestry Standard would no longer allow a monoculture plantation.30 He challenged “the idea … that productive forestry does not deliver biodiversity benefits.” But he agreed that “undoubtedly, a native broadleaf woodland would deliver more biodiversity benefits”.31

19.It is important to consider the properties of the site chosen for tree planting. In the 1970s and 1980s, much of the planting was on peatlands. Some of these peatlands are now being restored.32 This is because planting on peatlands (or wildflower meadows) disturbs the CO2 stored there and it damages that habitat. Other habitats are important for sequestering and storing carbon, and planting trees in these areas may do more harm than good. For example, converting land from productive agriculture or commercial forestry to forests planted primarily as a permanent carbon store could increase reliance on overseas imports for food or timber. This creates the risk that environmental degradation is merely “offshored.”33 Shifting emissions to other parts of the economy can make accounting for the overall impact of an intervention difficult. Studies have suggested that if you accounted for imports, 46% of the UK’s overall emissions would be associated with goods manufactured overseas but consumed in the UK.34

20.The long-term storage of carbon is another important consideration. Nature-based solutions schemes must be resilient to pests and diseases. Ash dieback, which is likely to kill 80% of one of the UK’s most common trees, illustrates how devastating diseases can be.35

21.Trees will also need to be resilient to future climate change. The Government is aware of this risk, cautioning “under a hotter, drier climate removal mechanisms may alter substantially. Traditional approaches to woodland management and selection of planting stock (species and seed origins) may no longer provide the level of removals expected, with difficult cultural discussions around selection of species and cultivars for future climates.”36 We heard that more biodiverse forests are more resilient because “some species are going to succumb to them [pests and diseases] but, because there is diversity, other species can quickly take their place and refill the woodlands.”37

22.Trees sequester carbon at different rates at different stages of their life, and this sequestration profile depends on the species of the tree. As they grow, trees sequester carbon rapidly, before becoming saturated when they are fully mature. This must be considered when determining which tree will sequester and store carbon over which timescale.38 Consequently, a way of sequestering more carbon in existing forests is to lengthen rotations and allow trees to grow for longer before harvesting them. Sir Harry Studholme, Former Chair of the Forestry Commission, thought this could increase carbon stock more quickly than new plantations.39 However, these forest management techniques would not be incentivised under tree planting schemes that focus on meeting the 30,000 hectare target for additional tree planting.

23.The evidence base for how much forests sequester carbon is more developed than for other habitats, but uncertainties remain. Professor Henderson, Chief Scientific Adviser, Department for Environment, Food and Rural Affairs, described the evidence for the carbon storage and sequestration of trees as “not perfect but … pretty good.”40 Dr Bonnie Waring, Senior Lecturer, Grantham Institute on Climate and Environment, Imperial College London told us there was “tremendous uncertainty” around the carbon sequestration of soils, which will be discussed below, and:

“Predictions of how much carbon a woodland can capture are based on our measurements of the stem. We know much less about allocation to roots and branches, particularly outside the most commercial species, and that could be important.

The other thing is that we can model carbon uptake fairly well in, essentially, plantations where there is a single species in the same age cohort. We have much less data to model what a naturally regenerating forest would look like with lots of different species all at different ages.”41

24.Many of the estimates of greenhouse gas emissions from different areas of land rely on emissions factor methodology. Emissions factors predict emissions from an area of land by sorting areas into categories, using estimates of emissions per unit area from a limited number of studies for land for each category, and multiplying the area by the emissions factor. It is a useful method for providing an estimate on a national scale but leaves uncertainty as to the condition of an individual site. Moving from broad emissions factors towards more comprehensive greenhouse gas accounting would improve the accuracy of the data but requires much more on-the-ground monitoring. In many cases, emissions factors themselves are uncertain, and based on limited data.

25.There is uncertainty around the long-term fate of the CO2 sequestered in trees. Forests that survive are a stable carbon sink into the future. For commercial forestry, long term carbon sequestration depends on how the wood is used once it is harvested. Wood used in construction will store its carbon for the relative long term, while wood used to make paper will release its carbon into the atmosphere quickly as the paper decomposes. We heard that, at present, “between a third and a half of the timber harvested is going into long-term storage”.42

26.Faster growing trees sequester carbon more quickly, but surviving, old growth, mixed woodlands are large stores of carbon. The scale of the contribution that fast growing, commercial, forestry can make to net zero by 2050 is significant, but it depends on how harvested wood is used.

27.We recommend that a life-cycle analysis be undertaken to calculate the carbon benefits of tree-planting. The fate of the carbon must be monitored beyond harvesting: it is not enough to plant a tree and consider that carbon “sequestered”. When deciding which trees to plant, the Forestry Commission must consider factors including resilience to climate change, disease, the risk of fire and potential release of carbon, carbon storage potential, including in the soils, and biodiversity benefits.

Peatlands

28.Peatland ecosystems are wetland habitats that contain a layer of semi-decomposed organic matter, or peat. The organic matter is semi-decomposed because the waterlogged, nutrient poor, and often acidic, conditions prevent it from being broken down. Peat forming plants consist mostly of sphagnum moss. Peaty soils are defined in England and Wales as those with a 30cm layer of soil comprised of more than 30% organic (carbon-containing) matter.43 Peatland covers 3 million hectares in the UK—around 12% of the UK’s land area.44 Most of the UK’s peatland is in Scotland.

29.Peatland in the UK consists of three main types: blanket bog, raised bog, and fens.45 Bogs are peatland areas that are mainly fed by rainwater, which makes them nutrient-poor and acidic. Raised bogs are relatively small areas in the lowlands where peat has accumulated to a depth of more than 10m. Blanket bogs consist of extended areas where the peat has formed a layer of 0.5m over lowland, or upland, areas. Fens are areas of peatland that are fed by both groundwater and rainwater, and therefore contain nutrients from the underlying rock. The UK is among the top 10 nations in the world in terms of peatland area, containing 9–15% of Europe’s peatland and about 13% of the world’s blanket bog.46 A distinction is often made between lowland and upland peats. Lowland peats are predominantly fens, which are good for agriculture due to their nutrient content.

30.The partially decomposed organic matter contains large stocks of carbon, which makes peatlands the most carbon-dense terrestrial systems on the planet. Peatlands store at least 550 gigatonnes of carbon worldwide, which is twice the amount of carbon stored in the biomass of all vegetation, including forests, while covering an area amounting to only 10% of the area covered by forests.47 When the peat is disturbed, the carbon locked in the organic matter can be released into the atmosphere as CO2. Globally, emissions from degraded peatland contribute 5.6% of all human-caused emissions (1.3GtCO2).48

31.It is not known how much carbon is stored in UK peats. One study estimated 11700MtCO2e, but the inadequate mapping of peatlands makes this a speculative figure.49 Despite the uncertainty, there is consensus that peatlands are the UK’s largest natural carbon store, holding roughly 40% of the UK’s soil carbon.50

32.The Royal Society told us “peatlands in a near-natural state have a moderate carbon sequestration effect.”51 This occurs as new layers of carbon-rich peat form on top of the peatland. But the UK’s peatlands are highly degraded—only 20% are in a near-natural state, and much of them no longer form new peat.52 Forty-one per cent are in a semi-natural state, but they have been affected by drainage, managed burning, livestock grazing, and peat harvesting. Woodland accounts for 16% of peatlands, the majority of which has been drained and planted with conifers; 15% is covered by cropland and grassland; and 0.15% is subject to peat extraction for horticulture.53 This degradation means that peatlands, instead of sequestering carbon, are emitting around 21.3MtCO2e each year.54 Dr Rebekka Artz, Senior Research Scientist, Ecological Sciences, James Hutton Institute, told us that this added 3.5% to the UK’s total emissions and that it is “the same as … the entire forestry carbon sink at present … all but cancelling out the uptake by the forestry sector.”55

33.The causes of this degradation vary but are mainly: drainage for agriculture or forestry, air pollution, fire (managed or otherwise), and the extraction of peat for horticulture.56 These activities disturb new peat formation and they release the carbon stored in old peat. Draining peat lowers the water level, allowing air to penetrate the peat and to convert stored carbon into CO2. Bare peat associated with fires or cultivation loses particles of peat, which can erode into streams. In oxygen-rich stream water, the peat can release its carbon as CO2. Similar processes account for CO2 emissions from soil erosion and heavily tilled agriculture. Dissolved organic carbon in water, to which eroded peatlands can contribute, must be removed by water treatment plants for most uses.

34.Different methods of land use affect peat differently. In the UK, most emissions come from lowland peats used for cropland. They account for just 7% of peatlands, but they are responsible for 32% of emissions from all peatlands.57

Figure 6: Peatland area breakdown by peatland category58

Figure 7: Greenhouse gas emissions breakdown by peatland category

Source: Office for National Statistics, UK natural capital: peatlands (22 July 2019): https://www.ons.gov.uk/economy/environmentalaccounts/bulletins/uknaturalcapitalforpeatlands/naturalcapitalaccounts [accessed 12 January 2022] *Note that near-natural peatlands do not emit greenhouse gases, so do not appear on this chart. While they account for a substantial fraction of peatland area they do not account for any of the emissions associated with peatlands.

35.Restoring peatlands is a nature-based solution to climate change. Once peatland is rewetted and restored, CO2 emissions are significantly reduced, due to the return to anaerobic conditions for the peat. Eventually, active peat formation and sequestration of atmospheric carbon can recommence. But the real carbon benefits in the short term are in protecting the stock that is there and preventing emissions. We heard that the sequestration rate for peatlands is relatively uncertain, but “there is really large vacant storage capacity in that landscape. Peatlands are intrinsically favourable to carbon accumulation given the right conditions.”59 Restoration also improves biodiversity, and it can reduce water purification costs by improving water quality. Peatlands can contribute to flood prevention, as they absorb water and slow its flow across land.60 In contrast, when peatland has been drained for agriculture “the idea is to remove the water as quickly as possible, and it all collects downstream in the towns.”61

36.Due to the potential of peatlands to keep carbon in the ground, sequester atmospheric carbon, and provide co-benefits, the Government has ambitious restoration targets. The Government aims to restore 35,000ha by 202562 and 280,000ha of English peatland by 2050.63 Since 1990, 110,000ha of peatland has been restored in the UK.64 The Office for National Statistics estimated that the cost of restoring all UK peatlands to near-natural condition would be £8.4–21.3 billion, but that this would deliver carbon benefits of £109 billion.65 The World Wildlife Fund and Royal Society for the Protection of Birds suggested that 23–34MtCO2e of emissions would be prevented by peatland restoration to 2030 and 63–122MtCO2e by 2050.66

37.The big challenge presented by this ambitious policy is balancing competing demands for land use. As set out above, lowland peats cover a small area and are responsible for a large proportion of the UK’s emissions from all peatlands. They are an obvious target for restoration. But they are also among the most productive agricultural lands in the UK. The East Anglian Fens hold 50% of the Grade 1 agricultural land in England and they support a food supply chain worth over £3 billion.67 Changes to these sites would require addressing the trade-off between the UK’s agricultural needs, economic concerns and the carbon benefits of restoration.

38.The main uncertainties in relation to the condition of peatlands include where the peat is, how extensive it is and its depth. Richard Lindsay, Head of Environmental and Conservation Research, Sustainability Research Institute, explained: “we do not know what the peatland floor is like in the UK … because we have not developed the technology to be able to show just how much peat there is in any given area … we cannot see the depth. We do not actually know the extent either.”68

39.Dr Artz added that it is unknown “what condition our [the UK’s] peatlands are in”.69 Peatland condition is estimated by sorting peatland into categories, based on satellite proxy data. Emissions are then calculated by multiplying these areas by emissions factors. Uncertainties arise from the process by which peatland is sorted into categories. These categories are based on a relatively small set of direct measurements at specific sites. Emissions factors are also uncertain. The most-recent estimates of emissions from UK peatlands, using revised emission factors for peatlands, discovered they were so degraded, that they are a net source of carbon, rather than a sink, as had previously been believed.70 These are not small uncertainties.

40.Another uncertainty surrounds the effect of methane emissions. When a bog is rewetted, microorganisms return, which emit methane as a by-product when they break down organic material. If this were to result in methane emissions, depending on the relative effects of the methane and CO2 emissions, it could change the net climate impact of rewetting. This is because methane is also a greenhouse gas; indeed, it is a more potent greenhouse gas than CO2 before it breaks down into water and CO2. Professor Chris Evans, Biogeochemist, UK Centre for Ecology and Hydrology, explained “you start to get methane emissions once your water table gets really close to or above the surface. You can over-rewet a peatland … to the point that you will start to see methane, in climate terms, outweighing the carbon benefits.”71 However, he explained that this occurred when a restoration was poorly managed. For a well-managed restoration, he did not consider methane emissions to be a “deal breaker.”72

41.Restoring degraded peatlands will reduce their emissions. Dr Artz told us that “over the long term, peatlands, if they are in their natural or intact state, are net carbon sinks. We are not yet sure whether restored sites will be able to do the same over the long-term period, but there are ongoing research projects.”73 Professor Chris Evans speculated that there “could be quite a large carbon gain there,” but cautioned that this needs to be monitored.74 We heard suggestions for programmes of long-term, on-the-ground, peatland monitoring using a network of sites to determine the effectiveness of restoration techniques.75 Earth Observation techniques, including satellite observation, can be helpful in determining peatland condition. We heard in written evidence that Interferometric Satellite Radar (InSAR) can measure the surface motion of peatlands, which provides information about the water depth of peatland.76

42.Managing the drainage depth of peatlands used for agriculture could also reduce emissions. Professor Evans told us about research that suggested that “if you halved the average drainage depth in agricultural peatlands … you could reduce emissions by about 3.5 megatonnes of CO2 equivalent per year … That is in itself about 1% of UK greenhouse gas emissions.”77 These are opportunities that should be explored.

43.Nature-based solutions must be resilient to a changing climate, and we heard this has not been established for peatlands. Climate change will lead to more rain. Dr Artz explained that it is possible “there are issues with increased methane production during those periods of temporary inundation, but that is probably a relatively minor factor if the water table can be engineered in restoration projects … to be far enough away from the surface to avoid significant methane emissions.”78 Climate change will result in more summer droughts and there “is some emerging evidence … that certainly younger restoration projects are not yet able to modulate their water table in the same way that a fully self-regulating natural peatland ecosystem is able to … One potential danger is that these restored sites may be less resilient to future droughts in particular.”79 The Committee on Climate Change found in its recent advice report to the Government on UK Climate Risk that the effects of higher temperatures and droughts on peatland could be severe. It found that emissions from peatlands could increase substantially in a world that is 4 degrees warmer without further restoration.80

Agricultural lands and grasslands

44.Of the UK’s land, 72% (17.3 million hectares) is managed for agricultural purposes. Of this, 31% (6 million hectares) is suitable for crops, 10 million hectares (60%) is grazing land, mostly grasslands, and 1 million hectares (6%) is woodland.81 Agriculture accounted for around 10% of the UK’s emissions in 2019 and reducing these emissions will be critical for reaching net zero by 2050.82

45.What is the definition of nature-based solutions in agriculture? Professor Chris Collins, Professor of Environmental Chemistry, University of Reading, provided his preferred definition: “solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social and economic benefits and help build resilience.”83 Dr Lynn Dicks, University Lecturer in Animal Ecology, University of Cambridge, refined this to include only “things that would naturally occur there and native species only.”84 This means that practices such as minimum tillage, or improved crop rotations to reduce reliance on chemical fertilisers, which may be beneficial, are not considered to be nature-based solutions.

46.Some agricultural land will have to be taken out of production to meet Government targets for afforestation and peatland restoration: this is “land sparing”. Other agricultural nature-based solutions are “land sharing”: approaches where the land remains productive from a farming perspective but is managed differently or incorporates new features. There is evidence that land sparing approaches are better for biodiversity and carbon sequestration than land sharing approaches, but they involve more substantial land use change and a trade-off in reduced food production.85

47.There are several proposed agricultural nature-based solutions in the UK. Innovative farming practices could reduce emissions from agricultural peat. Paludiculture refers to farming on wetlands. By partially rewetting peatland, CO2 emissions would be reduced (or cease, in the best case) but certain crops could still be grown. Professor Chris Evans cautioned against expecting too much from this approach: “Paludiculture/wetland agriculture … does not really produce food at the moment and it is not as profitable, so it is a real challenge. It is an ongoing research need … if we can find a solution I think everyone would be happy, but we are not there yet.”86

48.Hedgerows can be planted or allowed to expand, or trees can be included in them. Hedgerows sequester some carbon in the vegetation and soil through plant growth, they provide a habitat for wildlife, and they provide a corridor for wildlife to move along and genetically mix, alongside other co-benefits.87 Features such as ponds and meadows can be expanded within agricultural land to enhance biodiversity.88 Agroforestry is an umbrella term referring to the practice of mixing trees with a productive crop or grazing land (silvopasture). The trees sequester more carbon than the crops or grazing grasses, they reduce soil erosion, and they diversify the ecosystem. These are land-sharing approaches.

49.Untilled, species-rich grasslands sequester carbon. They provide a habitat for pollinators, they improve the health of grazing animals and they reduce flood risk by absorbing water.89 Written evidence from Plantlife says that over the last century, 97% of the UK’s species rich grasslands have been destroyed by agricultural processes.90

50.There is no formal target for emissions reductions from the agricultural sector. The agricultural sector is combined with the forestry and other land-use sectors in the Net Zero Strategy. The strategy includes an indicative pathway that net emissions from agriculture, forestry and other land use should fall by 27–43% by 2035 and 70–80% by 2050.91 The Government will be introducing environmental land management schemes to replace existing subsidies. The Government anticipates that Environmental Land Management schemes and other farming offers will reduce greenhouse gas emissions from agriculture by “up to a total of 6MtCO2e in Carbon Budget 6 (2033–7) in England”, but has not set out how contributions from different schemes will add up.92 A range of environmentally friendly practices will be encouraged by the Environmental Land management schemes. We cover these in more details below.

51.We have outlined the uncertainties in relation to peat, many of which apply to agriculture on peatland. There are uncertainties that are specific to agriculture and grasslands. Dr Dicks told us “there is good evidence that carbon storage is higher with agroforestry, which is mostly because of the carbon stored in the trees.”93 Studies found that silvopasture does not sequester as much carbon as replacing the land with a forest, but the land is still productive. Dr Dicks noted that the studies have focused on silvopasture, rather than trees mixed with crops. Overall, she considered the evidence “established but incomplete because there are not very many studies.”94

52.We heard that there is a small carbon sequestration benefit to hedgerows and field margins. Field margins also sequester a small amount of carbon in the soil. Dr Dicks said there was “well-established evidence that you get good climate carbon sequestration from hedgerows and field margins.”95 Combining the three practices, the National Farmers’ Union estimated that carbon storage in soils, hedges and trees could increase by 9MtCO2e a year.96

53.Dr Lisa Norton, Agroecology researcher, UK Centre for Ecology and Hydrology, told us that “converting arable to grassland is a good way of storing carbon. It can very quickly build up carbon in soils under grassland; even temporary leys97 in the arable land have been shown to help with that.”98 She argued that, in the short term, converting arable land to grassland sequesters carbon more quickly than converting it to forest. But she identified evidence gaps around the permanence of the carbon and how grazing animals affect the rate and permanence of sequestration.99 Dr Mike Morecroft from Natural England agreed that grasslands were “very important” but thought there had been “remarkably few” studies on their sequestration rate, “particularly in semi-natural grasslands.”100 Plantlife called for long-term studies to understand carbon sequestration by grasslands, with a particular focus on carbon in different depths of soil.101

54.A major uncertainty for nature-based solutions and agriculture is the question of whether they will reduce yield. Growing less food in the UK risks increased reliance on imports from overseas, where the agricultural emissions and environmental damage may be greater.102 Part of the Committee on Climate Change’s path to net zero relies on agricultural land (perhaps as much as 22%) being taken out of production so it can sequester carbon. To do this without offshoring emissions, the Committee says people must eat less land-intensive food (i.e. meat and dairy), food waste must be reduced and agricultural productivity must increase.103

55.The evidence is mixed on whether nature-based solutions reduce agricultural productivity. We heard from Dr Dicks that, for agroforestry, one study found an increase in yield, another a decrease. In the vicinity of a hedge that borders a field, there is a slight drop in yield, due to the shade and the increased competition for water and soil nutrients. But, further into the field, there is an increase. This may be because the hedges reduce soil erosion. Overall, fields with increased hedgerows have a higher yield. The evidence for the effect of field margins on yields is “unresolved”; “there is usually an increase in yield, but it does not go very far into the field”.104 There are likely to be co-benefits that may increase yield, since flowery field margins support pollinators and insects that predate on crop pests, but there is insufficient evidence. We heard that uncertainties remain because “there are not enough long-term studies, so if the effect takes four, five or 10 years to build up, you do not see it in a three-year study.”105 Dr Lisa Norton echoed the call for better long-term data, asking Government “to reconsider funding the UK Countryside Survey, which UKCEH [UK Centre for Ecology and Hydrology] used to run alongside Defra [Department for Environment, Food and Rural Affairs] … It has been going since 1978 and provides useful contextual information about how the landscape is changing in relation to current land use and climate change.”106 The Committee welcomes the Government’s renewed commitment to this survey.107

56.Further research and development will be needed to establish the impacts of new agricultural techniques on carbon storage and sequestration. We heard that, in some cases, the testing of novel or recycled fertiliser that could sequester additional CO2 is hampered by regulatory barriers.108

Soil

57.Soil carbon storage is important for many of the habitats described above. It is increasingly recognised as a vital carbon store and as a potential sink. Soil health underpins the sustainability of agriculture, forests and grasslands. The Sustainable Soil Alliance told us that there are around 9.8 billion tonnes of carbon in Britain’s soils. This is about “80 times more carbon … than in the above ground vegetation.”109 It estimated that agricultural soils have lost over half of their carbon from human activity. The Government concludes that “the available evidence indicates that soils sequester approximately 1 MtCO2e/yr, although healthy soils deliver a range of ecosystems services and could unlock further emissions savings across agricultural systems in particular.”110

58.Due to the relationship between the productivity of land and soil health, improving soil condition is a co-benefit of other nature-based solutions. Hedgerows, for example, reduce soil erosion. The mycorrhizal network in woodland soils is increasingly recognised as vital to overall woodland health. The Government has recognised that:

“Encouraging the uptake of sustainable soil management practices for agriculture and other sustainable land uses that improve soil health will in turn underpin a range of environmental, economic and societal benefits, including, food production, biodiversity, carbon storage and flood mitigation.”111

59.There is uncertainty about the potential of soils to sequester carbon. For example, the Royal Society Greenhouse Gas Removal report estimated that 1–31Mt CO2e/yr of greenhouse gas removals could be achieved if the majority of farms adopt soil carbon sequestration practices.112 In particular, there is uncertainty on how soil at different depths is affected by different practices and on the effectiveness of practices intended to increase the carbon content of soil. Professor Henderson also told us that it is difficult to measure carbon in soils because of the variation across small spatial scales as well as with depth.113

60.For example, it is now recognised that tree planting on deep peats is harmful, but it is less clear whether, on shallower peaty soils, the carbon sequestered by the tree would offset carbon released by the disturbance when planting. Dr Mike Morecroft from Natural England told us “soil is often not studied to depth. A lot of the data that we draw on are at a 15-centimetre depth of soil. Soil is much deeper than that in many places.”114

61.We recommend that the Government sets a target for emissions reductions from the agriculture, forestry and other land use sector in line with the Committee on Climate Change’s recommendations and interim targets.

62.We recommend that, as part of the agricultural transition, research and development is conducted on farms to better understand carbon emissions from farms and the practices that can reduce them. The Department for Environment, Food, and Rural Affairs should fund on-farm research projects and it should monitor them to ensure research is conducted to an appropriate standard. This could be funded through tax credits and grants. The Department should investigate and address any regulatory barriers to conducting this research and development.

63.We recommend that the Forestry Commission should keep its policy on tree-planting on peaty soils under review. The policy may need to be strengthened if evidence about the net carbon balance of planting shows that it is negative.

Marine environments

64.Marine, coastal and ocean (hereafter “marine”) ecosystems are an important store for carbon. Carbon stored in marine and coastal ecosystems is sometimes referred to as blue carbon.115 The location, extent and potential importance of marine environments in climate change mitigation are less well understood than land habitats. This means that the possible scale of marine nature-based solutions is also less well-understood. Well-known marine habitats include coral reefs and mangroves. The marine habitats in the UK are less known, but they support a unique biodiversity and they may contain significant stocks of carbon. Our report focuses on habitats in UK waters, but many of the recommendations apply to the important blue carbon habitats in the British Overseas Territories.

65.Shelf sediments are the layers of sediment on the ocean floor. Although sediments are likely to sequester CO2 very slowly, they cover such a large area that, overall, they will sequester a significant amount of carbon.116 More importantly, they “contain huge stores of carbon”.117 Estimates are uncertain, but there may be as much as 1900MtCO2e stored in the top 10cm of the sediments in the UK’s exclusive economic zone,118 with sequestration rates between 0.4–1MtCO2e/yr.119 Disturbance of marine sediments, through activities like bottom trawling, results in carbon emissions. It is not yet certain how much CO2 is released by these processes, but there are growing calls for the carbon stocks in marine sediments to be better protected.120 Thirty eight per cent of the UK’s seas are in Marine Protected Areas, but in only 5% of these bottom trawling is banned.121

66.Kelp is a marine alga. It is a seaweed that forms underwater forests. Carbon is sequestered when dead kelp ends up in ocean sediments. The extent of the habitat is uncertain: estimates range from around 40,000ha to 80,000ha. Restoration projects are in their early stages in the UK, so their effect is unclear.122 The Department for Environment, Food and Rural Affairs estimated a sequestration rate of 0.27 Mt CO2e/yr across all kelp in the UK, but this is uncertain.123 As co-benefits, kelp forests improve water quality, they provide habitats (specifically nurseries) for marine life and they may reduce coastal erosion. Seaweed aquaculture could be used in producing foods, medicines, bioplastics or biofuels.

67.Saltmarshes are coastal ecosystems that are flooded at high tide but are exposed at low tide, and they are a unique ecosystem. Saltmarshes sequester carbon as the organisms they support are buried in the sediment. There are about 44,100 hectares of saltmarshes in the UK; which represent around 30% of the saltmarshes in Europe.124 Since 1870, 85% of the UK’s saltmarshes may have been destroyed.125 Saltmarshes have an estimated carbon stock of 48MtCO2e in the UK, with a sequestration rate of 0.24MtCO2e/yr.126 They can also improve water quality, improve flood defences and support biodiversity. The Steart Marshes provide an example of a saltmarsh restoration project.127

68.Seagrasses are underwater plants that form meadows on shallow sediments. Their UK range is estimated at 7,000–9,000ha, which is one of the largest seagrass stocks in Europe.128 Seagrasses have declined by as much as 92%, due to disease and the pollution of coastal waters.129 There may be up to 410 tonnes of CO2e/ha in these seagrasses, depending on the species, the water quality and the sediment type, with a total stock, including the sediment, of 3.3MtCO2e.130 The annual sequestration rate is uncertain, but it is estimated at 0.02MtCO2e/yr.131 Seagrasses provide co-benefits of improved water quality, a habitat for economically valuable fish and increased biodiversity.

Table 1: Estimates for sequestration and storage of CO2e across different marine habitats, UK-wide

Marine habitat	Storage (t CO2e/ha)*	Sequestration rate (t CO2e/ha/yr)*	Approximate Extent (ha)
Sediments132	25	0.005–0.013	75,000,000
Kelp	N/A	4.5	60,000
Saltmarsh	1090	5.44	44,100
Seagrass**	410	0.000003	8,000

Sources: Parliamentary Office for Science and Technology, Blue Carbon, PostNote 651, September 2021; Written evidence from Department for Environment, Food and Rural Affairs (NSD0042); Natural England, Natural England Research Report (NERR094) Carbon storage and sequestration by habitat: a review of the evidence, second edition (20 April 2021), Section 6.6, pp 162–164: http://publications.naturalengland.org.uk/file/6726246198411264 [accessed 12 January 2022]

*Storage and sequestration rates have been divided by the approximate extents to determine storage and sequestration rates per hectare. **Seagrass carbon storage estimates include the sediment beneath the grass.

69.The evidence base for marine environments is sparse. The extent of many of these habitats is unknown, as is the rate at which they sequester carbon. Professor Hilary Kennedy, Emeritus Professor, School of Ocean Sciences, Bangor University, referred to being “an order of magnitude out on knowing [the] total area” of seagrasses.133 Estimates for other habitats also have large uncertainty ranges. Without knowing their extent, it is impossible to know what the stock of carbon is in each environment. As the Government told us, “we do not yet have sufficient data to accurately quantify the baseline and potential contribution of coastal habitats to emissions reductions in the UK”.134

70.There are not enough studies globally, and especially studies in the UK, to know the sequestration rates of UK coastal habitats. Dr Mike Morecroft of Natural England told us that the numbers for the sequestration rate of seagrass came from “one study on the other side of the Atlantic”.135 Professor Ian Bateman identified the interaction between terrestrial land use and seagrasses as another uncertainty.136 There are numbers around the sequestration and carbon stock of kelp and marine sediments, but Professor Kennedy did not consider them reliable.137

71.There are no specific targets for carbon emissions from marine ecosystems. The uncertainties are such that “the coastal environment does not currently contribute to carbon accounting and reporting due to a lack of appropriate data.”138 There is no blue carbon inventory setting out the carbon stocks of coastal and marine habitats in the UK, as there is for ecosystems on land.139 The Scottish Government has established a research programme to create an inventory for its blue carbon.140 Work is under way to include saltmarshes in forthcoming assessments, with other marine ecosystems to be included at an earlier stage. The Government is “actively exploring” the potential of marine environments to contribute to nature-based solutions and it is committed to strengthening the evidence base. The paucity of evidence means there are not specific targets for restoration.

72.There are large gaps in the evidence pertaining to carbon sequestration and storage in marine habitats. Saltmarshes and seagrasses are better understood, but uncertainties remain. The understanding of other habitats for nature-based solutions, such as, kelp forests, shelf sediments and algae, is less mature.

73.We recommend that the Department for Environment, Food and Rural Affairs supports research that focuses on establishing the current and historical extent of marine habitats, their carbon sequestration rates, and their long-term potential for carbon storage.

74.We recommend that a blue carbon mapping exercise for the UK exclusive economic zone be undertaken, learning from the Scottish Blue Carbon Forum. This should involve collaboration between Natural England, the Crown Estate, the Marine Management Organisation, academics, and other relevant public bodies.

75.We recommend that the Marine Management Organisation establishes research programmes to investigate the cause of the decline of marine habitats, such as seagrasses, and the potential effects of eliminating bottom trawling on carbon sequestration in the marine environment.

Urban

76.There are nature-based solutions that could be deployed in urban spaces. In cities and towns trees could be planted, urban wildlife reserves could be created and gardens might be allowed to grow “wilder”. Due to the limited available space, urban nature-based solutions are likely to be justified more on their co-benefits than on their potential carbon sequestration. Nevertheless, urban nature can store a large amount of carbon, with stock estimates of around 610MtCO2e in UK cities.141

77.Urban trees could improve adaptation to climate change, because the cooling effect of shade and the transpiration from plants will become more important as the climate warms. Green roofs, where plants, rather than building materials, are exposed to the sun, could help to reduce the urban heat island effect and they could mitigate extremes of temperature in cities.142 This would reduce reliance on air conditioning, which has high environmental costs.143 There is also a potential for nature to mitigate flood risk in cities and to reduce the impact of exposure to pollution. Bright Blue pointed to the biodiversity benefits of connecting isolated pockets of nature via wildlife corridors.144

78.The Government has supported urban tree-planting and has committed to funding community forests.145 The Government recognises that these forests can support “urban and peri-urban regeneration in some of the most deprived areas, delivering multiple social, economic and environmental outcomes.”146 The Government referred to the role that nature-based solutions could play in “natural flood management or urban cooling”.147

Need for further research

79.It is hoped that nature-based solutions will help to mitigate climate change. For nature-based solutions to fulfil their promise, they need to be based on robust scientific evidence. There are a number of gaps that need to be addressed—some are specific to habitats and others are more general. But the restoration of nature has benefits that are certain. It is important that a lack of evidence does not lead to a lack of action. Lord Goldsmith of Richmond Park, Minister for the Environment and the Pacific, told us that scientific uncertainty should not be used as “an excuse not to do things that we know are good”.148

80.While it is important that gaps in the evidence base are filled, the gaps should not act as a barrier to the large-scale adoption of nature-based solutions. The exact impact of nature-based solutions will be known only after they have been tried and monitored in the long-term, but evidence already indicates a positive impact. Given the urgency of the climate and biodiversity crises, there is no time to waste. The fact that it is not possible to quantify exactly the carbon loss due to marine shelf sediment disturbance, or to other activities, should not prevent the protection of these habitats.

81.We recommend that, where there are gaps in the evidence, policy should adopt a precautionary approach, weighted in favour of nature.

82.There is uncertainty about the long-term sequestration potential of nature-based solutions across habitats. Nature-based solutions that are not resilient to adverse weather, human activity, a changing climate, or pests and disease risk being ineffective and releasing any carbon they sequester. Monitoring will allow lessons to be learned from schemes that succeed, and from those that fail.

83.Monitoring technologies such as Earth Observation are potentially important. However, they cannot substitute for direct measurements on the ground. Uncertainties have direct implications for policy. They are greater for ecosystems that are less well-understood than woodlands and peatlands. Emissions factors are useful for estimating the contribution of habitats to greenhouse gas emissions across the UK. But nature-based solutions are inherently local and must be understood on a local level.

84.We recommend that long-term research and monitoring be supported and overseen by the relevant departments and their public bodies, including Natural England and UK Research and Innovation, to ensure schemes are resilient and deliver as promised. The research and monitoring programmes should support direct and indirect measurements of greenhouse gas fluxes on a range of representative sites for key habitats in the UK to address uncertainties concerning the timescale and duration of carbon storage and sequestration for all habitats.

Other conclusions and recommendations

85.The Government’s focus has been mostly on large-scale land sparing approaches, such as large-scale tree planting and peatland restoration, for which the evidence base is strongest, rather than land sharing approaches and improved management of ecosystems. Land sparing is likely to sequester more carbon than land sharing, but it may entail more trade-offs.

86.We recommend that research programmes be established to fill uncertainty gaps in the impact of land sharing techniques, including hedgerow planting, silvopasture and agroforestry and the effect of these practices on soil carbon storage and sequestration.

87.Restoring nature is often more complex and costly than protecting it. Restored ecosystems may take a long time to recover biodiversity and carbon stores, if they ever do. Policy should not assume that it is possible to ‘recreate’ in another place the natural systems that are destroyed.

88.We recommend that the Government makes it a priority to protect the natural ecosystems that remain wherever this is possible to ensure the significant stores of carbon in these habitats are not emitted.

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