Negative Emissions Technologies (NETS): Feasibility Study - Technical Appendices

Appendix 9. Non-engineered nets

Greenhouse Gas Removals (GGRs) can be divided into two categories, land-based and engineered removals. The focus of this report is engineered removal technologies; however, an overview of several land-based GGRs is provided in the section below, including reforestation, soil carbon sequestration, peatland restoration and wood in construction.

Although land-based GGRs have the potential to make a major contribution to meeting carbon reduction targets, such as the 2050 Net Zero target, they alone are not sufficient. This is due to competing land, as well as societal resistance to land-use changes on the scale that would be required. Whilst impacts vary greatly between each land-based GGR option, they collectively may negatively affect biodiversity, alter water/land use, or result in reversibility of carbon store, e.g., due to improper long-term management.

Forests and forestry management

Afforestation, reforestation, and forest management are various land-based GGRs that consider carbon removals through woodland expansion and forest management. They are based on the principle that by increasing forest area, the amount of CO₂ absorbed from the atmosphere increases. The maximum technical potential of this GGR is 26.5 MtCO₂/year by 2050, which is the highest of the land-based GGRs, however, still notably lower than engineered GGRs^[101].

The technology readiness level (TRL) of afforestation is 9 as it is robust, well evidenced, and already widely practiced throughout the world^[101].

Deployment of afforestation and reforestation has several limiting factors; these include land availability, the supply of tree seed and saplings, and capacity to plant large areas. Further, there are sustainability issues associated with this GGR, such as risk of biodiversity loss, greater water demand, and land competition with food production. Afforestation is already commercially deployed in the UK, and there is potential to grow existing capacity with afforestation targets. However, to meet these targets, early deployment is required, along with appropriate selection of tree species, planting age, and yield class, as these factors directly affect carbon sequestered.

It should be noted that carbon can move from this GGR to others, due to biomass supply for biochar, BECCS and wood in construction. Additionally, GGR afforestation competes with biochar and bioenergy feedstock (for BECCS) for land.

Peatland/ peatland restoration

There has been notable damage to UK peatlands, through forestry, drainage, agriculture, and burning, which has caused significant emissions of greenhouse gases due to the depletion of the carbon store. Peatland habitat restoration as a GGR method involves the re-establishment of functional, and hence carbon-accumulating, peatland ecosystems in areas that have been degraded to the extent they no longer sequester CO₂. The maximum technical potential of this GGR in 2050 is 4.7MtCO₂/year; this figure is based on restoration of 750 kha of the most degraded peatlands in the UK^[101]. This value is one of the lowest of all land-based GGR methods, and noticeably lower than engineering GGRs. However, based on modelling, very high rate of GGR per unit year are attainable in the period following effective peat restoration.

The technology readiness of peat restoration has been assessed to be TRL 9 as it is well established and widely implemented across the UK^[101].

According to the Sixth Carbon Budget Balanced Pathway (under the Climate Change Committee), 39 MtCO₂/year of nature-based sinks will be required in the UK by 2050, meaning that the UK will need to plant 300,000 hectares of mixed woodland by 2030, accelerating to 850,000 hectares by 2050^[252]. The Scottish Government has committed to funding the restoration of 250,000 hectares of peat by 2030 with funding of £100 million to Scottish Forestry as well as £30 million to Forestry and Land Scotland to expand Scotland's national forests by 18,000 hectares per year until 2024^[252].

Peatland restoration brings additional benefits such as biodiversity and improved habitat restoration, however, there are uncertainties associated with the GGR method. Flood regulation, water supply, and drinking water quality may be affected by various degrees depending on area, causing significant impact. Furthermore, as peat restoration has not yet been demonstrated at the large field scale, there is a risk that the restoration is not successful, meaning that CO₂ uptake and methane suppression is not as predicted.

Soil carbon sequestration

Soil carbon sequestration is a GGR method that considers how the carbon content of soil can be increased through land-use or land-management change. It is more relevant to agricultural land use, and hence has greater impact on cropland and grassland. The maximum technical potential of this GGR is 15.7 MtCO₂/year by 2050, which again is considerably lower than engineered GGRs^[101].

The technology readiness of soil carbon sequestration as a GGR was assessed to be TRL 8^[101]. There are several reasons as to why the TRL is not higher, and this is mainly due to a lack of consensus on the magnitude and effectiveness of land use and management change. Furthermore, this GGR encompasses a complex range of potential management practices that are dependent on socio-economic and environmental context.

The deployment of soil carbon sequestration takes place predominantly on agricultural land used for food production, as well as temporary and permanent grassland. It involves either applying compost/crop residues to fields, reducing soil disturbance by switching to low-till or no-till practice, changing planting schedules, or managing grazing of livestock. Due to the existing farming infrastructure and technology that already exists, uptake of soil carbon sequestration can be immediate and widely deployed in the 2020s.

However, there are uncertainties and limitations to deployment of this GGR, such as the reversibility risk. After approximately 20 years soil becomes saturated, and once saturated, it is assumed that land will require indefinite maintenance to avoid CO₂ being re-emitted. Another key challenge of soil carbon sequestration is the Measurement, Reporting, and Verification (MRV) aspect, as cost-effective measurement of changes in soil carbon is difficult at the field and farm scale. Lastly, there is limited evidence of efficacy in the UK context.

Wood in construction

Wood in construction as a GGR method is defined as the increased use of domestically produced wood in buildings to permanently store carbon. This has the potential to increase the amount of biogenic carbon stored in harvested wood products (HWP). Due to several limitations, the maximum technical potential in the UK of this land-based GGR is 3.3 MtCO₂/year by 2050, which is significantly less than any other engineered GGR^[101].

The technology readiness of wood in construction has been assessed at TRL 9, despite the variation seen between different products and applications, as there are significant commercially available and mature options within this GGR^[101].

To achieve an increase in the HWP carbon pool whilst only utilising the existing UK capacity for HWP production, the lifetime of HWP must be increased through diverting wood use to long-life applications. Long-life products are those which are still in the pool after 20 years, and ideally, 70 years, and applications include uses in construction, such as timber carcassing.

The UK harvests approximately 4 million oven dried tonnes (M odt) per year of softwood, 90% of which gets used in shorter life applications, and the remaining 10% for construction. The use of timber-frames in construction has increased over the years, with 50,000 homes being built per year with timber in the UK and figures suggest that the total demand for HWP in construction could reach 4.7 M odt/year in 2050^[101].

The uncertainties associated with this GGR are limited, however, mostly surround its feasibility. There are uncertainties in the UK’s ability to source enough domestic timber of appropriate quality, as well as consumer preference. Additionally, utilising more wood in construction requires adjustments to building requirements, safety, and quality assurance to enable sufficient scale.