Collectively, these mitigation requirements pose significant challenges to many existing land-use practices, especially given the increasing demand for the goods and services that those practices currently provide. Beyond much greater contributions to mitigating climate change, land uses also need to support climate change adaptation responses and efforts to reduce risks from natural disasters. All approaches need to consider not just climate impacts per se, but how actions taken can best support ecologies and provide biodiversity benefits.
3.3.1 Turning land-use emissions net negative: carbon dioxide removal
Meeting internationally agreed climate targets will not only require rapid reductions in greenhouse gas emissions – including, notably, from AFOLU – and the preservation of existing carbon stores and sinks. It will also very likely require significant additional removals of carbon dioxide from the atmosphere: most climate models suggest global emissions must stabilize and start declining by around 2030, and turn net negative by 2070, to meet the 2015 Paris Agreement’s target of keeping global temperature rises to well below 2°C relative to pre-industrial levels. As the majority of the global economy will only be able to achieve carbon neutrality at best, and as some residual sectors will find it impossible to reduce emissions to zero, meeting these objectives will require significant areas of land for sequestration and CDR. Notwithstanding the possibility that this requirement will be tempered to a small extent if novel land-sparing CDR approaches are rapidly scaled up, it is clear that land use as a whole will need to achieve net negative emissions rather than ‘just’ net zero emissions.
The feasibility and scale of CDR requirements will be determined to a large extent by the residual levels of fossil fuel, industry and agricultural emissions that need to be offset. Mitigation throughout the economy will require the reduction and reshaping of demand for goods and services, efficiency improvements in many areas of daily life, and the electrification and decarbonization of supply-side processes. If mitigation is delayed or insubstantial, then significantly more CDR will be needed. If decarbonization is rapid and expansive, then it will be possible for CDR to play a lesser role. Nonetheless, the vast majority of 1.5°C- and 2°C-compatible emissions pathways in climate scientists’ integrated assessment models (IAMs) assume very significant deployment of negative emissions technologies (NETs) by the end of the century.
Among the principal NETs included in IAMs is bioenergy with carbon capture and storage (BECCS). This involves burning carbon dioxide-absorbing biofuels, capturing the emissions and storing them in long-term underground reservoirs. Even under the more conservative 2°C scenarios previously elaborated in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), 90 per cent assumed a role for BECCS. And half of all 2°C scenarios relied on BECCS – alongside afforestation and reforestation – to remove at least one-third of all cumulative carbon emissions between now and 2100 (a volume equivalent to over three-quarters of the remaining 2°C carbon budget). Achieving this would require significant proactive use of CDR by around 2030.
BECCS presents considerable difficulties for balancing global land use. The area required for growing additional energy crops implies reduced availability of land for food production, or for preservation as natural habitats.
Under the more recent and ambitious 1.5°C scenarios, reliance on CDR is even more acute and the need for its introduction more urgent. As the IPCC’s sixth assessment cycle special report on climate change and land, published in 2019, concluded: ‘All assessed modelled pathways that limit warming to 1.5°C or well below 2°C require land-based mitigation and land-use change, with most including different combinations of reforestation, afforestation, reduced deforestation and bioenergy (high confidence).’
As discussed in Chapter 5, BECCS presents considerable difficulties for balancing global land use. The area required for growing additional energy crops implies reduced availability of land for food production, or for preservation as natural habitats. Depending on the energy crop used and the efficiency of production, the extent of BECCS deployment suggested by many 2°C scenario models may require the equivalent of anywhere from half to five times the current land area used to grow the world’s entire current cereal harvest (720 million ha).
Despite the heavy reliance of IAMs on BECCS, along with afforestation and reforestation, for their modelled greenhouse gas removals, many other CDR approaches – some nature-based, some technological – have the potential to contribute to stabilizing the climate. These options vary considerably in their feasibility, degree of readiness, co-benefits, trade-offs and impacts on land use. Many technological solutions present comparable resource use challenges – for instance, requiring large amounts of energy and water. Nature-based CDR options such as afforestation and reforestation could be similarly expansive in terms of land area needed, and risk being easily reversed at some future date. None of the options offer a panacea or are sufficient on their own, and it is likely that many of them will need to be deployed in some degree.
3.3.1.1 Nature-based sequestration solutions
Afforestation and reforestation (AR) – which involve planting new trees and restoring felled or degraded forests – can increase carbon stocks either through rewilding or as part of sustainable forestry operations. For example, marginal lands could be afforested to provide construction timber as a substitute for concrete, creating an additional pool of carbon that would reside in the built environment while forest regrowth sequestered additional carbon. Alternatively, such lands could be reverted to closed-canopy forests to provide long-term carbon sequestration and storage, climate regulation, and other ecosystem services and (potentially) biodiversity benefits. These co-benefits depend on the type of afforestation and reforestation chosen: restoring landscapes to maximize biodiversity and ecological resilience is preferable to developing large-scale homogeneous plantations that may have carbon and timber benefits but less ecological value. The sequestration potential (which comes not only from the trees themselves but also from improving soil quality) is also greater if the lands are restored to natural forest rather than repurposed for mixed uses such as agroforestry, plantations or rotational logging.
As well as providing near-term sequestration, some forms of afforestation will retain the option of providing BECCS feedstocks in the second half of this century, if these are still required. Afforestation and reforestation offer relatively cheap means of delivering negative emissions, with negligible energy requirements. But, depending on how and where they are implemented, they can compete for land and water with food (or biofuel) production, while albedo effects also limit the latitudes at which this strategy is effective: forests are not very reflective of sunlight, and so – especially at temperate and boreal latitudes – often absorb more radiation than alternative land covers do, thereby warming the Earth’s surface. Afforestation and reforestation could also have a similar water intensity to that of BECCS. However, unlike with BECCS, the potential for carbon dioxide storage is limited by the fact that trees become saturated with carbon over time if not harvested and replanted. Areas used for afforestation and reforestation are also vulnerable to wildfires and deforestation, with the consequent risk that they could go from being net negative carbon sinks to net positive sources of carbon.
Soil carbon sequestration (SCS) involves increasing soil carbon content through actions such as agroecology, agroforestry, conservation agriculture and landscape management. It has co-benefits for agricultural resilience and productivity, food security, biodiversity, water cycling, and climate change mitigation and adaptation. Increasing attention is being paid to SCS as a result of the international ‘4 per 1000’ initiative, launched by France when it hosted the 2015 UN Climate Change Conference in Paris (COP21). The initiative aims to increase agricultural soil carbon content at an aspirational rate of 0.4 per cent (2–3 gigatonnes of carbon – GtC) per year. However, the technical and economic feasibility of increasing soil carbon content at the scale envisioned has been called into question. Some argue that the required increase in nitrogen uptake by plants is unrealistic. Others point to a variety of constraints on universal adoption of best management practices; bottom-up estimates of the maximum biophysical potential on cropping and grazing land suggest that around 10–30 per cent (8–28 GtC) of the remaining global theoretical SOC sink potential could be filled. Nonetheless, in particular locations, especially where existing soil carbon content is low, best management practices could achieve an annual increase in SCS of up to 1 per cent for 20 years.
As with afforestation and reforestation, annual increases in SOC will decline as carbon saturates the storage medium (i.e. soils, in this case). Moreover, just as afforestation and reforestation require complementary efforts to halt deforestation, so the potential adoption of SCS will occur in a context in which most agricultural soils are losing rather gaining carbon. If soil carbon losses, such as through peat drainage, are not stemmed, they are likely to negate and outweigh the benefits of increasing soil carbon content elsewhere. Nonetheless, even if the potential of SCS is not as extensive as sometimes suggested, reducing soil carbon losses and increasing SCS offer a relatively low-cost, ‘no regrets’ means of mitigation with significant co-benefits for soil quality and food security.
Habitat restoration is a closely related solution that aims to restore carbon-dense habitats such as peatlands, and coastal and marine habitats such as salt marshes, mangroves and seagrass beds (‘blue carbon habitats’), to increase their absorption of atmospheric carbon dioxide. A recent estimate suggests that habitat restoration approaches, including in woodlands, could compensate for up to a third of the UK’s carbon emissions. Because the focus is on the habitat as a whole, such approaches typically support greater biodiversity alongside increased carbon uptake. This in itself can be crucial to maximizing carbon capture – emerging evidence on coastal habitats, for example, suggests a full trophic system with intact predator populations is required to maximize carbon-cycling potential.
Biochar, a charcoal formed from the thermal decomposition of biomass in the absence of oxygen, can be buried in soils to improve soil fertility and increase the carbon saturation limits of soils, as additional carbon is stored in the biochar.
3.3.1.2 Technological solutions
Direct air carbon capture and storage (DACCS) involves capturing carbon dioxide from the atmosphere using a chemical agent and storing the carbon dioxide in underground reservoirs. DACCS is significantly more attractive than BECCS from a land-use perspective, but it is very energy-intensive (so does not feature prominently in cost-optimizing IAMs). That equation may improve for DACCS if carbon-neutral renewable energy becomes abundant, as marginal electricity costs decline, and as DACCS is able to exploit its potential to use surplus electricity generated on a daily basis. Because direct air capture can occur anywhere, there are options to co-locate facilities with cost-effective renewable energy generation and carbon dioxide storage infrastructure (such as saline aquifers). Currently, direct air capture and use technologies can produce synfuels that have the potential to make significant contributions to decarbonizing aviation and maritime transport. Moreover, land-use needs (required primarily for photovoltaic arrays) in such cases would be minimal, in contrast to those for producing first- and second-generation biofuels. But to get to net negative emissions, DACCS installations will require economies of scale that are only likely to materialize with a carbon price upwards of $100 per tonne of carbon dioxide (tCO₂).
Enhanced weathering (EW) takes advantage of the carbon-fixing that naturally occurs in silicate rocks over geological timescales. By pulverizing rocks to massively increase their exposed surface area and then spreading them on agricultural soils, EW enables a vast acceleration in the chemical reactions with air and water that convert carbon dioxide into stable carbonates. Deploying EW on existing croplands, especially when using industrial silicate waste, offers opportunities to improve food and soil security and better align agriculture and climate policy. However, as with other solutions, it also requires appropriate regulatory and incentive frameworks. Scaling deployment could be particularly challenging for EW since it depends on widespread application by many smallholders and requires public acceptance of a balanced trade-off between local mining activities and global carbon sequestration. (See Figure 20 for further details on the costs and benefits of EW.)
