Land Use & Agriculture
Greenhouse gas emissions from the land use and agriculture sector are made up of nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2). Methane is produced primarily by livestock in their digestive processes, and from manure. N2O is released when nitrogen is added to the soil, either as mineral fertiliser or nitrogen-fixing crops. The bulk of it (60%) comes from fertilised grazed grassland and manure handling and relatively little from arable cropland (15%) (Brown & Jarvis, 2001). Carbon dioxide is produced from tilled or disturbed soils and from the energy used in agriculture.
Methodology: background assumptions
In the zerocarbonbritain2030 scenario we accept 15% of our food needs as imports from the EU, and about 7.5% from the tropics, but apart from this our food needs must be met domestically. This restriction is to enable the creation of the scenario. Apart from this small amount of trade, the landmass of mainland UK has been treated as a separate entity.
Energy used in agriculture is assumed to be decarbonised - this is dealt with in other chapters. The effect is to reduce the greenhouse gas intensities of land use products by approximately 20% for livestock and about 45% for crops.
Preserving carbon reservoirs has been given high priority. Soil stores a lot of carbon, particularly in peat-lands and to a lesser extent in grasslands. This carbon can be released if the land is disturbed or converted to tilled arable land. In order to avoid this there is no new arable land in the scenario, and peat-lands are especially protected. Woodlands are another reservoir, and in the zerocarbonbritain2030 scenario existing woodlands are preserved and carefully managed.
Switching from products with high greenhouse gas and land intensities to those with lower intensities enables us to achieve two goals; we reduce greenhouse gas emissions from agriculture and at the same time release land for other uses. Livestock products, particularly those from sheep and cows, have much greater land and greenhouse gas intensities than plant products.
Using the released grassland to grow biomass for energy allows us to supply the demand for storable solid, liquid and gas fuels in other sectors. For these we use energy silage from forage-type grasses, short rotation woody crops and miscanthus. These are perennial crops and so growing them on grassland need not cause a loss of soil carbon. They are also low nitrogen users and hence the emissions from growing them are very low. In addition, using some of the released grassland to grow biomass for carbon sequestration allows us to sequester enough carbon to cover the residual emissions from all sectors.
Some technical changes in land management also allow us to reduce greenhouse gas emissions and increase carbon sequestration. More organic matter is incorporated into soils than is current normal practice, and nitrogen is handled better to reduce N2O release.
In the zerocarbonbritain2030 scenario abundant food for the population is produced but livestock products are reduced to 20-30% of their present quantity. Cow and sheep stocks in particular are much reduced. The levels of egg, poultry and pig-meat production are only a little lower than today because they use little land and we can feed them on high-yielding crop products and food wastes. Plant protein is greatly increased; at the moment the ratio of meat to plant protein is about 55:45, and in the scenario it is to 34:66. This proportion of livestock products matches recommendations for optimum dietary health. Essentially the livestock sector switches from quantity to quality production.
In the zerocarbonbritain2030 scenario over 70 million tonnes of biomass for energy is produced. This is used in the following ways: 16 million tonnes for biogas (bio-synthetic gas), mainly used to back up the electricity grid; 18 million tonnes of woody biomass for CHP; and 27 million tonnes to create kerosene, petrol and diesel using the Fischer-Tropsch process to power those parts of the transport sector for which there is currently no alternative to liquid fuels.
After appropriate management changes, land can remove CO2 from the air and sequester it in soil or above-ground biomass. Carbon can also be sequestered in products. Although neither of these can accumulate carbon indefinitely, they can provide us with a “window” of around 20-30 years, after which other methods of sequestering carbon may be available. These might include deeper soil sequestration or new technologies.
After food needs have been met, 43% of the remaining “productive non-food” land is dedicated to growing biomass for carbon sequestration. Carbon is also sequestered in soils through best practice management, encouraged though financial incentives. Below are the final figures for carbon sequestration in zerocarbonbritain2030, adjusted for uncertainty.
About 10 million tonnes of CO2e per year is sequestered in long lasting biomass products such as buildings and other wood products.
About 23 million tonnes of CO2e per year is sequestered in engineered biomass silos.
Carbon management of existing woodland is improved and 1.37 million hectares of new woodland is planted. This increases CO2e stored in-situ in standing timber by an estimated 12 million tonnes a year (Read et al., 2009).
A soil sink of around 9 million tonnes CO2e per year is achieved through best practice on all soil types (Brainard et al., 2003; Klumpp, 2009; Weiske, 2007; Worrall et al., 2003).
4.3 million tonnes of biochar a year is created and incorporated into soils (Sohi et al., 2010) providing sequestration of around 14 million tonnes per year. Biochar is charcoal that is used as an agricultural amendment. It cannot easily be broken down by decomposers and so may have potential as a more permanent net negative process.
In the scenario a healthy diet is provided for the population on only 29% of the land currently used for food production, supplemented by low-carbon imports. It provides a much higher degree of food security than at present. Total greenhouse gas emissions from agriculture are reduced to a fifth of their current quantity, leaving a total of 17 million tonnes of CO2e. Meanwhile the sector provides enough biomass to fulfil the fuel needs of the other sectors and to sequester carbon at a rate of 67 million tonnes of CO2e year. These “negative emissions” match the residual emissions from other sectors to meet the scenario’s ultimate goal, of a zero carbon Britain.
Brainard J, A. Lovett & I. Bateman (2003) "Social & Environmental Benefits of Forestry: Phase 2: Carbon Sequestration Benefits Of Woodland", Report to Forestry Commission, Centre for Social and Economic Research on the Global Environment, School of Environmental Sciences, University of East Anglia. Available at: http://www.forestry.gov.uk/website/pdf.nsf/pdf/carbonseqrep0603.pdf/$FILE/carbonseqrep0603.pdf [Live: March 2010].
Brown, L. & S. Jarvis (2001) “Estimation of Nitrous Oxide Emissions from UK Agriculture”, IGER Innovations. Available at: http://www.aber.ac.uk/en/media/chapter_10.pdf [Live: March 2010].
Klumpp, K. et al. (2009) “Grazing triggers soil carbon loss by altering plant roots and their control on soil microbial community”, Journal of Ecology, 97 (5), pp. 876-885.
Read D. J. et al. (eds) (2009) Combating climate change – a role for UK forests: The synthesis report. An assessment of the potential of the UK’s trees and woodlands to mitigate and adapt to climate change, Edinburgh: The Stationery Office (TSO).
Sohi S. P. et al. (2010) “A review of biochar and its use & function in soil”, Advances in Agronomy, 105, pp. 47-82.
Weiske, A. (2007) “Potential for carbon sequestration in European agriculture, Impact of Environmental Agreements On The Cap”, final version 16 February 2007, specific targeted research project no. SSPE-CT-2004-503604. Available at: http://www.ieep.eu/publications/pdfs/meacap/D10a_appendix_carbon_sequestration.pdf [Live: March 2010].
Worrall, F. et al. (2003) “Carbon budget for a British upland peat catchment”, The Science of the Total Environment, 312(1), pp. 133-146.