Carbon: greenhouse gas emissions from agriculture
Carbon has become shorthand for greenhouse gases – a range of natural gases and particles – usually water vapour and carbon dioxide, including dust. Find out more about current legislation and the goals set by government to reduce emissions.
Greenhouse gases can absorb infrared heat energy reflected by the earth. When their molecules re-emit this energy, some of it is redirected back towards the earth as a heating (or greenhouse) effect. This is essential to keep the planet's surface habitable, since without them the earth's surface would be too cold to support life as we know it.
Global temperature and greenhouse gas concentrations are linked, and follow a natural cycle. However, since the industrial revolution, human activity - particularly the burning of fossil fuels, has increased the concentration of greenhouse gases in the atmosphere to levels higher than those seen since prehistoric times. This recent increase in CO2 levels leads most of the scientific community to believe that we are beginning a period of significant global warming.
Governments around the world have therefore committed to finding ways to reduce greenhouse gas emissions, most recently in the 2016 Paris Agreement.
Emissions from global agriculture
In global terms, agriculture is a significant contributor to greenhouse gas emissions. The IPCC's Special Report on Climate Change and Land (2019) estimates that agriculture is directly responsible for up to 8.5% of all greenhouse gas emissions with a further 14.5% coming from land use change (mainly deforestation in the developing world to clear land for food production).
The two biggest sources of greenhouse gases from agriculture are:
• the release of nitrous oxide from agricultural soils
• methane from livestock and manures
While energy use in agriculture (for example, diesel for cultivation) is important, its contribution to greenhouse gas emissions is much lower, less than 1.5% of total emissions.
Greenhouse gas emissions from domestic agriculture
In the UK, emissions from industry remain a sizeable proportion of our total carbon footprint, and those from agriculture and horticulture account for about 10% of all UK greenhouse gas emissions.
Legislation and targets
In domestic legislation, the Climate Change Act 2008 committed the UK to reduce greenhouse gas emissions by 80% from a 1990 baseline by 2050. This led to UK agriculture adopting the Greenhouse Gas Action Plan (GHGAP), aiming for a reduction of 5.2 Mt CO2e by 2020 – we missed, achieving only about a third of this objective. This is a problem for agriculture, as all other industry sectors have made bigger cuts in their emissions.
In 2019, the Climate Change Act 2008 was further amended, moving the targeted reduction to 100%, i.e., Net Zero by 2050. To achieve this, our land-based industries can contribute strongly by adopting new practices both to reduce emissions and to sequester carbon from the atmosphere.
How can emissions be reduced?
In broad terms, many of these practices will involve optimising soils, maximising vegetation growth and, wherever possible, ceasing management of unproductive areas. This is because semi-natural vegetation and woodland stores more carbon in the plants and soil than improved grassland, and markedly so where heavily carbon-depleted arable soils are taken out of production.
How are greenhouse gases calculated and measured?
Greenhouse gases are often presented in units of carbon dioxide equivalent (or CO2e). This is a common unit of measurement that allows comparisons to be made between emissions of different greenhouse gases.
Each greenhouse gas can have a different impact on the Earth’s climate. This is because each behaves differently in the atmosphere, namely in their ability to absorb heat and the length of time they exist for in the atmosphere (or their “lifetime”).
CO2 is the most abundant greenhouse gas emitted by human activity, and so is used as a “benchmark” against which emissions of other greenhouse gases are compared.
Non-CO2 greenhouse gases are converted into CO2e by multiplying the quantity of gas emitted by the global warming potential (GWP) of that gas. The GWP of a gas is its heat-absorbing ability compared to CO2 over a fixed timeframe. As CO2 is the benchmark, it always has a GWP of 1.
A timeframe of 100 years (GWP100) is currently the most common method used to calculate emissions of GHGs. The GWP100 value for methane is 28, meaning that methane has 28x more global warming potential than CO2. Nitrous oxide has a GWP100 value of 298, meaning it has 298x the warming potential of CO2.
However, there is debate among scientists as to whether the use of GWP100 misrepresents the impact that “short-lived” GHGs, like methane, have on the Earth’s climate. This is important when considering the impact of agricultural emissions, which are mostly of methane, in comparison to emissions from other sectors that are mostly CO2, such as transport.
Figures last updated: 26 May 2021
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