What are Greenhouse Gases (GHGs)

What are greenhouse gases?

Greenhouse gases (GHGs) are gases in the atmosphere that warm the Earth by absorbing energy and slowing the rate at which energy escapes the atmosphere.

$The sun radiates heat to earth. Some of this heat is absorbed by the planet, some is captured in the atmosphere and some is refracted back into space. The Earth’s atmosphere is made up of greenhouse gases. This is called the greenhouse effect. Without it our planet would be unihabitable. Greenhouse gases are released into the atmosphere through both natural and human-induced activites and these are also know as emissions. As emission levels rise, so to does the warming effects.$

On-farm greenhouse gas sources and sinks

The main greenhouse gases emitted on farms are:

Methane (CH₄)
Nitrous oxide (N₂O)
Carbon dioxide (CO₂).

Methane (CH₄) accounts for around 69% of agriculture sector emissions in NSW, excluding emissions from on-farm energy use and land clearing. Methane produced by ruminant livestock such as cattle and sheep during the digestion process, referred to as enteric (from the intestines) methane, is the main source of agricultural emissions. Methane is also produced under anaerobic conditions when organic matter decays (e.g., from manure deposited in paddocks, from pasture-based stock or manure in constructed ponds/anaerobic lagoons used to treat effluent.

Nitrous oxide (N₂O) emissions, in an agricultural context, are generated by adding nitrogen (N) to soils through the use of nitrogenous fertilisers, the decomposition of residues, and the deposition of livestock urine and manure. Nitrous oxide is produced from these N sources by microbial activity in water-logged soils as a component of the nitrogen cycle.

Carbon dioxide (CO₂) Depending on the use and management of land, carbon dioxide can be sequestered from the atmosphere and stored in vegetation and soils (carbon sinks). It can also be released into the atmosphere as a result of vegetation clearing and bushfires, combustion of fossil fuels (carbon sources) or the use of soil carbon as an energy source by soil microbes. When leaves and branches fall off plants or when plants die, the carbon stored in them is either released into the atmosphere or is transferred into the soil. Limestone or dolomite used to improve soil acidity results in carbon dioxide emissions as the carbonate reacts with acids. Urea is a common, highly soluble fertiliser, the use of which also results in carbon dioxide emissions.

Other greenhouse gases include water vapour and fluorinated gases such as hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride. These gases are not emitted in significant quantities from the agriculture and land sectors so are not discussed in this community of practice.

Global warming potential of greenhouse gases

Greenhouse gases differ in their capacity to contribute to global warming because they vary in potency (how much energy they absorb) and the length of time after being emitted that they stay in the atmosphere. To account for these differences, carbon dioxide (CO₂) is used as a reference and the warming impact of each greenhouse gas is quantified as carbon dioxide equivalents (CO₂e), commonly based on the relative contribution to warming over 100 years, referred to as the Global Warming Potential (GWP)100.

Methane is short-lived but potent. The global warming potential of methane is approximately 28 times that of carbon dioxide.
Nitrous oxide is long-lived and potent. The global warming potential of nitrous oxide is 265 times that of carbon dioxide.
Carbon dioxide has a GWP100 of 1. Carbon dioxide is the reference point for comparison to other greenhouse gases.

The Global Warming Potential of greenhouse gases is periodically reviewed and updated by the Intergovernmental Panel on Climate Change (IPCC). The figures shown here are from the IPCC’s Fifth Assessment Report completed in 2014.

Carbon dioxide equivalents are used to compare the different potency of greenhouse gases. These are updated with the IPCC findings and each new report delivered. It is determined that carbon dioxide is 1 unit. Therefore nitrous oxide is 265 times more powerful than carbon dioxide and methane is 28 times. E.g. 1 tonne of methane produced is equivalent to 28 tonnes of carbon dioxide

Scope 1, 2 and 3 emissions

Greenhouse gas reporting categorises emissions into 3 scopes to enable consistent reporting and accounting across sectors. For a farming operation, the emissions in each scope include:

Scope 1 - Direct

Direct greenhouse gas emissions generated on farm as a result of farming activities. Enteric methane, a by-product of ruminant livestock digestion, is the main source of scope 1 agricultural emissions. Other examples of scope 1 farm emissions include greenhouses gas emissions from manure, fertiliser use and the combustion of fuel in plant and machinery.

Scope 2 - Electricity

Greenhouse gas emissions from off-farm generation of electricity used on-farm.

Scope 3 - Pre-farm sources

Greenhouse gas emissions from pre-farm sources not captured in Scope 2. These greenhouse gases are generated during the production and transport of purchased materials such as feed, fertilisers, herbicides, veterinary products and fuel used on-farm.

An image that depicts the scopes of emissions on-farm (direct emissions, off-farm electricity and pre-farm inputs)