Net Zero: a glossary of terms

CO2, CH4, GWP, N2O, CO2e? It’s not all about carbon – and the most important emission from agriculture isn’t carbon dioxide. See below our glossary of terms regarding carbon and Greenhouse Gas (GHG) emissions.

Changes made to activities in response to climate change.

The baseline is the state against which change is measured.

For farming and the environment, this could be the initial measurement of carbon emissions, sequestration, carbon stores (e.g. in your soil), or other measures like biodiversity. Baselining refers to the act of getting these measurements.

Solid, lots of carbon atoms, stored in soils and plants – and fossil fuels (very old plants). Often (incorrectly) used as shorthand for carbon dioxide (CO2) – beware of confusion!

Gas, each molecule of which has one carbon atom and two oxygen atoms. Global warming potential of 1. Usually emitted by respiration and combustion.

  • CO2 is the yardstick - with a global warming value of 1
  • Methane (CH4) - Global warming potential 28 times the impact of CO2 (over 100 years - see GWP), Particularly relevant to livestock (enteric fermentation and manure management) and paddy rice systems
  • Nitrous oxide (N2O) – Global warming potential 298 times the impact of CO2 (over 100yrs). Occurs in fertiliser manufacture and the breakdown of fertilisers and other organic matter in the soil

Money made available for storing or sequestering carbon.

Occurs when carbon moves between two systems such as plant material and the atmosphere. Sequestration is the net flow of carbon out of the atmosphere, whilst a system that emits more carbon to the atmosphere than it sequesters is a carbon source.

A way of estimating the impact that a production process has on climate change.

A commodity market for buying and selling carbon credits. 1 carbon credit = 1 tonne of CO2 sequestered, or CO2e emissions avoided. For more information, see our carbon markets page.

Plants and soil store carbon, but it is carbon dioxide (CO2) which is found in the atmosphere and contributes to climate change. One tonne of carbon is equal to 3.67 tonnes of CO2, so if you have 10 tonnes of carbon stored, that’s equivalent to 36.7 tonnes of CO2 emissions.

The removal of carbon dioxide from the atmosphere and storage in another system, such as vegetation. If the carbon dioxide sequestered is more than the carbon dioxide emitted, the store is increasing and is known as a carbon sink.

The amount of carbon contained in soils. This varies enormously, ranging from sandy arable soils with under 40 tonnes of carbon per hectare to peat soils with up to 300 t/ha.

A market used to manage greenhouse gas emissions; instead of cutting their own emissions to meet mandatory targets, companies can pay someone else to cut theirs, or to sequester carbon.

The process by which the climate is changing over periods of many years; for example, rainfall patterns may change in terms of both amount and seasonality. Climate is the overall sum of weather, which is the natural variability which we experience day-to-day.

A factor that quantifies the emissions or removals of a greenhouse gas per unit activity. Since real emissions can’t usually be measured, factors based on a sample of measurement data can be used instead to calculate emissions. For example, emissions from fuel use = emissions factor for burning fuel x amount of fuel used (the ‘activity’).

Emissions factors are always an average figure under a given set of conditions. They are used in different tiers - see “Tier 1,2 and 3” below.

Describes how much impact a gas will have on atmospheric warming over a period of time compared to carbon dioxide. Each greenhouse gas has a different atmospheric warming impact, and some gases remain in the atmosphere for longer than others.

The internationally accepted standard measure uses 100 years, known as GWP100. Alternatives include GWP20, modelling impact over 20 years, and GWP*, which models the impact of methane, accounting for its faster breakdown time.

The release of gases whose molecules reduce the rate that solar heat energy in the Earth’s atmosphere is lost back into space as infrared radiation.

Methane is one of the two main greenhouse gases emitted by agriculture, with 28 times the impact of carbon dioxide (CO2e). It is largely a product of livestock production, mostly from enteric fermentation in ruminants but also from slurry, manure and waterlogged land.

Enteric methane exists within natural cycles, and the methane emitted will be cycled or sequestered in due course.  However, increased ruminant numbers at a global level are having an impact both through higher emissions that have to be cycled, and land use change.

The process of finding ways to reduce the greenhouse gas emissions created by human activities.

Net zero means that the UK’s total greenhouse gas (GHG) emissions would be equal to or less than the emissions the UK removed from the environment. This can be achieved by a combination of reducing emissions and removing emitted GHGs through methods such as sequestration.

Net Zero within a farm business is when GHG emissions are equal to carbon sequestration. 

Nitrous oxide is the other of the two main greenhouse gases emitted by agriculture, mostly from cropped land – it has 298 times the global warming potential (CO2e) of carbon dioxide, and is mostly created by fertiliser production and its breakdown in the soil, together with the decay of other organic matter.

Energy generated from naturally replenished resources such as sunlight, wind, water and biomass.

An IPCC method of categorising emissions associated with a business. Scope 1 is direct emissions (e.g. from running a tractor). Scope 2 is indirect emissions from generating the energy bought by the business. Scope 3 is all other indirect emissions (e.g. the emissions from making and transporting fertiliser to the farm).

The carbon footprints of farms will come under Scope 3 for food processors and retailers.

Carbon sequestration is the net transfer of carbon from the atmosphere to the land where it is stored in soils and vegetation.

Also soil organic carbon (SOC) is carbon stored in organic matter in the soil. It comes from decomposing plant material and is vital for soil health. About 58% of soil organic matter is carbon.

These refer to the methods used to calculate greenhouse gas emissions and sequestration, with Tier 1 being the basic method and Tier 3 being the most detailed. Tiers 2 and 3 are generally considered to be more accurate.

Part of this also refers to the emissions factors (see above) used in the calculations (carbon calculators). Generally speaking, Tier 1 factors are international average figures, Tier 2 is national level averages, and Tier 3 factors are for a specific place, project or product.

For example, if you were calculating emissions from cattle: a tier 1 emissions factor would assume all the cows in the world are the same breed/size etc and therefore emit the same methane. A tier 2 emissions factor would treat all the cows in the UK as emitting the same. A tier 3 emissions factor could be even more granular, e.g. giving an average for a specific region and/or system.

While higher-tier methods and factors are the most accurate, they can be the most difficult to achieve. For agriculture, most carbon footprint calculators use tier 2 for emissions and tier 1 for sequestration.   

The most important greenhouse gas of all – but only a contributor to global warming under very specific circumstances.


Useful links

AHDB work on the environment