A greenhouse gas emission inventory is a compilation of historical greenhouse gas emissions from sources, such as transport, power generation, industry and agriculture, from 1990 to the most recent year for which data is available.

A greenhouse gas emission projection is an estimate of what emission levels are likely to be in the future. They are based on key assumptions such as economic growth, fuel prices and Government policy.

Essentially the inventory data provides a summary of past emissions whereas the projections attempt to estimate the emissions in the future.

National Emission Inventories; The United Nations Framework Convention on Climate Change (UNFCCC) and its Kyoto Protocol provide the basis for international action to address climate change. Parties to the convention and its Kyoto Protocol are committed to developing and publishing the national emission inventories of greenhouse gases (GHGs) which is a key element of assessing progress towards meeting commitments and targets.

The EPA compiles Ireland's national greenhouse gas emission inventory on an annual basis. This inventory is submitted to the European Commission and UNFCCC each year by 15 January and 15 April respectively.

Emissions data for the following gases is reported on an annual basis: carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perflurocarbons (PFCs), sulphur hexafluoride (SF₆), and nitrogen trifluoride (NF₃).

Emissions are classified into the ten following sectors; agriculture, transport, energy industries, residential, manufacturing combustion, industrial processes, F-gases, waste, commercial services and public services.

Greenhouse gases other than CO₂ (i.e. methane, nitrous oxide and F-gases) may be converted to CO₂ equivalent using their global warming potentials (GWPs). The GWP of a gas is a measure of the cumulative warming over a specified time period usually 100 years, by a unit mass of this gas. This is expressed relative to carbon dioxide (CO₂) which has a GWP of 1. The mass emission of any gas multiplied by its GWP gives the equivalent emission of the gas as carbon dioxide. This is known as CO₂ equivalent. This makes it possible to sum up the emissions and contribution of Greenhouse Gas (GHG) to climate change and determine options to address climate change.

These gases comprise HFCs (hydroflurocarbons), PFCs (perfluorcarbons), SF₆ (sulphur hexafluoride) and NF₃ (nitrogen trifluoride). They are much more potent than the naturally occurring greenhouse gas emissions (carbon dioxide, methane and nitrous oxide).

Global warming potential of greenhouse gases in the inventories and projections;

Carbon dioxide (CO2) GWP = 1

Methane (CH4) GWP = 28

Nitrous oxide (N2O) GWP = 265

Hydrofluorocarbons (HFCs) GWP = 4 to 12,400

Perfluorinated compounds (PFCs) GWP = 6630 to >17,400

Sulphur hexafluoride (SF6) GWP = 23,500

Nitrogen trifluoride (NF3) GWP = 16,100

The mass emission of any gas multiplied by its GWP gives the equivalent emission of the gas as carbon dioxide. This is known as CO₂ equivalent. This makes it easier to sum up the emissions and contribution of Greenhouse Gas (GHG) to climate change and determine options to address climate change.

The EU's Effort Sharing Decision (Decision No 406/2009/EC) sets targets (see row G of table in latest emissions data tab) for the non-Emissions Trading Scheme sector for  EU members states including Ireland for 2020. Ireland is required to deliver a 20% reduction in non-ETS greenhouse gas emissions by 2020 (relative to 2005 levels). In addition, Ireland has binding annual emission limits for the period 2013-2020 to ensure a gradual move towards the 2020 target. The non-ETS sectors cover those that are outside the EU Emissions Trading Scheme and includes the following sectors: agriculture, transport, residential, commercial, waste, and non-energy intensive industry.

Million tonnes (Mt) CO2eq =1,000 kilotonnes

1 kilotonne=1,000 tonnes

The EU emissions trading system (EU ETS) is a cornerstone of the European Union's policy to combat climate change and its key tool for reducing industrial greenhouse gas emissions cost-effectively. The first - and still by far the biggest - international system for trading greenhouse gas emission allowances, the EU ETS covers more than 11,000 power stations and industrial plants in 31 countries, as well as airlines.

Installations and aircraft operators covered by the EU ETS are those which carry out activities listed in Annex I of the EU ETS Directive. Emissions occurring from the activities listed in Annex I are referred to as ETS emissions, and these are excluded from the Effort Sharing Decision targets. Emissions from activities occurring outside of the EU emissions trading system are referred to as ESD emissions or Effort Sharing Decision (ESD) emissions. agriculture and transport accounted for 72.9% of total ESD emissions in 2018.

Land use, land-use change, and forestry (LULUCF), also referred to as forestry and other land use (FOLU), is defined by the United Nations Climate Change Secretariat as a "greenhouse gas inventory sector that covers emissions and removals of greenhouse gases resulting from direct human-induced land use such as settlements and commercial uses, land-use change, and forestry activities." It covers the following categories forest land, cropland, grassland, wetlands, settlements, other land and harvested wood products.

Reporting of the LULUCF sector

The UNFCCC reporting guidelines on annual inventories for Parties included in Annex I to the Convention (Decision 24/CP.19)provide guidance on the estimation and reporting of anthropogenic emissions by sources and removals by sinks of greenhouse gases not controlled by the Montreal Protocol caused by activities relating to land use, land use change and forestry. The guidance stipulates that reporting under the UNFCCC covers all anthropogenic emissions and removals from the lands included in the LULUCF sector (land-based approach). In principle, this approach applies a wall-to-wall comprehensive inventory of anthropogenic sources and sinks of greenhouse gases over the land units subject to activities relating to land use, land use change and forestry.

The WEM scenario is a projection of future emissions based on the measures currently implemented and actions committed to by Government. To become part of the WEM scenario a policy or measure must be in place by the end of 2021 (the latest inventory year) and the projected emissions reduction is commensurate with the resources or legislation already in place or committed to Government Departments or Agencies. For example, the WEM scenario includes a measure where the Carbon tax increases annually and reaches €100 per tonne by 2030. This policy is considered to be implemented because annual Carbon tax increases have been committed to in legislation (Finance Act 2020).

The WAM scenario is the projection of future emissions based on the measures outlined in the latest Government plans at the time Projections are compiled. This includes all policies and measures included in the WEM scenario, plus those included in government plans but not yet implemented. For example, the WAM scenario includes the target of 945,000 Electric Vehicles on the road by 2030 in the Climate Action Plan 2023. The full amount of this ambition is not currently in the With Existing Measures scenario as actions still remain to be taken that would deliver it.

Further information on the policies and measures for the individual sectors that are included in both With Existing Measures and With Additional Measures scenarios in the latest emissions projections is available in Ireland’s Greenhouse Gas Emissions Projections 2022-2040.

The data submitted in the current year is the inventory for x-2 years so for example in 2022 the EPA will submit to the EU and UNFCCC, the 1990-2020 time series. All reports and infographics are based on the 1990-x-2 timeseries and the most recent year of data. A provisional estimate of emissions is produced in quarter four of the year before submission and the website will be updated with this provisional data as soon as it is available.

Decarbonisation means reduction of carbon. What is meant is the conversion to an economic system that sustainably reduces and compensates the emissions of carbon dioxide (CO₂). The long-term goal is to create a CO₂-free global economy.

A low-carbon economy, low-fossil-fuel economy, or decarbonised economy is an economy based on low-carbon power sources that therefore has a minimal output of greenhouse gas emissions into the atmosphere, specifically carbon dioxide. A low-carbon economy is simply an economy that causes low levels of GHG emissions compared with today's carbon-intensive economy. 'Carbon' refers to carbon dioxide, the GHG, which contributes the most to climate change. The low-carbon economy can be seen as a step in the process towards a zero-carbon economy.