Greenhouse gas emissions: What are they and how are they measured?

Aug 25, 2021

The goal of the Paris agreement on climate change is to limit global warming to well below 2°C, and preferably to 1.5oC above pre-industrial temperatures of the 1700s. This requires countries to reduce net greenhouse gas emissions as soon as possible in order to achieve a climate-neutral world by the middle of this century. Unlike 131 other countries, Australia is yet to commit to net-zero by 2050.

If you have been brave enough to check out the recent IPCC (Intergovernmental Panel on Climate Change) report on the science of the climate system and climate change, you will appreciate that it’s an amazingly complex field.

This article simply gives an overview of the GHGs (greenhouse gases) we need to deal with and how countries document their emissions over time.

International reporting requirements on greenhouse gas emissions

Australia produces annual Greenhouse Gas Inventory reports to fulfil its international obligations under the United Nations Framework Convention on Climate Change (UNFCC) and the more recent Paris agreement. These inventories are a record of estimated emissions and removals of greenhouse gases (GHGs) in specified sectors of human activity.

Removals of GHGs from the atmosphere, which offset emissions, are often called ‘sinks’. The most familiar example is vegetation which absorbs carbon dioxide from the atmosphere. ‘Net zero emissions’ refers to achieving an overall balance, where GHG emissions are offset by removals.

Removals, however, are far outweighed by emissions. Think of the (so far largely unsuccessful) efforts being made to capture and store carbon dioxide emitted from power stations. Revegetation efforts around the world need to accelerate and land clearing dramatically reduced, but the current levels of emissions can never be offset by removals. So concerted effort is required to reduce GHG emissions.

Australia’s emissions estimates, prepared by the Commonwealth, are consistent with UNFCC reporting requirements and IPCC guidelines. Annual reports date back to 1990. Australia prepares these for individual states and territories as well as the country as a whole. Quarterly updates are also released.

Draft reports are reviewed by the National Greenhouse Gas Inventory Committee (NGGIC) which comprises representatives of the Australian, state and territory governments. I represented Victoria on this committee in the 1990s.

Which GHGs must be reported?

Greenhouse gases (GHGs) subject to UNFCC reporting requirements are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3) and some synthetic chemicals such as perfluorocarbons (PFCs) and hydrofluorocarbons (HFCs). Unlike oxygen and nitrogen, which make up most of the atmosphere, GHGs absorb heat energy.

Each GHG has a different impact on global warming, with some much more potent than others. Hence, the concept of a Global Warming Potential (GWP) was developed to allow assessment of the relative importance of the gases and aggregation. (A GWP is somewhat like a currency exchange rate, allowing GDP in different currencies to be converted to $US, for example.)

The GWP of a specific GHG depends on its capacity to absorb heat energy and how long it remains in the atmosphere. A GWP is expressed relative to CO2 which is defined as having a GWP of 1. This table shows the GWPs of the most common non-CO2 gases. We see, for example, that methane is 28 times more potent than carbon dioxide.

Greenhouse gas (GHG) Global Warming Potential (GWP) – 2020/21 onwards
Carbon dioxide (CO2) 1
Methane (CH4), 28
Nitrous oxide (N2O) 265
Tetrafluoromethane (CF4) 6,630
Hexafluoroethane (C2F6) 11,100
Sulphur hexafluoride (SF6) 23,500
Hydrofluorocarbons (HFCs) Depends on the HFC


Greenhouse gas inventories use GWPs to aggregate estimated emissions of different gases from each source, and hence the totals are expressed as CO2-e (CO2 equivalent).

Australia’s emissions profile – what are we emitting?

Based on IPCC reporting guidelines, Australia summarises GHG emissions according to five broad sectors as follows. We can split energy into four sub-sectors:

  1. Energy:
    • Electricity generation (e.g. from coal fired power stations),
    • Stationary energy, other than electricity generation (e.g. gas/LNG combustion by industry and households),
    • Transport (including road, rail, domestic aviation), and,
    • Fugitive emissions (leaks and other irregular emissions of gases, mostly through coal, gas and oil extraction, processing and transmission).
  2. Industrial processes and product use: such as cement and aluminium production. (Aluminium production, for example, involves chemical reactions that produce CO2 and trace amounts of other greenhouse gases such as sulphur hexafluoride.)
  3. Agriculture: includes enteric fermentation (the process by which livestock produce CH4 by digestion) and N2O from fertilisers. Around 70% of CO2-e emissions in this sector come from enteric fermentation.
  4. Land use, land use change and forestry (LULUCF): covers forests, cropland, grassland and settlements, and conversions between these land uses. As well as CO2, emissions of CH4 and N2O are significant.
  5. Waste: Mostly CH4 emissions from landfill and wastewater.

Net emissions for 2019 are shown in the table and the bar chart below.

Sector Megatonnes (Mt) CO2-e % of Total
1. Energy 432.0 82.2%
Electricity generation 176.2 33.5%
Stationary energy (excl. electricity) 101.5 19.3%
Transport 99.8 19.0%
Fugitive emissions 54.5 10.4%
2. Industrial processes and product use. 31.6 6.0%
3. Agriculture 72.9 13.9%
4. LULUCF -24.8 -4.7%
5. Waste 13.5 2.6%
TOTAL 525.1 100%

Many may not appreciate that only about one third (33.5% in 2019) of Australia’s net emissions derive from electricity generation. Another 19% comes from energy used directly from fuels such as LNG.

When we include emissions from another familiar sector, transport, (19.3%), that still leaves almost 30% from activities we have heard less about, for example agriculture which contributes almost 14%. 

How are emissions estimates put together?

Total GHG emissions over a year must be estimated from each source within each of these sectors. It is a highly complex process. There are two main types of data needed which are then combined to calculate total emissions. To illustrate, we give two examples:(a) brown coal power stations and (b) rams, whose emissions differ from other classes of sheep, such as wethers.

Emission factors: These express the quantity of the GHG emitted per unit of activity. For our examples: (a) CO2 emissions per PJ (petajoule) of brown coal, (b) daily CH4 emissions per animal in a class of sheep (e.g. rams), in each season and in each state (feed types vary in different parts of the country).

Activity data: This data reflects the total level of the source of emissions. Our examples: (a) For the brown coal power station, we use total energy content from the fuel combusted, (b) For sheep, we use the number of sheep in each class (e.g. rams), per season, per state.

To calculate total emissions for a source category: (a) We multiply the total PJ content of the brown coal from a particular power station (activity data) by the relevant emission factor. This process is repeated for other power stations and then aggregated. (b) We multiply the number of rams in each state and season by the relevant emission factor for that state and season. These totals are aggregated across seasons and states and the process is repeated for other classes of sheep.

Inventory reports frequently refer to ‘top-down’ versus ‘bottom-up’ methods of estimation. ‘Top-down’ refers to the use of more aggregated activity data and default emission factors, so calculating total emissions is relatively simple. A ‘bottom-up’ approach reflects the use of disaggregated or even site-specific (e.g. an individual factory) activity data and/or emission factors.

Quite often, both approaches are used, providing an independent cross-check of results. As an example: the Australian Energy Statistics (energy use by economic sector and fuel) provides a comprehensive and detailed ‘bottom-up’ quantification of energy use in Australia. They are reconciled with ‘top-down’ statistics of all major fuels in Australia, collected from the suppliers of those fuels, i.e. the coal, oil, gas and electricity industries.

How has Australia been tracking?

The chart below (Figure 2.1 in the 2019 National Inventory Report) shows the trend in aggregated net emissions (in Mt CO2-e) from 1990 to 2019. The Australian government uses 2005 as the base year on which to defend its progress in reducing emissions.

(Preliminary 2020 emissions estimates, strongly influenced by the global pandemic, are not discussed here.)

Following a decline to the mid-1990s, emissions rose to a peak in 2007 and have declined gradually since. Overall, total emissions in Australia have decreased by 15.2% from 2005 to 2019, but not at all compared to 1995.

It is worth exploring this interactive chart that shows underlying trends for different sectors in the country, as well as the eight states and territories, to 2019. Key points for each sector:

  1. Energy emissions have grown steadily (by 47%) since 1990. They dropped slightly between 2009 and 2014 but have risen again since.
    • A separate interactive chart from the latest quarterly report splits the broad energy sector into the four sub-sectors. You can see that the drop in emissions associated with electricity generation is being outweighed by continued growth in the other three sectors.
    • Electricity generation emissions dropped from a peak in 2008 and are now back down to about 1998 levels. This decline has been driven by the increase in the use of renewables and corresponding decline in the use of coal.
    • Stationary energy (excluding electricity generation) emissions have risen steadily since 1990.
    • Fugitive emissions (mostly methane) have grown strongly (37% since 2005) reflecting increases in open cut coal mining, gas and LNG production. This growth has been concentrated in WA, NT and QLD.
    • Transport: emissions since 2005 have increased by 22%. Road transport makes up the bulk at 84.2%, with domestic aviation 8.4%. Passenger vehicles (cars) contribute more than half of road transport emissions with continuing growth reflecting population growth and increased travel overall, as well as increased fuel consumption due to a switch to heavier, less fuel-efficient vehicles.
  2. Industrial processes and product use emissions grew by 2.4% between 2005 and 2019. Much of the increase is associated with halocarbons (HCFs), synthetic gases used to replace ozone-depleting refrigerants now banned under the Montreal Protocol.
  3. Agriculture emissions have dropped by 12.5% since 2005. Levels fluctuate significantly from year to year, due to changes in stock numbers resulting from climatic and seasonal events (droughts, floods) and in response to fluctuations in international and national markets. While sheep numbers have declined, cattle numbers and crop production (and fertiliser use) have increased.
  4. LULUCF net emissions have dropped by 130% since 2005. In Australia, this sector became a net sink from 2016, i.e., more CO2-e emissions are now captured and stored than emitted. Forestry and croplands are net sinks, so they more than offset emissions associated with the clearing of forests.

The significant drop in net emissions is due to a big reduction in land clearing since 1990, reduced logging of native forests, and the expansion of forest cover. But recent underlying trends are not so encouraging for the future, reflecting the flattening out of net emissions from this sector since 2015:

      • Land clearing rates are rising again after a steep drop from 2005.
      • Areas of new forest plantations are now growing at a much smaller rate, compared to large increases in areas planted between 2001 to 2009.
      • Areas of native forests harvested have been relatively stable since 2009.
      • Looking at the individual jurisdictions, by far the biggest contribution from this sector comes from Queensland. Only that state and NT are net emitters.
  1. Waste emissions have decreased by 13.6% since 2005, mostly due to increased recovery of methane from landfill. But this sector makes up only about 2% of total net emissions.

The overall drop in net emissions since 2005 is essentially due to actions in the LULUCF sector, the growth in renewables in the energy sector and the small contribution from the waste sector. Emissions from most other sectors have grown.

In an article tomorrow, I look at Government projections of Australia’s emissions to 2030 and explain why Australia is not on track to meet net-zero by 2050.

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