How are greenhouse gases measured, estimated, and reported?

Estimating and reporting emissions

Countries have pledged to reduce greenhouse gas emissions in response to the Paris Climate Agreement to limit global temperature rise to well below 2 ˚C.

To report on emissions reduction progress to the United Nations Framework Convention on Climate Change (UNFCCC), countries determine their emissions using a ‘bottom-up’ inventory of emissions sources.

Australia’s National Greenhouse Accounts and National Greenhouse Gas Inventory contributes to the UNFCCC information, and guides and tracks Australia’s implementation of reduction targets.

Australia has been estimating and reporting on greenhouse gas emissions since 1991.

The methods used to estimate Australia’s emissions have improved over time and will continue to evolve as new information and estimation techniques emerge.

The Department of Industry, Science, Energy and Resources compiles and accounts for Australia’s greenhouse gas emissions from 1990 to the present.

Australia’s emissions are calculated using national inventory data and other data sources. Sources of information include the:

• Australian Bureau of Agricultural and Resource Economics and Sciences (ABARES) livestock population, crop production and rice cultivation data
• Australian Bureau of Statistics demographic data
• Australian Energy Market Operator electricity market data
• Bureau of Infrastructure, Transport and Regional Economics transport data
• Bureau of Meteorology climate summaries
• Department of the Environment and Energy statistics.

CSIRO contributes information on emissions via the Global Carbon Project, a global consortium of scientists studying the impact of human activities on the carbon cycle and associated changes in climate.

Measuring concentrations

A complementary approach to determining emissions using the Australian Government’s bottom-up inventory approach is a ‘top-down’ approach, which uses measurements of atmospheric greenhouse gas concentrations to model the trajectories of gases and hence infer emission sources.

Measuring the amount of each greenhouse gas in the atmosphere acts as a check against bottom-up derived emissions estimates.

The Cape Grim measurement station in Tasmania, a joint responsibility of the Bureau of Meteorology and CSIRO, was established in 1976 to monitor and study global atmospheric composition.

The 45 years of data from Cape Grim are more valuable than ever in understanding how our atmosphere is changing. All data are publicly available.

CSIRO maintains a collection of air from Cape Grim to preserve a record of atmospheric composition for future analyses.

Since 2010, CSIRO has operated a Northern Territory Baseline Air Pollution Station near Darwin that measures greenhouse and other gases.

These observations are complemented by air extracted from Antarctic ice cores, extending the record of global greenhouse gas concentrations back thousands of years.

Changing emissions

Between 1990 and 2019 (1990 was used as a reference year in the Kyoto Protocol, the first international agreement to reduce greenhouse gas emissions), total global greenhouse gas emissions increased by 60 per cent.

China’s fossil-fuel carbon dioxide emissions increased by 420 per cent, the USA by 3 per cent, while the European Union decreased emissions by 25 per cent.

Over the same period, Australian fossil-fuel carbon dioxide emissions grew by 47 per cent, balanced to some extent by a decrease in emissions from land clearing.

In 2020, lockdowns and travel restrictions due to COVID-19 caused a temporary and unprecedented drop in global greenhouse gas emissions. However global fossil CO₂ emissions in 2021 are returning towards their 2019 levels.

It’s expected to be a 5.3% increase to 36 billion tonnes of CO₂ emission in 2021, relative to 2020.

This follows a slowdown in global emissions before the pandemic: between 2016 and 2019, emissions from 64 countries declined while emissions from 150 other countries increased, so global emissions were still growing, but a bit slower.

Data from 2020 shows that global fossil-fuel carbon dioxide emissions decreased by about 7 per cent, or 2.4 billion tonnes of carbon dioxide. Such a large drop in global annual emissions is unprecedented. The second-largest decrease was 0.9 billion tonnes of carbon dioxide in 1945, after the second world war.