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Human activity causes carbon dioxide (CO2) and other greenhouse gases to be emitted into the atmosphere.

Scientists know that the source of CO2 in the atmosphere is the result of human activity (e.g. burning fossil fuels to produce electricity, transport, and industrial processes). This is because they can analyse the different isotopes of carbon in atmospheric CO2 to understand their sources.

Measurements of carbon-13 and carbon-14, relative to carbon-12, confirm that the increase in CO2 concentration since 1800 originates principally from fossil fuel and land clearing emissions.

Electricity production involving the burning of fossil fuels is the predominant contributor to Australia's CO2 emissions.

How excess CO2 emissions are added to the atmosphere

Carbon naturally flows in and out of the land, ocean and through living things as part of the carbon cycle. Land based plants and phytoplankton in the oceans take in CO2 in a process called photosynthesis. Soils also store, cycle and emit carbon as part of the carbon cycle. About 30% of the excess CO2 added to the atmosphere is absorbed by oceans, where the CO2 is dissolved, leading to ocean acidification.

In total, Earth’s oceans, land and plants absorb 190.2 billion tonnes of CO₂ per year from the atmosphere.

On the other hand, animals – including humans – breathe out CO₂ in the process of respiration. When plants and animals die, their stored carbon is also released as CO₂. Natural processes such as respiration and decay, forest fires and volcanic eruptions add an additional 190.2 billion tonnes of CO₂ to the atmosphere per year.

Excess carbon dioxide emitted by human activity is impacting the natural carbon cycle in the environment that has occurred for millions of years.
Humans generate CO2 when burning fossil fuels such as gas, petrol, oil, and coal. This adds an additional 9.1 billion tonnes of CO₂ to the atmosphere each year.

Plants and soils take up 2.8 billion tonnes of this extra carbon, while the oceans take up 2.2 billion tonnes.

The remaining 4.1 billion tonnes of CO2 stays in the air, increasing the atmospheric concentration of carbon dioxide.

Sources of global CO2 emissions

Emissions of CO2 from fossil fuels make the largest contribution to climate change.

About 90 per cent of the world’s carbon emissions comes from the burning of fossil fuels – mainly for electricity, heat and transport.

In 2022, most of the world's fossil fuel carbon emissions came from coal (40 per cent), oil (32 per cent), natural gas (21 per cent), cement (5 per cent) and flaring and other smaller sources (2 per cent).

Just four regions accounted for about two-thirds of global fossil-fuel carbon emissions in 2021: China (31 per cent), the USA (14 per cent), the EU27; 7 per cent), and India (7 per cent).

Industrialised countries represent just 20 per cent of the world’s population but account for 80 per cent of cumulative CO₂ emissions since the beginning of the industrial revolution.

Eraring coal-fired power station in Newcastle. ©  Credit: Nick Pitsas

Australia's contribution to global greenhouse gas emissions

Australia is the world’s 14th highest emitter, contributing just over 1 per cent of global emissions.

The Australian Government tracks the nation’s individual greenhouse gases emissions, as well as CO₂ equivalent through the National Greenhouse Gas Inventory.

CO₂ equivalent is calculated from the atmospheric concentrations of CO₂, methane, nitrous oxide and the suite of synthetic greenhouse gases.

According to the December 2020 update, Australia emitted 499 million tonnes of CO₂ equivalent, which marked a 5 per cent decrease as compared to 2019.

Energy production is the largest contributor to Australia’s carbon emissions. This is followed by transport, agriculture, and industrial processes. Specifically:

  • energy (burning fossil fuels to produce electricity) contributed 33.6 per cent of the total emissions
  • stationary energy (including manufacturing, mining, residential and commercial fuel use) 20.4 per cent
  • transport 17.6 per cent
  • agriculture 14.6 per cent
  • fugitive emissions 10.0 per cent
  • industrial processes 6.2 per cent
  • waste 2.7 per cent.

Greenhouse gas emissions are also influenced by changes in land use. For example, clearing land for agricultural use or pastures means that the carbon stored in plants and trees is lost to the atmosphere because the vegetation will not be allowed to regrow.

Today's CO2 concentrations in the atmosphere

As a result of increasing emissions, the global atmospheric CO₂ concentration has increased by 50 per cent since pre-industrial times, rising from 277 ppm in 1750 to 416 ppm in June 2023.

The CO₂ concentration in the atmosphere today is much higher than the natural range of 172 to 300 ppm that existed for hundreds of thousands of years.

In fact, CO₂ concentrations now are likely to be the highest they have been in at least the past 2 million years.

It was estimated that 37.5 billion tonnes of CO2 were emitted by the end of 2022.

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