At a glance
Greenhouse gases (GHGs)—carbon dioxide (CO2), methane, short-lived tropospheric and stratospheric ozone, nitrous oxide and synthetic GHGs—together with water vapour, and natural and industrial aerosols, influence Earth’s energy balance. Human activity, primarily the burning of fossil fuels during the past 250 years, has caused well-quantified increases in the concentrations of GHGs in the atmosphere, resulting in significant increases in positive radiative forcing, which has a warming effect on climate. CO2 levels in our atmosphere have increased by 43 per cent from pre-industrial (1750) levels, and methane levels have increased by 152 per cent. The contributions of CO2 and methane to radiative forcing have increased since 1995.
Australia’s emissions of CO2 per person in 2013 were nearly twice the average of countries in the Organisation for Economic Co-operation and Development. This reflects Australia’s heavy reliance on fossil fuels as a primary energy source and, particularly, the role of coal in the production of electricity. The energy sector continues to be the dominant source of Australia’s GHG emissions, accounting for 76 per cent of net emissions in 2015. Within the energy sector, 52 per cent of emissions arise from electricity generation and 17 per cent from transport. The energy sector contributed 93.9 per cent of CO2 emissions in the 2015 inventory, whereas the agriculture sector contributed 59.3 per cent of methane emissions and 72 per cent of nitrous oxide emissions.
Between 1990 and 2015, Australia’s GHG emissions decreased by 27 per cent per person, and, in 2012, Australia met its commitments under the Kyoto Protocol by limiting increases in net GHG emissions to 103 per cent of its 1990 levels from 2008 to 2012. However, climate projections suggest that GHG emissions will continue to put pressure on our climate.
Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system. The evidence for human influence has grown since IPCC AR4 2007. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. (Stocker et al. 2013a)
The energy balance of Earth’s atmosphere is influenced by the presence of trace levels of GHGs, including carbon dioxide (CO2), methane, short-lived tropospheric and stratospheric ozone, nitrous oxide and the synthetic GHGs (e.g. chlorofluorocarbons—CFCs and hydrofluorocarbons—HFCs). Water vapour (a major GHG), and natural and industrial aerosols are also important in the atmospheric energy balance, as are clouds and the level of incoming solar radiation. The effect of climate change ‘drivers’ such as solar radiation, GHGs, aerosols and surface albedo (reflectivity) on the energy balance is termed ‘radiative forcing’ (see Box ATM1).