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 (Figure ATM1). This has resulted in significant increases in positive radiative forcing, which has a warming effect on the climate (Figure ATM2).
The CSIRO global GHG-observing network recorded a global CO2 atmospheric concentration of 399.5 parts per million (ppm) for 2015. This represents an increase of 43 per cent from pre-industrial (1750) levels of 278 ppm, determined using data obtained from air extracted from ice cores (Etheridge et al. 1996, Hartmann et al. 2013). The global growth rate of CO2 in the atmosphere is increasing: in the 1990s, it averaged 1.5 ppm per year; in the 2000s, it averaged 1.9 ppm per year; and, in the 2010s (to 2015), it averaged 2.3 ppm per year. This increase is largely because of increasing consumption of fossil fuels (coal, oil, gas) (Raupach & Fraser 2011, CSIRO unpublished data). The concentration measured at the Cape Grim Baseline Monitoring Station reached 400 ppm in May 2016.
CSIRO observations show that the global average methane concentration in 2015 was 1834 parts per billion (ppb), which is an increase of 154 per cent above the estimated pre-industrial level of 722 ppb (Etheridge et al. 1998, Hartmann et al. 2013). The global methane growth rate slowed in the 2000s (2.3 ppb per year) compared with the 1990s (4.9 ppb per year) (Raupach & Fraser 2011, CSIRO unpublished data), likely because of reduced emissions from global natural gas production and distribution. However, it increased again in the 2010s (7.8 ppb per year), possibly because of the increased global use of natural gas, and increased tropical and Arctic wetland emissions (Hartmann et al. 2013).
CSIRO observations show that global nitrous oxide levels in 2015 were 328 ppb, an increase of 21 per cent from the pre-industrial level of 270 ppb (Hartmann et al. 2013, CSIRO unpublished data). The global nitrous oxide growth rate was relatively constant in the 1990s (0.72 ppb per year) and the 2000s (0.77 ppb per year), but has increased significantly in the 2010s (0.95 ppb per year) (Raupach & Fraser 2011, CSIRO unpublished data). Increased use of nitrogenous fertilisers in global agriculture is the main cause of the increase in nitrous oxide (Park et al. 2012, Hartmann et al. 2013).
Several synthetic GHGs, including fluorinated GHGs—such as CFCs, HFCs, perfluorocarbons (PFCs) and sulfur hexafluoride (SF6)—are emitted from a range of industrial processes, and from business and domestic use1 (Raupach & Fraser 2011). Although these gases are present in the atmosphere in only trace amounts, they are long-lived and have global warming potentials (measured in watts per square metre per ppb) that are thousands of times that of CO2 when assessed on a 100-year timescale (Myhre et al. 2013, Rigby et al. 2014). They can therefore contribute significantly to global warming in the medium to long term, and are included in the set of synthetic GHGs (HFCs, PFCs, SF6) covered by Annex A of the Kyoto Protocol. The production and consumption (and hence emissions) of CFCs, other ozone depleting substances and, recently (2016), HFCs are covered by the Montreal Protocol on Substances that Deplete the Ozone Layer and its subsequent amendments (see Box ATM2).