Overview

Measurements clearly show that there has been a global warming trend over the past 100 years. The mean worldwide temperature has risen an average of 0.13^oC per decade since 1900, while Australia’s mean annual temperature has increased by about 0.9^oC since 1950. Sea levels have also risen by 1.8 mm per year over the past 100 years.

In its Fourth Assessment Report (2007), the Intergovernmental Panel on Climate Change (IPCC) concluded that most of the increase in global average temperatures in the past 60 years was probably caused by the increase in greenhouse gas (GHG) concentrations in the atmosphere. The main gases involved are carbon dioxide, methane and nitrous oxide. They are released by human activities such as burning carbon-containing fuels, intensive agriculture and deforestation.

The increase in global average temperature is predicted to accelerate over the next century from the effects of GHGs already in the atmosphere and from future emissions. The IPCC’s Fourth Assessment Report predicts global mean temperature rises of 0.1^oC to 0.7^oC by 2020 and 1.1^oC to 6.4^oC by 2100. It predicts a sea level rise by 2100 of 18 cm to 59 cm, with a possible additional contribution from melting ice sheets of 10 cm to 20 cm. The wide range in the forecasts can be attributed to both scientific uncertainty and uncertainty over how effective policies will be at reducing future GHG emission levels.

Clearly, the effects of such sea-level rises on the Australian coastline will be significant. Other likely impacts of climate change include increased bushfire risk; more frequent droughts; more intense storms; ocean acidification and coral bleaching affecting the Great Barrier Reef; a decline in production from agriculture and forestry over much of southern and eastern Australia; and significant loss of biodiversity in ecologically rich areas such as Kakadu, south-west Western Australia and the Wet Tropics of Queensland. There are important security implications in some of the global effects of climate change, and these are discussed in a separate brief.

The United Nations has indicated that, without additional action, GHG emissions are projected to rise by 25–90 per cent by 2030. It notes that governments could implement policies to stabilise the level of GHGs in the atmosphere and limit the rise in global temperature. Modelling has indicated that stabilising GHG levels below 490 parts per million (ppm)—the most ambitious target that was assessed—‘would require global carbon emissions to peak by 2015 and to fall to 50–85 per cent of 2000 levels by 2050’. This could limit global mean temperature increases to 2°C to 2.4°C above pre-industrial levels—an improvement on the range of 2.8°C to 3.2°C that is predicted to occur if GHGs stabilise below 590 ppm.

Policies are needed both to reduce anthropogenic GHG emissions and to deal with the impacts of climate change. See separate brief for analysis of the options.