Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and in land surface properties alter the energy balance of the climate system.
These changes are expressed in terms of radiative forcing, which is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system.
Global atmospheric concentrations of carbon dioxide (the most important anthropogenic greenhouse gas), methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture.
The understanding of anthropogenic warming and cooling influences on climate leads to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming.
Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. At continental, regional and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves and the intensity of tropical cyclones.
Observational evidence shows that many natural systems are being affected by regional climate changes, particularly temperature increases, and a global assessment of data shows that it is likely that anthropogenic warming has had a discernible influence on many physical and biological systems, although many effects on natural and human environments are difficult to discern due to adaptation and non-climatic drivers. Not all impacts are negative - Beneficial impacts in northern polar regions would include reduced heating costs and more navigable sea routes.
More specific information is now available across a wide range of systems, sectors and regions concerning the nature of future impacts, for example, in Australia:
o As a result of reduced precipitation and increased evaporation, water security problems are projected to intensify by 2030 in southern and eastern parts of the country;
o Significant loss of biodiversity is projected to occur by 2020 in some ecologically-rich sites including the Great Barrier Reef, the Queensland wet tropics, the Kakadu wetlands, southwest Australia, subantarctic islands and alpine areas;
o Ongoing coastal development, and population growth in areas such as Cairns and southeast Queensland, are projected to exacerbate risks from sea-level rise and increases in the severity and frequency of storms and coastal flooding by 2050;
o Production from agriculture and forestry by 2030 is projected to decline over much of southern and eastern Australia due to increased drought and fire; and,
o Although the region has substantial adaptive capacity due to well-developed economies and scientific and technical capabilities, there are considerable constraints to implementation and major challenges from changes in extreme events, and natural systems have limited adaptive capacity.
Magnitudes of impact can be estimated systematically for a range of possible increases in global average temperature. Anticipated impacts due to altered frequencies and intensities of extreme weather, climate and sea level events are very likely to change, and some large scale climate events have the potential to cause very large impacts, especially after the 21st century (for example, substantial sea level rises from widespread deglaciation of ice sheets). Climate changes are very likely to impose net annual costs, which will increase over time.
Mitigation technologies and practices are currently commercially available in a range of sectors. For example, in the Energy Supply sector, there is the potential for improved supply and distribution efficiency, fuel switching from coal to gas, nuclear power, renewable heat and power (hydropower, solar, wind, geothermal and bioenergy), and early applications of carbon capture and storage. In the Transport sector, there is the potential for more fuel efficient vehicles, hybrid vehicles, cleaner diesel vehicles, biofuels, shifts from road transport to rail and public transport, non-motorised transport (cycling, walking), as well as land-use and transport planning. Mitigation technologies and practices are also currently commercially available in the Buildings, Industry, Agriculture, Forestry and Waste sectors, and new mitigation technologies (currently under development) are projected to be commercialised in all of these sectors before 2030. Health benefits from reduced air pollution, as a result of actions to reduce greenhouse gas emissions, can be substantial and may offset a portion of the mitigation costs.
A wide variety of national policies and instruments are available to governments to create incentives for mitigation action. For example:
o Regulations and standards generally provide some certainty about emission levels, but may not induce innovations and more advanced technologies;
o Taxes and charges can set a price for carbon, but cannot guarantee a particular level of emissions, whilst tradeable permits will establish a carbon price, but fluctuations in that price make it difficult to estimate the total cost of complying with emission permits.
o Financial incentives (subsidies and tax credits) stimulate the development of new technologies, but economic costs are generally higher than for the foregoing instruments.
o Voluntary agreements between industry and governments are politically attractive, but the majority of such agreements have not achieved significant emissions reductions.
Policies that provide a real or implicit price for carbon could create incentives for producers and consumers to significantly invest in low greenhouse gas emission products, technologies and processes.
Supplementary URL: http://www.bom.gov.au