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Climate Variability and Change (continued)

Findings and implications

A global average warming of about 0.6C has occurred since 1860. Observed mean surface air temperature over Australia has increased by 0.76C from 1910 to 1999, with most of this increase occurring after 1950. Minimum temperatures have increased more than maximum temperatures during this period (0.96C versus 0.56C), giving a decrease in the day-to-night (diurnal) temperature range.

The frequency of extreme warm days and nights in Australia has increased while that of extreme cool days and nights has decreased over the second half of the 20th century. More significant trends are associated with increases in minimum temperatures. For areas affected by frosts, the annual number of frost days declined by an average of 5.6 from 1957 to 1996 and the average length of the frost season shortened by around 40 days. Such an increase in hot days and a reduction in cool days could lead to greater heat stress on humans, livestock, ecosystems, agriculture and building materials, increased frequency of bushfire, higher energy demand for air-conditioning and increased demand on water supply.

Concentrations of greenhouse gases have risen significantly during the 1900s. Future increases in these concentrations are likely to lead to global warming and associated climate change. Model simulations of global climate indicate that the average temperature of the earth will increase between 1.4 and 5.8C between 1900 and 2100. This represents a warming rate of 0.1 to 0.5C per decade. Despite substantial efforts in the past few years to reduce both emissions and land clearing, Australia's net greenhouse gas emissions have grown since 1990. Australia's total emissions are small compared with those of other OECD countries, although per capita emissions are high compared to other countries. New programs targeting renewable energy and large-scale greenhouse gas abatement should constrain further growth in emissions.

Most of Australia is likely to warm by 0.4 to 2.0C by 2030 and by 1.1 to 6.0C by 2070. Increases in temperature, changes in rainfall (predominantly decreases) and increases in moisture stress are likely to be the result of global warming.

An increase in tropospheric temperature and a decrease in stratospheric temperature are consistent with simulations of several models of global climate under increased greenhouse gases. However, large uncertainties are associated with the observational record of upper-air temperature and satellite measurements and also with model simulations. Continued monitoring of surface, tropospheric and stratospheric temperature with ground-based thermometers, balloons and satellites can give better estimates of global temperature change.

Australian rainfall has increased slightly in the 1900s in most regions. Heavy rainfall events have also increased. Floods associated with an increase in heavy rainfall events could have a significant effect on agriculture, industries and infrastructure if this trend continues. There is also evidence of an ocean basin-wide Interdecadal Pacific Oscillation that contributes to decadal climate variability. Increases in temperature and floods associated with heavy rainfall events on decadal and century time scales can have significant effects on the savannah ecosystem in tropical Australia. Some animal and plant species may come under increasing stress, causing long-term change in species composition.

Floods and droughts in Australia are closely associated with ENSO variations. Any changes to the characteristics of ENSO under enhanced greenhouse conditions will have major economic effects. However, the direction of change to ENSO under climate change is not yet clear.

No significant trends in cyclone numbers and intensities have been observed in the Australian region. Some climate models project increases in the intensities of tropical cyclones and other extreme events as a result of global warming.

Changing frequencies of drought and fire associated with higher temperatures could have increasing effects on agriculture, forestry and biodiversity. Climate change will affect (both positively and negatively) the recent trends of increasing crop yield. Considerable changes in management may be required to minimise cost, maximise benefits and ensure sustainability.

The strong link between ENSO and sea level on the interannual time scale suggests that any changes to ENSO behaviour under conditions of climate change could have substantially affect the coastal community and urban infrastructure. Sea level rise has strong implications for the sustainable development of planned infrastructure including coastal development, ports, bridges and urban centres. Continued monitoring of climate and sea level around Australia and in the Pacific is essential for future estimates of sea level rise and ENSO-related effects.

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