Global warming has taken place over the last century, and there is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. It is likely that the 1990s was the warmest decade in the last 1000 years, at least in the Northern Hemisphere. Other observations are consistent with this observed warming, including a rise in global average sea level and ocean heat content, and decreases in snow cover and ice extent both in mountain glaciers and Arctic sea ice. Recent evidence suggests that a predicted slow-down in the deep ocean circulation driven by variations in temperature and salinity may also be occurring.
Statistically significant associations between increases in regional temperatures and observed changes in physical and biological systems have been documented in freshwater, terrestrial, and marine environments on most continents. Surface and satellite-based observations support this view.
Global average sea level is projected to rise by 2100 within a range 9 to 88 cm, about half by thermal expansion of sea water and about one quarter from melting of glaciers.
Australian average temperatures have risen by 0.7 ºC over the last century. Rainfall has increased over the last 50 years over north-western Australia, but decreased in the southwest of Western Australia, and in much of south-eastern Australia, especially in winter. The changes are consistent with an observed increase in mean sea level pressure over much of southern Australia in winter. Effects on runoff are potentially serious as evidenced by a 50% drop in water supply to the reservoirs supplying Perth since the 1970s and near-record low water levels in storages in much of south-eastern Australia in 2002-03 due to low rainfall and high temperatures in the south-east since 1996.
Attribution of the rainfall changes is under lively discussion within the scientific community. In the case of the south-west of Western Australia,...