Observed Changes in Australian Climate and Ecosystems

Australian average temperatures have risen by 0.7 ºC over the last century, and the warming trend appears to have emerged from the background of natural climate variability in the second half of the 20th century. Rainfall has increased over the last 50 years over northwestern 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, a combination of natural variability and a trend due to the enhanced greenhouse effect is considered to be the likely cause, although recent papers suggest that stratospheric ozone depletion may also be causing a southward shift of the westerlies and associated rainfall systems. Northern Hemisphere aerosol effects may also have played a part via changes to atmospheric dynamics, but as aerosol lifetimes in the atmosphere are short and precursor sulfur emissions are being curtailed, this effect is probably of diminishing importance. If rainfall decreases are due to anthropogenic effects they may well continue, necessitating "informed adaptation" to a reduced water supply.

It is at least as difficult, with the current state of knowledge, to attribute changes in Australian ecosystems to climate change, as other local causes are possible in many cases. However, a number of observed changes in vegetation, wetlands, terrestrial vertebrates, marine birds and coral reefs are consistent with regional warming trends.