Detecting and attributing Australian climate change: A review

Abstract

In climate change science, detection is the demonstration that climate has changed in some defined statistical sense. A change is detected in observations if its likelihood of occurrence by random chance due to internal, natural climate variability is small. Attribution is the process of establishing the most likely causes for the detected change with some defined level of confidence. Evidence of a human influence on the global climate has accumulated steadily during the past two decades, based on such detection and attribution studies. This paper is a review of detection and attribution studies of Australian climate trends. The major Australian climate trends observed over the past 50 years or so are: • Mean maximum (daytime) temperature has increased over most of Australia, with cooling in the northwest (very strong in summer) and along the south coast of Western Australia (in most seasons). • Mean minimum (night-time) temperature has increased over nearly all of the country except for cooling in some parts in the inland northwest (in all seasons except spring, although the location of the cooling varies between seasons). • Annual rainfall has increased in the northwest (a summer phenomenon), decreased in the southwest (a winter phenomenon) and along and inland from the east coast (Queensland in summer; New South Wales in winter). • Pan evaporation has declined about three per cent since the mid-1970s. Detection and attribution studies of Australian climate indicate that: • The widespread warming is very likely to be due to increased greenhouse gas concentrations. • The rainfall decrease in southwest Western Australia is likely due to a combination of increased greenhouse gas concentrations, natural climate variability and land-use change. • The increased summer rainfall in northwest Australia may be due to increased aerosols resulting from human activity, especially in Asia. • The apparent decline in pan evaporation is mainly due to changes in instrumental exposure. • No study has attributed a cause to the rainfall decrease along the east coast. The highest priority for new detection and attribution studies would appear to be the decline in east coast rainfall, because of the large population and high economic value of this region, the dearth of relevant studies, and the magnitude of the apparent change. A more comprehensive, Australia-wide, formal detection and attribution study to determine how firmly we can conclude that human activity has affected Australian rainfall in general, is also a high priority.