Detecting, Understanding and Attributing Climate Change

A background report on research priorities

Prepared for the Australian Greenhouse Office by
Neville Nicholls, School of Geography and Environmental Science, Monash University, 2006

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Executive summary

Climate has always varied, since well before humans evolved. Now, however, we are changing the climate. Climate change is defined as a change that can be identified (e.g. using statistical tests) by changes in the mean and/or the variability of one or more climate variables (e.g. rainfall, temperature) that persists for, typically, decades or longer.

Societal responses to climate change can be improved if we can determine the cause of the change. This requires ‘climate change detection and attribution’ studies. Detection is the process of demonstrating that climate has changed in some defined statistical sense, without providing a reason for that change. A change is detected in observations if its likelihood of occurrence by random chance due to internal, natural climate variability alone is small. Attribution is the process of establishing the most likely causes for the detected change with some defined level of confidence.

Evidence of human influence on the global climate has accumulated steadily during the past two decades. A recent review of such global studies concluded that: “Externally driven climate change has been detected by a number of investigators in independent data covering many parts of the climate system, including surface temperature on global and large regional scales, ocean heat content, atmospheric circulation, and variables of the free atmosphere, such as atmospheric temperature and tropopause height.”

The major Australian climate trends observed over the past 50 years or so are:

Mean maximum (day time) temperature has increased over most of Australia, with cooling in the north-west (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 north-west (in all seasons except spring, but the location of the cooling varies somewhat between seasons).
Annual rainfall has increased in the north-west (in summer), and decreased in the south-west (in winter) and along and inland from the east coast (Queensland in summer; New South Wales and Victoria in autumn and winter).

Detection and attribution studies of Australian climate indicate that:

The widespread warming is very likely to be due to increased greenhouse gas concentrations in the atmosphere.
The rainfall decrease in south-west 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 north-west Australia may be due to increased aerosol particles in the atmosphere resulting from human activity, especially in Asia.

The highest priorities for new detection and attribution studies are, in order:

The decline in east coast and south-east rainfall.
Changes in atmospheric circulation across the Australian region.
Possible changes in drought frequency and/or intensity.
Possible changes in tropical cyclone frequency and/or intensity.
Changes in extreme temperatures.
Ocean changes around Australia (temperature, sea level, circulation, chemistry).