Geoscience Reference
In-Depth Information
potential to offer benefits now and lay the foundation for addressing projected
changes. Actions that range from incremental steps to transformational changes
are essential for reducing risk from climate extremes.
Projected changes in Australian extreme weather
Projected changes in extreme events at the regional scale are inherently difficult
to simulate in global climate models. Finer resolution regional climate models
with a better representation of climate processes are usually required to improve
the reliability of such projections.
There is limited information about projected changes in extreme weather over
Australia for a Four Degree World. Most information is available for changes
associated with less than 4°C global warming because (a) projections tend to
be based on simulations for low to medium and high emission scenarios (A2,
A1B and B1), rather than very high emissions (A1FI), and (b) many projections
were focussed on the period 2030 to 2070, rather than 2090 and beyond. In the
following discussion, the emission scenarios and years are noted, along with the
approximate magnitude of global warming.
Extreme temperatures
Changes in the number of days over 35°C have been estimated for 15 sites for
2030 and 2070 (CSIRO and BoM, 2007). The warmest scenario considered was
for A1FI emissions in 2070, which has a global warming range of 1.74 to 4.64°C.
The upper end of this global warming range gives an annual mean warming of
4-5°C over most of Australia, reaching 5-6°C in central Australia (CSIRO and
BoM, 2007, Appendix A). Under these conditions, the present (1971-2000)
average annual number of days over 35°C increases substantially by 2070, e.g.
Sydney increases from 3.5 days at present to 12, Canberra from 5 to 26 days,
Melbourne from 9 to 26 days, Adelaide from 17 to 47 days, Perth from 28 to 67
days and Brisbane from 1 to 21 days (
Table 3.1
). Large decreases in extremely
cold days are also simulated (CSIRO and BoM, 2007).
Pitman and Perkins (2008) assessed projected changes in the annual average
intensity and frequency of the 99th percentile of maximum temperature (the
hottest 1 per cent of events, hereafter referred to as Tmax
99
) over Australia,
based on 16 climate models. At each grid-cell, only climate models that
simulated twentieth century climate variability with relative accuracy were
considered. Projections included the high (A2) emission scenario in 2100, which
has a corresponding global warming of 2.0-5.4°C (IPCC, 2007).
The intensity of Tmax
99
increases by 3-4°C over most of northern Australia,
4-5°C over most of southern Australia and more than 5.0°C along the south
coast and southeast coast. The average frequency of Tmax
99
increases from
4 times per year at present to 5-10 times over most of southern and eastern
Australia, and 10-30 times over most of northern and western Australia.
