By BlaIr TrewIn, climatologist, National Climate Centre, Australian Bureau of Meteorology, Melbourne
The first eight months of 2005 have been Australia"s warmest such period on record. Averaged across the country, mean temperatures for the January to August period have been 1.01°C above the 1961-90 average and 0.02°C above the previous record set in 1998.
The record for a full calendar year is 0.80°C above normal, also set in 1998 (which cooled to near-normal conditions in November and December after a very warm start). Typical post-1990 conditions in the remainder of 2005 would suffice to break it. 1998"s warmth was partly in response to the very strong 1997-98 El Niño event, whereas last year and this year have seen neutral conditions in the Pacific Ocean.
Daytime maximum temperatures have been particularly high, especially in autumn, which was also very dry. Maximum temperatures are currently running 1.30°C above the 1961-90 average, well ahead of the previous record, and statewide records have also been set in NSW, South Australia, Western Australia and the Northern Territory.
Around 20 per cent of Australia has seen record maximum temperatures (see map), while about 70 per cent of the country has seen temperatures in the "very much above average" (top 10 per cent of all years) range, and no part of Australia has been significantly cooler than average.
Overnight minimum temperatures have been 0.72°C above normal, the fourthwarmest on record. This is significant in that it has happened in a year which has been much drier than normal (74 per cent of average and eighth lowest on record), particularly up until May. Dry years are normally characterised by high maximum and low minimum temperatures, as the diurnal temperature range is larger under clear skies.
The current record-holder for highest mean minimum temperatures, 1973 (1.30°C above normal), was also a year with rainfall well above normal. Most of Australia has seen above-normal minimum temperatures, although Victoria has been slightly below normal.
The most extreme month of 2005 to date was April. It was the warmest April on record over more than half of Australia, by large margins in many cases, and in terms of the temperature difference from normal (2.58°C), it was the most extreme month ever observed in Australia.
All eight months of 2005 have been warmer than normal, with August (0.25°C above) being the only month which was even near normal.
Reliable data on monthly and seasonal temperatures goes back to 1950. Annual temperatures are available back to 1910 - prior to that date, the station coverage, particularly through outback Australia, is too sparse and many observations were made using instruments that are not compatible with current standards.
The annual data suggest that, in addition to the post-1950 monthly data described above, no year in the 1910-50 period had January to August temperatures anywhere near 2005 levels. The 2002-05 period has also been much warmer than comparable previous longterm drought periods in eastern Australia, such as 1938-46, placing additional moisture stress on agriculture through increased evaporation.
Australian temperatures have warmed by about 0.7°C since 1910, with a similar warming trend in the surrounding oceans. With such a long-term warming trend, the chances of records being set in any individual year increase substantially, and more records can be expected to fall with projected warming over the next century.
Cooler-than-normal years are still not unknown (2001 was the most recent), but they are becoming increasingly rare.
For more information on australian climate variability and change: www.bom.gov.au/silo/products/cli_chg/