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Climate index: summer brings lower highs but higher lows

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The highest summer temperatures recorded in Australia this year were below average for the first time since 2012, the latest Australian Actuaries Climate Index reveals, but an increase in the lowest temperatures indicated ongoing climate warming.

Australia’s summer was mild with moderate temperatures and unusually light levels of wind. The overall index, which is updated each quarter, showed the lowest value since 2016.

That was in stark contrast to several consecutive summers of extreme weather where high temperatures reached records and contributed to significant damage from bushfires.

“Even in the absence of extreme weather, the index provides valuable insight, such as the low temperature index showing continuing warming despite apparently mild conditions,” Actuaries Institute President Jefferson Gibbs said.

Low temperatures have long exhibited an increasing trend, with 34 of the last 35 seasons bringing a rise in the coldest temperatures recorded. In the Southern Slopes region of Victoria, which includes Melbourne, a new record was reached on the low temperature index this summer.

“While this may be experienced as benign or even pleasant weather and does not result in immediate damage, it indicates that the climate in Australia continues to warm,” the Actuaries Institute says.

Extreme wind gusts were the third lowest on record for Australia as a whole during summer while the Murray Basin and South and South Western Flatlands (East) clusters, which cover parts of SA, Victoria and NSW, experienced the lowest extreme wind gusts index value ever.

A global trend towards decreased wind speeds, known as terrestrial stilling, is thought to be due to changes in large scale atmospheric circulation and an increase in surface roughness. In the long term it may affect wind power generation, while possibly reducing the risk of wind damage to property.

The index shows changes in the frequency, or rate of occurrence, of extreme high and low temperatures, heavy precipitation, dry days, strong winds and changes in sea levels across 12 regions that are climatically similar.

Each season is compared to the same season in previous years and against a reference period of 1981-2010.

Last summer, an above reference-period rainfall average was recorded for seven of the 12 clusters across the country, though no new records were set in December, January or February as extreme rain was less frequent than in previous summers.

The index is designed to help inform actuaries, policymakers, companies and the general public about climate trends in Australia.

The focus is on the extremes, which are related to risks such as inland and coastal flooding, cyclones, drought, and heatwaves.

“The indices are intended to add to our understanding of how certain risks may be changing as a result of climate change,” the Institute says.

For the high temperature, the index measures the change in the proportion of days in a month on which the maximum and minimum temperatures exceed the 99th percentile of the reference period distribution for the relevant day. The results for the maximum and minimum temperatures are then averaged.

The low temperature measure is calculated using a similar process by looking at the frequency above the 1st percentile.

Both the high and low temperature components therefore record if there is a warming in temperature.