Farmer Steve Osborn cooling off at his pumpkin farm in Maitland, NSW. Credit:Peter Stoop "The increase in hot records is likely to be related to the warming from greenhouse gases," Dr Lewis, a research fellow at the Australian National University, said. Given the level of gases such as carbon dioxide and methane are going to rise further, "in the future, it's likely we'll see an increase in [the hot-cold record] ratio", she said. The ratio of temperature extremes was more marked during the cooler seasons, with a faster drop of record cold days than at other times of the year. Night-time warm temperature records were also being broken more frequently than daytime ones, the researchers found.

The beach. Credit:Wayne Taylor Heat on cue The paper's release this week came as Perth on Wednesday had its earliest day of 31 degrees or warmer in data going back to 1897, the Bureau of Meteorology said. The 32.1 degrees beat the previous date for such heat by nine days, said Glenn Cook, senior climate liaison officer at the bureau. The nearest equivalent was 32.3 degrees recorded in Perth on September 18, 1988. Dr Lewis said the past 15 years had shown an acceleration of hot day records even with more La Nina events, which tended to favour cooler conditions.

"The only year in the past 15 years to produce more cold records than warm records was 2011, which was impacted by two La Nina events at the beginning and end of the year that brought extensive rainfall," Dr King, from Melbourne University's School of Earth Sciences, said. "However, the dominance of cold records in 2011 was not caused by an increased number of cold records, which were around average, but by the lack of heat records for that one year," he said. One direction When the models removed the increase in carbon dioxide and other greenhouse gases as a factor, the ratio of hot days and cold ones was basically equal, Dr Lewis said. David Karoly, a climatologist at Melbourne University, said the data showed an increase in the frequency of extremes - mostly hot ones.

"That shifting climate is clearly in one direction - and that's to a warmer climate," Professor Karoly said. According to the Bureau of Meteorology, almost all of southern Australia had its hottest temperatures over any 15-year contiguous period on record during 2000-2014: Globally, the last record cold year was in 1909, with the record hottest year broken 18 times since then, including in 2014. With the powerful El Nino now dominant in the Pacific, 2015 and 2016 may both raise the bar for record annual heat, climatologists say. "If this trend continues with the continuing rise in greenhouse gases, there may come a time when the chances of a new record cold year being set in Australia will be close to zero," Dr Lewis said.