WEATHER extremes, including the Victorian and Queensland floods, were not caused by global warming, one of Australia's most eminent climate scientists says.

Prof Neville Nicholls told an international earth science conference in Melbourne that extremes such as the heatwave that accompanied the Black Saturday bushfires, similar heat in Pakistan and Russia, and the devastating tornados that have ripped through parts of the US this year, are, in many cases, unprecedented in modern times.

But global warming couldn't be blamed, he said.

"Global warming doesn't produce these events. However, it's pretty hard to avoid the conclusion that global warming has exacerbated the frequency and the intensity of these heatwaves," the Monash University expert said.

"Whenever these things happen, people ask: 'Was it caused by global warming?' " Prof Nicholls told the International Union of Geodesy and Geophysics conference.

"The short answer is no.

"They were all caused by well-known and reasonably well-understood weather and climate events, even with some predictability."

While global warming didn't cause the weather events, the professor acknowledged its part in making some of them more severe. But this year's floods do not fit into that picture.

"It is much harder to make the connection to link those floods in Queensland in early 2011 to global warming," he said.

The culprit was a record-breaking version of La Nina.

"This is the largest recorded La Nina event seen in 120 years of recorded history," Prof Nicholls said.

His observations draw the inevitable question as to whether global warming has in turn caused the latest version of La Nina to be especially strong.

The answer to that evades Prof Nicholls. "We don't really know," he said.

Originally published as Bad weather link to global warming denied