"All the indications show that in fact we crossed back into that negative climate state in the year 2000/01 and I think that goes some way to explaining why this El Nino is impacting us as strongly as it is," Franks, a research engineer, told Reuters.

"The longer term 15 to 20 year, 25 year outlook..., is that we would expect a return to these higher frequency and magnitude of La Ninas. And they'll only be punctuated by the occasional El Nino, which will be stronger than average."

Associated with a warming of Pacific waters, the El Nino weather event occurs every few years, bringing drought to Indonesia, Papua New Guinea and Australia and excessive rainfall to western South America.

A severe drought in Australia this year, which has slashed grains output in the major food exporter, has been compounded by the return of El Nino, albeit a weak-to-mild manifestation compared to the 1997/98 episode.

La Nina is the reverse condition, bringing flooding to Southeast Asia and Australia and dryness to South America.

During the past century, the only time comprehensive weather records are available for Australia, the period 1910-45 was relatively free of either drought or floods, Franks said.

From 1945 to 1975, the island continent went through a period of "once a century" floods and severe dryness, with drought affecting agriculture particularly hard in 1964/65.

From 1975 until 2000, the weather had again been relatively stable.

But that appeared to have changed with the current drought and the next quarter century, if the historical weather patterns held, could be marked by enhanced El Ninos alternating with unusually frequent and powerful La Ninas bringing heavy rain.

As Australian farmers often complain, "if they're not in drought they're in flood", Franks said.