La Nina, global warming to blame for floods

Cyclists ride down flooded Montague Rd in West End, Brisbane. Photo / Getty Images

Experts blame a combination of a La Nina weather pattern and global warming for the magnitude of the Queensland flood disaster.

One scientist warns the catastrophe is only the start of things to come, saying what are described now as one-in-100-year floods could arrive every 20 years.

The La Nina effect, the inverse of the drought-inducing El Nino effect, results in higher than average sea temperatures in the Pacific Ocean leading to heavy rain.

Professor Will Steffen, executive director of the Australian National University's (ANU) Climate Change Institute, says it is likely the floods are climate change related.

"What we can say about the Queensland floods is there is a strong La Nina, which tends to give this heavy rainfall, but in addition to that there are very high sea surface temperatures."

Professor Matthew England, joint director of the Climate Change Research Centre at the University of NSW, says the temperatures are the highest ever recorded.

Rising sea temperatures, especially in northern Australia, are a key part of the climate system, says Prof England.

"Climate change has seen a warming of waters globally, and the waters north of Australia are an important part of the climate system for Australia's monsoon rains.

"They are at their warmest ever measured and we cannot exclude climate change from contributing to this warmth, (and) if it is very warm there this enhances evaporation into the atmosphere, creating moist air."

Prof Steffen agrees the temperature rise is a climate change phenomenon.

Sea temperatures have been rising for years, he says.

He cites a study in the US that looks at rainfall in a heavily saturated area over the past 100 years.

"(In the study) there's been a significant increase (in rain in the area) since 1980 consistent with a strong warming," Prof Steffen says.

The study shows the the effects of warming will make flooding of the type that has devastated parts of Queensland more common.

"There's definitely a risk and a growing risk that events of this type will become more frequent as the climate warms," Prof Steffen says.

"One-in-100-year events would become a one-in-20 or one-in-30-year event as the climate shifts ... we say with some confidence they are becoming more frequent and they will become more frequent in future."

Prof England says that climate change projections point to extreme weather becoming more common, but it is hard to know how much flooding Australia could get.

"Climate change projections are pointing to more frequent extreme events, that's to say more flooding events, more droughts and fires, but whether Australia as a nation sees many more flooding events or not is still a little bit more complex to pin down," he says.

But not all experts agree that global warming is a factor.

Environmental science Professor Neville Nicholls from Monash University believes the Queensland floods are not due to climate change but purely a result of La Nina.

"The main reason we're seeing this heavy rain is just this incredibly strong La Nina, and that's almost certainly a natural part of climate variability," he says.

Prof Nicholls says the evidence is inconclusive about the effect of global warming on the La Nina phenomenon.

"The question is, is it exacerbated by climate change or global warming? At the moment, we just can't say. No one has done the studies yet," he says.

"You would have to think the warming we've seen - about half a degree in the last 30 or 40 years - should have had some influence on this event, but we can't tell you reliably or credibly what that influence is."

- AAP