Why slower cyclones are bad news for us

INTERNATIONAL weather research has revealed Australia has some of the slackest tropical cyclones.

But contrary to what you might think that means, the finding is actually a bad thing.

Worldwide cyclones have become sluggish, slowing down 10 per cent over the past 70 years.

In Australia the figure is even higher being a "significant" 19 per cent slower.

The study published in Nature this week by US weather experts shows the problem weather systems are taking longer to travel across the planet.

This means the more time they spend above land, the more devastation they can wreak with rainfall and storm-induced damages.

The authors say the slowdown is likely to contribute to worsening destruction alongside the associated increase in rain rates caused by global warming.

They say while global warming is projected to increase the severity of the strongest tropical cyclones, warming may bring other more serious effects such as the general weakening of summertime tropical atmospheric circulation.

In a warming world where atmospheric circulations are expected to change, the atmospheric circulation that drives tropical cyclone movement is expected to weaken.

Scientist James Kossin said as little as a 10 per cent slowdown could double local rainfall and flooding impacts caused by 1C of warming.

Tropical cyclones have generally slowed more in the Northern Hemisphere where they are also known as hurricanes and typhoons and where more of these storms typically occur each year.

Kossin said the findings were of great importance to society.

"These trends are almost certainly increasing local rainfall totals and freshwater flooding, which is associated with very high mortality risk," he said.

He said Hurricane Harvey in Texas last year was a dramatic example of the consequences of a slow-moving or "stalled" tropical cyclone.

"The observed 10 per cent global slowdown occurred in a period when the planet warmed by

0.5C, but this does not provide a true measure of climate sensitivity, and more study is needed

to determine how much more slowing will occur with continued warming," he said.

"Still, it's entirely plausible that local rainfall increases could actually be dominated by this slowdown rather than the expected rain-rate increases due to global warming."