Get used to flash flooding, scientists say

As Australia’s east coast is ravaged by storms, University of New South Wales engineers have got some bad news — due to climate change, flash flooding from such events is becoming more likely.

In research published in the journal Geophysical Research Letters, Professor Ashish Sharma and Conrad Wasko analysed data from 1300 rain gauges and 1700 temperature stations across Australia to see how air temperature affected the intensity and spatial organisation of storms. They found that atmospheric moisture was more concentrated near the storm’s centre in warm storms than in cooler ones, resulting in more intense peak rainfalls in those areas.

“As warming proceeds, storms are shrinking in space and in time,” explained Wasko. “They are becoming more concentrated over a smaller area, and the rainfall is coming down more plentifully and with more intensity over a shorter period of time. When the storm shrinks to that extent, you have a huge amount of rain coming down over a smaller area.”

Wasko said scientists have long suspected that the intensity of rainfall would be boosted by climate change, as the warming air raises the carrying capacity of moisture. But while extreme rainfall has been rising, little was known about the mechanisms causing it. The study shows that storms are changing in spatial terms.

This Australian data has global implications, said Professor Sharma, as Australia is a continent that “spans almost all the climate zones in the world — Mediterranean, tropical, temperate, subtropical — everything except the Arctic or Antarctic. So the results hold a lot of value — we are finding the pattern repeating itself over and over, happening around Australia and around the world.

“Look at the incidents of flooding in Mumbai or in Bangkok last year — you see urban streets full of water,” he said. “You see it now in Jakarta and Rome and many parts of Canada. That’s because the stormwater infrastructure cannot handle the rain, and part of the reason there’s more rain is the increase in global temperatures.

“The increase is especially noticeable in urban centres, where there is less soil, unlike rural areas, to act as a dampener. So there is often nowhere else for the water to go, and the drainage capacity is overwhelmed. So the incidence of flooding is going to rise as temperatures go higher.”

In 2015, the authors released another study in Nature Geoscience showing that storms were also changing their ‘temporal pattern’ — that is, getting shorter in time, thereby intensifying. If both spatial and temporal changes in storms continue, there will be more destructive flooding across the world’s major urban centres, said Professor Sharma — and there are still unknowns to contend with.

“When we say that the storms are shrinking in space and shrinking in time, and we say floods will increase, we are making an assumption that the volume of water coming down is not changing,” said Professor Sharma. “That assumption is very conservative, because you would expect the air to hold more moisture. If you factor that in as well, there’ll be even more rainfall and more floods.”

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