Flooding, rainfall extremes in cities to rise as climate changes

Most urban centers have older stormwater infrastructure designed to handle rainfall patterns of the past, which are no longer sufficient. “The increase is especially noticeable in urban centers, where there is less soil, unlike rural areas, to act as a dampener,” said Sharma. “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.”

Wasko, lead author of the paper, 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 latest study shows that storms are changing in spatial terms.

It follows a study by the same authors in Nature Geoscience in June 2015 showing that storms were also changing their ‘temporal pattern’ - that is, getting shorter in time, thereby intensifying. When it comes to flash flooding, the amount of rain that falls over a period of time is much more important than the total volume of rainfall that a given storm delivers. This study was the first to show that climate change was disrupting the temporal rainfall patterns within storms themselves.

If both spatial and temporal changes in storms continue, as they are likely to do as the world warms, there will be more destructive flooding across the world’s major urban centers.

In their Nature Geoscience paper, the duo calculated that floods in some parts of Australia would likely increase by 40 percent, especially in warmer places like Darwin. “If you add the spatial pattern from this latest paper, you will probably increase this 40 percent number to maybe 60 percent,” said Sharma.

Earlier this year, a pivotal framework for infrastructure maintained by the Institution of Engineers, the Australian Rainfall and Runoff national guidelines, were updated for the first time since 1987, a process that took three years. It is now clear, said Sharma, that these will need to be adjusted, as the safety and sustainability of Australian infrastructure adapts to a warming climate.

And there are still unknowns to contend with, he added.

“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 Sharma. “That assumption is very conservative, because you would expect the air to hold more moisture. If you factor in that in as well, there’ll be even more rainfall, and more floods.”

— Read more in Conrad Wasko et al., “Reduced spatial extent of extreme storms at higher temperatures,” Geophysical Research Letters (25 April 2016) (DOI: 10.1002/2016GL068509)