Be prepared for more ‘extreme’ rainfalls — think biblical floods — U.S. climate scientists warn

 

A huge thunderstorm strikes Las Vegas in 2015. The reason for the expected heavier rainfalls is simple physics: warm air can hold more moisture than cold air.

Residents dig a car out of a driveway as a record snowfall fell for the second day in a row in London, Ont., Dec. 7, 2010.

Stranded passengers are rescued from a flooded GO Train following a massive rainstorm in Toronto, July 8, 2013.

A GO Train is stranded on flooded tracks following a massive rainstorm in Toronto, July 8, 2013.

Vehicles stranded by floods that followed heavy rain in Lacombe, Alta., on July 11, 2011.

 

In a research paper that reads like it was written by a horseman of the apocalypse, U.S. climate scientists are predicting more frequent extreme rainfalls, the kind of downpours that cause flooding, landslides and massive infrastructure damage, especially in cities where paved ground cannot absorb water.

The same goes for snowstorms and ice storms, according to the team from the National Center for Atmospheric Research, which also found the expected frequency of these “extreme precipitation events” increases five-fold in large parts of Canada and the western United States in the coldest winter months. The effect is dramatically lessened south of the 30th parallel, which runs through Houston, Texas.

The reason is simple physics, playing out in the chaos of continental weather patterns. Warm air can hold more moisture than cold air. That is why storms from the Arctic, or the famed Alberta clippers, tend to be windy but dry, while the ones that come up from the southern United States are warm and full of precipitation, and can become ice storms if they run into a cold front.

If climate change raises atmospheric temperature by one degree, the air can hold about 12 per cent more moisture. According to the study, this same increase would cause precipitation intensities to increase by about seven per cent.

This has not been directly observed in the wild, but it is solidly predicted in theory, and by the computer simulations based on 13 years of data that form the basis of this latest work. It adds up to what the authors call “a significant increase of flash flood hazards due to climate change.”

“Our results indicate that there is a clear need to increase societal resilience to short-term precipitation extremes,” they write in Nature Climate Change, published this week. “The increase in the intensity and frequency of extreme precipitation would cause major challenges for existing infrastructure systems.”

Environment Canada’s senior meteorologist David Phillips said the main effect of this increasingly moist warm air is that when it rains, more falls in less time, as in the kind of deluge Phillips called a “Texas gullywasher.”

One major recent example was Toronto’s 2013 rainstorm, which dropped more rain in two hours than the famously destructive Hurricane Hazel did in a whole day, while some of the city’s eastern and western suburbs got hardly anything. It was the most expensive weather disaster in Ontario’s history.

Canada is second coldest and first snowiest country in the world, so it is hard to convince Canadians that climate change “has some evil to it,” Phillips said. Many simply look fondly to a future of milder winters and longer summers. A bit of flooding might seem like a small price to pay.

But as he described it, this is not your grandfather’s flood, the classic river backup caused by melt water and ice blockage. It is a modern artifact of a highly urbanized society in an era of warming climate, when a single flooded road can leave people stranded and at risk of death.

“That’s the kind of thing that scares the bejeebies out of insurance people,” he said. “You could have floodier floods and droughtier droughts.”

Andreas Prein, the lead author of the study, told the New York Times there was some variation depending on region, and that some areas would not be as affected by these types of storms. For instance, while the Pacific Northwest is likely to see more intense winter storms, summer thunderstorms will probably not increase in intensity, he said.

Prein said the results of this study showed that many areas of the country need to build or restore infrastructure, including reservoirs and storm sewers, to handle the intense rainfall.

One area of particular concern is the Midwest, which is already becoming drier on average, especially in the summer, Prein said. However, he added, “the extreme events there are getting more frequent and more intense.”

It might seem as though big rainstorms would help alleviate drought, but the opposite can actually be true, Prein said. If a heavy rainstorm occurs during a drought, it can lead to soil erosion, washing away the plants that help the soil absorb moisture.

Without regular, gentler rain to restore the soil, he said, conditions will continue to worsen.

“Droughts and extreme rainstorms? This could be a very harmful combination,” Prein said.