Manhattan on Wednesday evening. Central Park recorded 8cm of rain in a single hour, a record. Photo / Stephanie Keith, The New York Times
Because of global warming, the heaviest storms can now produce huge amounts of rainfall in a short time.
The torrential rains Wednesday that deluged New York and New Jersey, killing more than two dozen, carried a stark warning about climate change: As the planet gets hotter, heavy rainstorms are dumping more water than ever before, threatening to devastate unprepared cities.
Climate scientists have long predicted that global warming would make certain parts of the world wetter overall, in part because a warmer atmosphere can hold more moisture. But simply looking at global averages can obscure a more important reality: The heaviest rainstorms are now more intense and can produce vastly more rainfall in short periods of time. Those extreme events are what can drive catastrophic flooding.
"Storm intensity is increasing much faster than the average change in precipitation," said Aiguo Dai, a professor of atmospheric science at the University at Albany, State University of New York. "And it's the intensity that really matters, because that's what we design our infrastructure to handle."
As the remnants of Hurricane Ida swept over New York City, Central Park recorded 3.15 inches (8cm) of rain in a single hour Wednesday night, smashing the previous one-hour record of 1.94 inches (4.9cm) set August 21 during Tropical Storm Henri. The sudden burst of rain paralysed the city, with cascades of water pouring into subway stations and shutting down much of the system for hours.
Across the continental United States, the heaviest downpours have become more frequent and severe in recent decades, according to the federal government's National Climate Assessment. In the Northeast, the strongest 1 per cent of storms now produce 55 per cent more rainfall than they did in the middle of the 20th century.
"There's a lot of fluctuation year to year, but over a longer period of time, the trend is becoming increasingly evident," Dai said. "This is exactly what both theory and climate models predicted."
Other parts of the world are also struggling with increasingly vicious downpours. In July, unusually heavy rains in Germany and Belgium caused rivers to burst their banks, washing away buildings and killing at least 230 people. That same month, days of torrential rain in Zhengzhou, China, submerged the city's subway system and caused at least 300 deaths in the region.
While scientists cannot always predict exactly when and where such rainstorms will occur, they understand how global warming is making them stronger. As temperatures rise, more water evaporates into the air from the oceans and land. And, for every 1 degree Celsius of global warming, the atmosphere can hold roughly 7 per cent more water vapour.
That means when a rainstorm does form, there is more water that can fall to the ground, sometimes within a very short period. Recent studies have detected an increase in hourly rainfall extremes in parts of the United States, Europe, Australia and Asia.
And if the planet keeps getting hotter, the threat of more intense rainfall will grow. Earth has already warmed roughly 1.1 degrees Celsius since preindustrial times, driven by greenhouse gas emissions from the burning of fossil fuels and deforestation. Without swift action to reduce those emissions, a recent report from the UN's Intergovernmental Panel on Climate Change warned, the planet could warm twice that amount or more.
That report explored the consequences for heavy rainstorms. Consider a severe rainstorm that, in the past, might have occurred just once a decade, on average. Today, that same storm is now 30 per cent more likely to occur and produce 6.7 per cent more rainfall, on average. If total global warming reaches 2 degrees Celsius, that same storm will produce 14 per cent more rainfall. The report predicted that heavy precipitation and flooding would very likely become more frequent across North America, Europe, Africa and Asia as temperatures rise.
More rain can often be a blessing for drinking water supplies and agriculture, as the Western US, which is grappling with a record drought, knows well. But too much of it coming down all at once can also have devastating impacts.
In Tennessee last month, intense thunderstorms caused rivers and creeks to quickly overflow, flooding homes and killing at least 22 people. In California this year, portions of Highway 1 collapsed into the Pacific Ocean after heavy rains unleashed torrents of mud and debris. In the Midwest in 2019, unrelenting downpours destroyed crops, stripped away topsoil and forced farmers to delay their plantings.
Whether a heavy rainstorm leads to destructive flooding, however, depends on a combination of factors: the amount of rainfall, the way that water flows and collects on the landscape and how all that water is managed. Over time, studies have found, the United States and other countries have managed to reduce their vulnerability to many types of dangerous flooding by building dams, levees and other protective measures.
Still, plenty of risks remain. Cities such as New York are often more vulnerable to sudden downpours because so much of their land area is paved over with impervious surfaces like asphalt, which means that runoff is channelled into streets and sewers rather than being absorbed into the landscape. In Houston, researchers have found that the transformation of open land into paved parking lots and housing developments helped worsen flooding after Hurricane Harvey in 2017.
New York's subway system, built a century ago, was also not designed to handle more extreme rainfall fueled by climate change.
The Metropolitan Transportation Authority has invested US$2.6 billion in resiliency projects since Hurricane Sandy inundated the city's subways in 2012, including fortifying 3,500 subway vents, staircases and elevator shafts against flooding. Still, this week's flash floods showed that the system remains vulnerable.
And as heavy rainfall increases, experts say, more will need to be done. That could include adding more green space in cities to absorb excess runoff, as well as redesigning sewer systems, roads and public transit networks to cope with heavier precipitation. It also includes updating flood-risk maps to account for climate change, so that people have a clearer sense of where it's risky to build and where they should buy insurance against flooding.
"Pretty much all the infrastructure we've built today was designed to deal with historical weather conditions, and that's no longer enough," said Jennifer Jacobs, a professor of civil and environmental engineering at the University of New Hampshire. "It's tough in places like New York City, because there's just not much room for the water to go, but we need to think more creatively about drainage and how we design our systems for higher levels of precipitation."
This article originally appeared in The New York Times.
Written by: Brad Plumer
Photographs by: Stephanie Keith
© 2021 THE NEW YORK TIMES
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