Biogeosciences Scientific Press Release

Small Storms Over Time Can Cost More Than Extreme Events

Researchers find rising long-term impact of climate change on American cities.

This news article was issued by the University of California, Irvine and the American Geophysical Union (AGU) as a press release on 21 February 2017.

WASHINGTON, DC — Global climate change is being felt in many coastal communities of the United States, not always in the form of big weather disasters but as a drip, drip, drip of nuisance flooding.

According to researchers at the University of California, Irvine, rising sea levels will cause these smaller events to become increasingly frequent in the future, and the cumulative effect will become comparable to extreme events such as Hurricane Katrina or Superstorm Sandy.

“Catastrophic storms get a lot of media attention and are studied, but we wanted to know more about the non-extreme events,” said Amir AghaKouchak, associate professor of civil and environmental engineering at the University of California Irvine (UCI) and co-author of a new study on cumulative hazards in Earth’s Future, a journal of the American Geophysical Union.

“These diffuse floods happen multiple times a month or year,” AghaKouchak said. “They don’t kill anyone, they don’t damage buildings, but over time they have extremely high-cost outcomes, and it happens without us realizing it.”

In Washington, D.C., for instance, the number of hours of nuisance flooding per year has grown from 19 between 1930 and 1970 to 94 over the last two decades. Projections suggest that there could be as many as 700 hours of nuisance flooding per year by 2050. The capital’s monuments, marinas, parks, public transportation infrastructure, roads and businesses could be affected. The UCI researchers found similar potential impacts in four other American cities: Miami, New York, Seattle, and San Francisco.

Climate change is driving the growth of cumulative hazards, the team noted. A full moon on a clear night triggering higher tides is enough to now cause flooding, because the ocean levels are so high.

“The frequency is going higher because of sea level rise,” he said. “We call it clear-sky flooding. There’s no rain, but if you have a higher than usual tide, the ocean level is already high, so you get flooding in these coastal areas.”

Graphic showing how higher tides are now enough to cause flooding.
This graphic shows how higher tides are now enough to cause flooding, because sea levels are so high. Credit: Jennie Brewton/UCI

While not catastrophic at the time, these episodes degrade infrastructure and can damage roads and building foundations. More immediately, nuisance flooding forces municipalities to spend resources to pump water out of streets. Communities suffer school closures, traffic interruptions and reverberating waves of cost and inconvenience. Degraded sewer infrastructure result in heightened public health risks.

Hamed Moftakhari, a UCI postdoctoral scholar and lead author of the new study, said people in often-hit regions are growing somewhat inured to the problem. “In a recent social science survey, people weren’t really interested in knowing the depth of the water, they just wanted to know how long they would be flooded,” he said. “Their main concern was finding out when they could get back to their schools and businesses.”

But public officials can’t afford to take cumulative hazards in stride, said Richard Matthew, professor of planning, policy & design at UCI and co-author of the new study. Policy makers faced with limited capital funds often defer action or make incremental improvements, the researchers note, when major investments may be critical to fortify their communities. The team created a Cumulative Hazards Index that could accurately pinpoint which locations would experience the greatest long-term risk.

“This index gives them a tool that could potentially help them decide to move beyond the convenient but potentially very costly strategies of deferral and incrementalism, and promote more transformative policies where these make sense,” Matthew said.

Brett Sanders, UCI professor of civil and environmental engineering and co-author of the study added, “the index is particularly useful for predicting future hot spots for nuisance flooding across the U.S., where adaptation measures are needed the most.”