Floods are already the most common and among the most deadly disasters in the United States. As global warming continues to exacerbate sea level rise and extreme weather, flood-prone areas around the country are expected to grow by nearly half in just this century. Here’s how climate change plays a role in flooding and how we can better keep our heads above water.
Flooding facts and causes
What causes a flood?
A flood, put simply, is the accumulation of water over normally dry land. It’s typically caused by the overflow of coastal or inland waters (like rivers and streams) or by an unusual accumulation of water from heavy or prolonged rains, storm surges, or sudden snowmelt. Often, the ways in which we manage our waterways (via dams, levees, and reservoirs) and the alterations we make to land also play a role in flooding. Increased urbanization, for example, adds impermeable surfaces (think roads and parking lots), altering natural drainage systems. Areas can be especially prone to flooding when stormwater infrastructure isn’t maintained or homes are built in areas susceptible to flooding known as floodplains. More and more, flooding factors are also linked to climate change.
Major types of floods
River flooding
This occurs when a river or stream overflows its natural banks and inundates normally dry land. Most common in early spring, river flooding can result from heavy rainfall, rapidly melting snow, or ice jams. According to the 2018 study “Estimates of Present and Future Flood Risk in the Conterminous United States,” published in the journal Environmental Research Letters, more than 40 million U.S. residents are at risk from flooding along rivers and streams. And even a single episode can wreak havoc on a massive scale: For instance, in 2019, a slow-motion disaster of intense spring flooding swelled the Arkansas, Mississippi, and Missouri rivers. Hundreds of miles of levees were topped or impaired, destroying homes and supersaturating cropland. According to the National Oceanic and Atmospheric Administration (NOAA), the inland flooding caused $20 billion in damage. Some of these losses stemmed from the impact to farmers who could no longer plant or transport their crops.
Coastal flooding
More than half of the U.S. population lives or works in areas susceptible to coastal flooding, which happens when winds from a coastal storm, such as a hurricane or nor’easter, push a storm surge (essentially, a wall of water) from the ocean onto land. A storm surge can produce widespread devastation, like that seen around New York and New Jersey when Hurricane Sandy arrived during high tide.
There are also increasing numbers of shallow, nondeadly floods caused by higher sea levels. These high tide floods (also known as “nuisance” or “sunny day” floods) occur when the sea washes up and over roads and into storm drains as the daily tides roll in. In places like Miami, increased nuisance flooding is raising concerns over climate gentrification, as wealthier residents looking to settle on higher ground are pricing out historically underserved BIPOC communities.
Flash floods
These quick-rising floods are most often caused by heavy rains over a short period—usually six hours or less. Flash floods can happen anywhere, and low-lying areas with poor drainage are particularly vulnerable. Also caused by dam or levee breaks or the sudden overflow of water due to a debris or ice jam, flash floods combine the innate hazards of a flood with speed and unpredictability. That’s why they’re responsible for the greatest number of flood-related fatalities. In late 2022 and early 2023, California was hit by deadly flash floods during powerful wintertime atmospheric rivers, which may become more powerful as climate change increases the amount of moisture they can hold.
Urban flooding
The term urban flooding refers specifically to flooding that occurs when rainfall—not an overflowing body of water—overwhelms the stormwater drainage capacity of a densely populated area. In 2021, Hurricane Ida, strengthened by warm air, shattered records across the Northeast. In New York City, the deluge made rivers of impermeable streets and subway stations. Eleven people living in basement-level apartments drowned as the floodwaters—with nowhere else to go—swiftly overwhelmed below-ground spaces.
Climate change and flooding
The Intergovernmental Panel on Climate Change (IPCC) has found that climate change “has detectably influenced” several of the variables that contribute to floods, such as rainfall and snowmelt. In other words, while our warming world may not be the only or most direct cause of any given flood, it exacerbates many of the factors that increase flood risk. That’s why mitigating climate change—and particularly, limiting global average temperature rise to within 1.5 degrees Celsius in this century—is an important way to avert some of the worst scenarios for sea level rise and escalating flood risks.
How does climate change lead to flooding?
These are some of the key ways that climate change increases flood risks.
Heavier precipitation
A warmer atmosphere holds—and subsequently dumps—more water. As the planet has warmed by 1.9 degrees Fahrenheit since the preindustrial revolution era, the United States has also become about 4 percent wetter, according to the federal Climate Science Special Report. The same report says that heavy precipitation events are projected to increase by 50 percent to as much as three times the historical average in just this century. This includes extreme weather like atmospheric rivers, which are air currents that become heavy with water from the tropics. Meanwhile, in regions with significant seasonal snowmelt, hotter temperatures can trigger more rain-on-snow events, with warm rains inducing faster and earlier melting—a phenomenon playing out in the western United States.
More-frequent hurricanes
Climate change is increasing the frequency of our strongest storms, which bring greater rains, including in places not known for flooding. In August 2023, Tropical Storm Hilary—the first storm of its kind to hit the West Coast in 84 years—broke rainfall records in Idaho, Montana, Nevada, and Oregon. Hurricane Harvey, which made landfall as a Category 4 storm in 2017 and soaked Houston homes and businesses with catastrophic floods, was the nation’s wettest storm in nearly 70 years. Researchers estimate that Hurricane Harvey dumped as much as 38 percent more rain than it would have without climate change. Just a month after Harvey, Hurricane Maria hit Dominica, Puerto Rico, and the U.S. Virgin Islands. The storm produced the most rainfall in the area of any weather event since 1956 and exposed the inadequacy of U.S. policies to respond to disasters.
According to the IPCC, future hurricanes are expected to be as much as 37 percent wetter near their centers and about 20 percent wetter as much as 60 miles away. In the Atlantic basin, an 80 percent increase in the frequency of Category 4 and 5 hurricanes (the most destructive) is expected over the next 80 years. And it’s not only the most severely rated storms that bring the greatest flood impacts; in fact, the rating system the government uses to categorize the severity of storms is based solely on wind speed, not rainfall—so even unrated storms can unleash lethal amounts of water.
That said, gustier winds can whip up greater storm surges, which are already higher because of sea level rise. It was Hurricane Katrina’s 28-foot storm surge that overwhelmed the levees around New Orleans in 2005. Winds can also increase the destructiveness of waves, causing storm surges to get bigger and penetrate further inland.
Higher seas
As ocean temperatures rise and the glaciers and ice sheets melt, global sea levels are rising—and directly contributing to coastal flooding problems. According to the Fourth National Climate Assessment, our oceans are approximately seven to eight inches higher than they were in 1900. The IPCC predicts seas around the world will rise anywhere from just under one foot to more than three and a half feet above 2000 levels by century’s end. NOAA’s projections also show that, due to regional factors such as currents bringing water to coastlines, areas along the East Coast could experience seas rising 2 feet higher as early as 2050. By then, damaging coastal flooding is expected to occur 10 times as often as it does today.
In addition to amplifying storm surges, sea level rise increases high tide flooding, which, according to NOAA, has doubled in the United States over the past 30 years. For example, by 2045, Charleston, South Carolina, could see as many as 180 tidal floods per year, compared with just 11 in 2014.
Continue reading at the source (Natural Resources Defense Council)
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