Flooding and Climate Change: Everything You Need to Know
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Floods are the most common (and among the most deadly) natural disasters in the United States. They have brought destruction to every state and nearly every county, and in many areas they are getting worse. As global warming continues to exacerbate sea level rise and extreme weather, our nation’s floodplains are expected to grow by approximately 45 percent by century’s end. Here’s how climate change plays a role in flooding, and how we can better keep our heads above water.
What is a flood?
A flood is the accumulation of water over normally dry land. It’s caused by the overflow of inland waters (like rivers and streams) or tidal waters, or by an unusual accumulation of water from sources such as heavy rains or dam or levee breaches.
Major types of floods
This occurs when a river or stream overflows its natural banks and inundates normally dry land. Most common in late winter and early spring, river flooding can result from heavy rainfall, rapidly melting snow, or ice jams. According to one study, approximately 41 million U.S. residents are at risk from flooding along rivers and streams.
More than 8.6 million Americans live 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—a wall of water—from the ocean onto land. Storm surge can produce widespread devastation. There are also increasing numbers of shallow, non-life-threatening 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.
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, although 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 and are responsible for the greatest number of flood-related fatalities.
Flash floods, coastal floods, and river floods can occur in urban areas, but the term “urban flooding” refers specifically to flooding that occurs when rainfall—not an overflowing body of water—overwhelms the local stormwater drainage capacity of a densely populated area. This happens when rainfall runoff is channeled from roads, parking lots, buildings, and other impervious surfaces to storm drains and sewers that cannot handle the volume.
Many factors can go into the making of a flood. There are weather events (heavy or prolonged rains, storm surge, sudden snowmelt), and then there are the human-driven elements, including how we manage our waterways (via dams, levees, and reservoirs) and the alterations we make to land. Increased urbanization, for example, adds pavement and other impermeable surfaces, alters natural drainage systems, and often leads to more homes being built on floodplains. In cities, under-maintained infrastructure can lead to urban flooding. More and more, flooding factors are also linked to climate change.
Climate Change and Flooding
Connecting climate change to floods can be a tricky endeavor. Not only do myriad weather- and human-related factors play into whether or not a flood occurs, but limited data on the floods of the past make it difficult to measure them against the climate-driven trends of floods today. However, as the IPCC (Intergovernmental Panel on Climate Change) noted in its special report on extremes, it is increasingly clear that climate change “has detectably influenced” several of the water-related variables that contribute to floods, such as rainfall and snowmelt. In other words, while our warming world may not induce floods directly, it exacerbates many of the factors that do. According to the Climate Science Special Report (issued as part of the Fourth National Climate Assessment, which reports on climate change in America), more flooding in the United States is occurring in the Mississippi River Valley, Midwest, and Northeast, while U.S. coastal flooding has doubled in a matter of decades.
How Does Climate Change Lead to Flooding?
These are some of the key ways climate change increases flood risks.
A warmer atmosphere holds and subsequently dumps more water. As the country has heated up an average of 1.8 degrees Fahrenheit since 1901, it has also become about 4 percent wetter, with the eastern half of the United States growing soggiest. In the Northeast, the most extreme storms generate approximately 27 percent more moisture than they did a century ago. Basically, because of global warming, when it rains, it pours more. Such was the finding of a study by the National Oceanic and Atmospheric Administration (NOAA) examining the record-breaking rainfall that landed on Louisiana in 2016, causing devastating flooding. The study determined that these rains were at least 40 percent more likely and 10 percent more intense because of climate change.
Looking forward, heavy precipitation events are projected to increase (along with temperatures) through the 21st century, to a level from 50 percent to as much as three times the historical average. This includes extreme weather events known as atmospheric rivers, air currents heavy with water from the tropics, which account for as much as 40 percent of typical snowpack and annual precipitation along the West Coast. Experts predict they will intensify, bringing as much as 50 percent more heavy rain by the end of this century.
Of course, heavier rainfall does not automatically lead to floods, but it increases the potential for them. And even moderate amounts of rainfall can cause serious damage, particularly in places where urban flooding is on the rise.
Meanwhile, in regions where seasonal snowmelt plays a significant role in annual runoff, hotter temperatures can trigger more rain-on-snow events, with warm rains inducing faster and often earlier melting. This phenomenon is playing out in the western United States, where, according to the IPCC, snowmelt-fed rivers, at least since 1950, have reached peak flow earlier in springtime. The combination of rain and melting snow can aggravate spring flooding as winter and spring soils are typically high in moisture and often still frozen, and therefore less able to absorb snow and rain runoff. Regions with higher rain-to-snow ratios, such as the Northwest, are expected to see higher streamflow—and higher flood risks.
Climate change is increasing the frequency of our strongest storms, a trend expected to continue through this century. 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 stronger storms bring greater rains. Indeed, 2017’s Hurricane Harvey, which made landfall as a category 4 storm and soaked some 200,000 Houston homes and businesses with catastrophic floods, was the nation’s wettest storm in nearly 70 years (see a related Houston flood map here). It was also slow and therefore able to dump more, a result of weakened atmospheric currents from a warmer atmosphere. Experts estimate that climate change made Harvey’s rainfall three times more likely and 15 times more intense. In 2018, Harvey was followed by Hurricane Florence (the second-wettest storm in nearly 70 years), which set at least 28 flood records in the Carolinas, according to the United States Geological Survey (USGS). Hurricane Maria, which hit Puerto Rico, Dominica, and the U.S. Virgin Islands in 2017, produced the most rainfall in the area of any weather event since 1956.
Even rainier storms are predicted for the future, with tomorrow’s hurricanes expected to be as much as 37 percent wetter near their center and about 20 percent wetter as much as 60 miles away.
Stronger storms can also produce gustier winds that whip up greater storm surge, which starts as much as eight inches higher than a century ago because of sea level rise. It was Hurricane Katrina’s 28-foot storm surge that overwhelmed the levees around New Orleans in 2005 and caused the vast majority of deaths. And it was the combination of storm surge and high tide that led to Hurricane Sandy’s inundation of coastal New York and New Jersey in 2012—flooding that could be as much as 17 times more frequent in the area’s coastal regions by 2100, according to one study. Storm surge and winds can also increase the destructiveness of waves, causing them to get bigger and penetrate further inland. According to the Federal Emergency Management Agency (FEMA), waves of just 1.5 feet have been seen to cause significant damage to coastal structures.
As ocean temperatures rise and the world’s glaciers and ice sheets melt (phenomena exacerbated by climate change), global sea levels are rising. Our oceans are approximately seven to eight inches higher than they were in 1900 (with about three of those inches added since 1993 alone)—a rate of rise per century greater than for any other century in at least the past 2,000 years. And while the IPCC predicts seas around the world will rise anywhere from one foot to more than four feet above 2000 levels by century’s end, NOAA’s projections show that, due to regional factors such as currents bringing water to coastlines, places such as the East Coast could see seas as much as 9.8 feet higher by 2100. (Check out NOAA’s interactive map that demonstrates where flooding will occur as sea levels rise.)
In addition to amplifying storm surge because the water starts at a higher level, sea level rise increases high-tide flooding, which has doubled in the United States over the past 30 years and is expected to rapidly worsen in the coming decades. According to the Fourth National Climate Assessment, for example, by 2045, Charleston, South Carolina could see as many as 180 tidal floods per year, compared with just 11 in 2014.
When flooding inundates a home or community, it upends lives and introduces a litany of potential short- and long-term consequences. The most obvious include loss of life (floods cause more than 100 U.S. fatalities annually) and vast property damage. Repairing and replacing flood-damaged roads, bridges, utilities, and other public infrastructure cost FEMA an estimated $48.6 billion between 1998 and 2014.
Between 2007 and 2017, the National Flood Insurance Program (NFIP) paid an average of $2.9 billion per year to cover flood-related losses, with individual years often costing far more. Following 2012’s Hurricane Sandy, for example, property owners filed approximately $8.8 billion in flood claims. Another $8.8 billion would be filed five years later, after Hurricane Harvey. And many times, the same homes are repeatedly flooded—more than 30,000 properties flooded an average of five times each have been covered under the NFIP. And not only has the NFIP been deeply in debt since Hurricane Katrina, but costly major floods are only becoming more common. An NRDC analysis found that in some cases, it would save money for the government to buy flood-damaged properties, demolish them, and not rebuild on the land. This would also allow families to move somewhere safer and avoid the hardship of additional floods.
Flooding also brings contamination and disease. Floodwaters can carry raw sewage, leaked toxic chemicals, and runoff from hazardous waste sites and factory farms. They can pollute drinking water supplies and cause eye, ear, skin, and gastrointestinal infections. When floodwaters recede, bacteria and mold may remain, increasing rates of respiratory illnesses, such as asthma. Flooding can also contribute to mental health problems, lead to economic loss (as in the form of lost business or wages), and uproot whole communities.
And while it is true that floods do not discriminate, affecting anyone in their path regardless of wealth or ethnicity, it is most often lower-income people, the elderly, and minority communities who suffer the greatest impacts. These populations are least likely to have flood insurance, access to transportation during an evacuation, cash on hand, or the ability to relocate.
Advance preparation for a flood can save your property or even your life. (See FEMA.gov for a comprehensive flood preparation list.) To stay safe, take these precautions:
- Visit FEMA’s Flood Map Service Center to find your community’s flood map and better understand your flood risk, then consider investing in (or renewing) a flood insurance policy.
- Plan and practice following an evacuation route, identify how you will communicate with family members and friends, and decide where you will stay in case of flooding. (For information on shelters, visit the American Red Cross website or text SHELTER + your zip code to 43362). Plan in advance for those with mobility issues, as well as for pets.
- Assemble an emergency supply kit that includes food, bottled water, first-aid supplies, medicines, and a battery-operated radio. Visit Ready.gov for a complete disaster supply checklist. And be sure to store copies of important documents in a waterproof place.
- Learn how to receive timely information about local weather conditions. Sign up for your community’s warning system, which may provide emergency notifications via text or email. (To find out what alerts are available in your area, do an internet search with your town, city, or county name and the word alerts.) The Emergency Alert System, NOAA Weather Radio, and USGS’s WaterAlert system also provide important emergency updates.
- Know your flood alerts. A flood or flash flood watch means flooding is possible and may or may not occur. A flood or flash flood warning means flooding is imminent or already happening; if you are in a flood-prone area when a flash flood warning is issued, seek higher ground.
- If you must evacuate, remember to take “the five Ps”: people (the most important), prescriptions, paper (birth certificates, passports, and other key documents), personal needs (clothes, phones, and phone chargers), and priceless items (irreplaceable mementos). If there is time, such as in the case of a slow-onset flood, consider moving valuables to higher levels; turning off gas, water, and electricity; and putting sandbags around your property. Avoid walking, swimming, and driving through floodwaters, which may not only be surprisingly powerful (a mere 12 inches of water can wash away a car) but also hide hazards, such as downed (and possibly live) power lines, broken glass or metal, and contaminants like sewage and chemicals.
Learn If a Home Is Flood-prone
According to FEMA, flooding is a factor in more than 90 percent of disaster-related property damage in the United States, with many homes repeatedly damaged by floods. But actually finding out if a property is flood-prone when house shopping can be difficult. Twenty-one states have no legal requirements that a seller disclose a property’s history of flood damage to a buyer. The other 29 states and Washington, D.C., have a variety of disclosure requirements, but some of them can actually make it harder for buyers to find out about a house’s flood history. The best strategy for potential buyers: Look at FEMA’s flood history maps, but know that they are only a starting point. (Many are outdated; more on that below.) A more low-tech option: Introduce yourself to your prospective neighbors and ask them about flooding in the area.
How to Protect a Home from Flooding
For repeatedly flooded homes, relocation may be the best option. But a wide array of measures exist to prevent or reduce flood damage to structures when relocation is not possible. These include keeping gutters and drains free of debris; installing a sump pump (with battery backup in case of power failure) for crawl spaces and basements; adding “check valves” in sewer lines to keep floodwater from backing up into the drains of your home; and safeguarding indoor utilities and outdoor equipment by elevating furnaces, water heaters, electrical systems (switches, sockets, circuit breakers, and wiring), generators, and air-conditioning units above flood levels. In areas where flooding is a regular occurrence, more drastic retrofits may include raising the entire structure of a house, wet floodproofing (which purposely allows water to flow into a structure and then back out), and dry floodproofing (the application of coatings and other sealing materials to walls to prevent floodwaters from entering a home).
As mentioned earlier, flooding is the most common natural disaster in the United States, occurring in 98 percent of the nation’s counties. It is costly, too, with just one inch of flooding capable of racking up more than $25,000 in damage to the average home. Despite this, a mere 15 percent of American homeowners had a flood insurance policy in 2018 (the average cost of which is about $1,000 per year), according to the Insurance Information Institute, a trade association. Because typical homeowners’ and renters’ insurance fails to cover flooding, this means that if there is an inundation, the vast majority of Americans must take out loans or pay out of pocket to repair or replace damaged items. This vast insurance gap has been created largely by holes in the nation’s federal flood insurance program, which often relies on outdated and inaccurate flood maps and provides little incentive for people to move to safer, flood-free areas. Of course, insurance doesn’t stop flooding, and simply increasing the number of people who have it doesn’t stop damage and destruction.
FEMA Flood Insurance
Administered by FEMA, the National Flood Insurance Program was created by Congress in 1968 to provide affordable flood insurance to homeowners, renters, and business owners. It is available in communities that adopt and enforce floodplain management ordinances and building requirements aimed at reducing flood risks. According to FEMA, nearly every U.S. community with serious flooding potential has joined the NFIP, with a full list of participating communities available via NFIP’s Community Status Book. At present, the federal program insures more than 5 million properties worth approximately $1.25 trillion in more than 22,000 communities.
The NFIP provides critical aid to victims recovering from a flood disaster, but it also incentivizes the rebuilding of homes in flood-prone areas (often multiple times) by providing little assistance to those who wish to move to higher ground. Indeed, it is estimated that more than 30,000 U.S. properties covered under the nation’s federal flood insurance program have faced inundation an average of five times each (with some homes flooded 30-plus times). And yet, for every $100 FEMA has put toward rebuilding properties through the NFIP, it has invested just $1.72 to relocate people. This constant cycle of post-flood rebuilding can lock people into a costly and dangerous situation. It also wastes billions of dollars, which in turn threatens the NFIP itself. The program is already $20.5 billion in debt, the result of paying out more in damages than it collects in premiums.
FEMA Flood Maps
Mitigating potential loss from future floods requires knowing where floods are most apt to occur. In the United States, this information is provided by FEMA, which produces maps of the nation’s flood zones. NFIP relies on these maps to assess flood risk, determine insurance rates, and establish floodplain management standards.
According to a 2017 report by the Department of Homeland Security’s Office of Inspector General, however, nearly 60 percent of FEMA flood maps are out of date. Despite a requirement that FEMA reassess its maps every five years, some community maps haven’t been reassessed in decades. When Hurricane Sandy hit in 2012, for example, many of the maps for areas that were flooded had not been updated in nearly 30 years. The flooding that resulted from the storm covered an area 65 percent larger than the flood-vulnerable area identified by FEMA.
Keeping flood maps up to date is critical because flood risks change as land use and other factors change. Floods, for example, become a greater risk when more pavement and other impervious surfaces are built over an area. As the inspector general’s report notes, FEMA’s old maps fail to reflect “true flood vulnerability” or accurately inform insurance rates to reflect a “real risk of flooding.”
Although climate change is increasing future flood risks, FEMA’s maps also typically fail to take into account the effects of global warming, such as sea level rise. Instead, they rely on historical data to determine future flood hazard projections. This can cause officials to designate areas as being “safe” for development today even when they are at risk of serious floods tomorrow.
FEMA Flood Zones
FEMA flood maps (officially called flood insurance rate maps) depict the high-, moderate-, and low-risk flood zones of communities nationwide and can be found at FEMA’s Flood Map Service Center. Moderate- to low-risk areas (called non-special flood hazard areas) are regions with less flood potential (though properties in these areas still account for more than 20 percent of NFIP claims). High-risk areas (also known as special flood hazard areas—or simply floodplains) are regions with a 1 percent (1 in 100) chance of being inundated by river or stream floodwaters of a certain magnitude in any given year. (The term 100-year flood refers to this, not a flood expected to occur just once every hundred years). But even a 1 in 100 chance of flooding each year equates to about a 1 in 5 chance a home will flood at some point over the life of a 30-year mortgage.
Among other things, FEMA’s floodplains determine how and where homes and other structures are built, as well as who is required to purchase flood insurance (coverage is mandatory if you live in a floodplain and have a federally backed mortgage). The problem is, many of FEMA’s mapped floodplains are inaccurate, as the floods caused by Hurricane Harvey demonstrated. Indeed, nearly three-quarters of Houston’s flood-damaged homes and apartment buildings sat outside of FEMA’s identified high-risk area. A 2018 NOAA analysis provides one explanation: Greater rainfall has made what used to be a 100-year flood event in Houston, by FEMA’s standards, more like a 25-year event. In other words, the scale of the city’s real floodplain is vastly underestimated. And another recent study published in the journal Environmental Research Letters suggests the extent to which FEMA flood zones are missing the mark: It found that some 41 million Americans live within a 100-year floodplain, roughly three times FEMA’s estimates.
Entirely preventing floods isn’t possible. But we can take steps to lessen their devastation.
Flood resiliency can come from water-smart improvements to buildings and green infrastructure, updated FEMA maps that reflect new climate realities, an overhaul of the NFIP to help more homeowners relocate to higher ground, and a reinstatement of the Obama administration’s 2015 federal flood protection standard. The latter, scrapped by President Trump in 2017, included, among other things, commonsense measures such as requiring FEMA to rebuild flood-damaged public infrastructure (police stations, schools, hospitals, and the like) to be safer and stronger than their pre-flood incarnations.
A 2017 poll showed that the vast majority of Americans support such flood-smart federal measures—and for good reason. For one thing, they can save enormous amounts of money (for example, it is estimated that for every $1 invested in riverine flood mitigation, taxpayers and the federal government save $7 in recovery costs). Moreover, such measures increase the odds that millions more Americans will stay safe—and dry.
Finally, curbing climate change is an important way to avert some of the worst scenarios for sea level rise and escalating flood risks. As the IPCC has made explicitly clear, limiting global average temperature rise to within 1.5 degrees this century will be critical to limiting many future weather extremes, including those that most contribute to flooding, such as heavy rainfall. And only ambitious climate action can make that happen. To that end, it is imperative that we meet the commitments laid out in the Paris Climate Agreement, as well as support key domestic policies, including the Clean Power Plan and automotive fuel efficiency standards, that would slash our nation’s climate-altering pollution.
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