Environmental Science Behind Increasing Flood Trends
How changes in sea levels, atmospheric temperatures, and sea surface temperatures lead to more frequent and severe floods.
Originally posted on First Street Foundation.
Flooding is a natural process that benefits the environment by offering erosion control, habitat protection, biological productivity, groundwater recharge, and more.(1) However, high frequency and excessive flooding can damage local ecosystems, natural habitats, and communities. For the 2019 season, the National Oceanic and Atmospheric Administration (NOAA) predicts that 200 million people in 25 states will be at risk from flooding.(2)
Not only can flooding pose a great risk to communities, but a costly one. The combination of winter flooding, flooding, and tropical cyclones represents more than 63% of the cost associated with all billion dollar or more natural disasters since 1980.(3)
Between 2000 and 2017, tidal flooding increased an average of 233% nationally.(4)
Floods caused by heavy downpours are also increasing in frequency across most of the United States, especially in the Midwest and Northeast where there has been the greatest increase in heavy rainfalls.(5)
To keep communities safe from the ever-growing threat of flooding, it’s important to not only understand the science behind flooding events, but what can worsen these events. The significant increase in flooding that has been seen over the last 20 years is largely due to changes in sea levels, surface temperature, and sea surface temperature, and as these environmental factors continue to change, so will flood patterns.
SEA LEVEL RISE
The ocean has risen 8.5 inches nationally since 1950 and is projected to rise another 4.5 inches by 2050.(6) Sea level rise can worsen floods by adding more water to already high tides; increased pressure from sea level rise can fill drainage systems, causing water to spill into the street and cause flooding even on days without rain. Higher seas also create a higher launching point for hurricane storm surge, allowing more water to come on land and travel further with more velocity.(7)
Sea level rise is governed by a number of contributing factors, with the four most prevalent contributors being ice melt, thermal expansion, changes in ocean circulation, and land subsidence.(8) Though a global threat, sea level rise varies by location. For example, the slowing of the Gulf Stream and sinking land contribute significantly to sea level rise on the East Coast, while the impact of ice melt and thermal expansion is felt across the coastal United States.
Thermal expansion is responsible for approximately one-third of global sea level rise, causing more than 6 inches of sea level rise since 1950.(6) As the ocean warms, seawater becomes less dense and expands, raising sea level. Thermal expansion has increased in the last 10 years, causing sea levels to rise 75% faster than previously.(6)
The loss of land-based ice causes approximately two thirds of global sea level rise.(6) Gaining mass through snow accumulation, glaciers, ice caps, and ice sheets lose their mass primarily through ablation and iceberg calving, which sends over 1,700 trillion pounds of glacial ice from the land into the sea each year.(9) This includes both continental (ice sheets and ice caps) and alpine (glaciers in mountains and valleys). Mountain glaciers and ice caps have been melting for hundreds of years, but research now shows that ice sheets in Antarctica and Greenland are melting at an accelerated rate, causing them to become of greatest concern to sea level rise.
Ice melt from Antarctica, Greenland, and glaciers is causing freshwater to disrupt the balance of the Gulf Stream. Freshwater is lighter than ocean water because it does not contain salt, therefore it does not sink as quickly and slows down the current of the Gulf Stream. Because the Gulf Stream is slowing, less water is taken from the East Coast, contributing to sea level rise. Studies suggest that short-term changes to the Gulf Stream can add 1–3 feet to tides over the course of a day or week.(10) If this occurs during already high tides or rain storms, it can produce extreme flooding.
Land subsidence is generally a small contributor to sea level rise, but in certain places, like Virginia, it can be responsible for nearly half. Sea levels are measured relative to land, so when land subsidence is prevalent it can drastically increase the amount of sea level rise in a given area. For example, in Norfolk, Virginia the land is sinking at a rate of approximately 1 inch every 10 years, which increases sea level rise because as the sea level rises, the ground also sinks.(11)
ATMOSPHERIC CHANGES
The atmosphere is 1.9 degrees (F) warmer than it was in 1950 and is projected to warm another 1.28 degrees (F) by 2050.12 This impacts the frequency and intensity of pluvial (precipitation) and fluvial (riverine) flooding.(7) As the atmosphere warms, there is more evaporation, which means there is more water available for rain. This extra moisture contributes to changes in weather patterns and storm systems throughout the United States.
Over the last 3–5 decades, extreme rainfall events have increased in volume, intensity, and frequency, and the amount of rain falling on the heaviest rain days has also increased. The increase has been felt most in the Northwest, Midwest, and upper Great Plains, with heavy precipitation events climbing to 30% above the 1901–1960 average.(13) Even rain events that were once considered rare are now increasing in frequency. For example, the National Weather Service recorded 10 of what were considered to be rare rain events between 2015 and 2016, all of which resulted in flooding. These types of rain events are generally expected to occur once every 500 years.(14)
When a precipitation event produces more water than the soil in a given areas can absorb, the extra water will create runoff that flows into rivers and can overwhelm culverts and storm drains, creating flash flooding events. Urban flooding and flash floods, which are directly linked to heavy precipitation, are expected to increase as heavy precipitation events continue to increase.(15)
INCREASING SEA SURFACE TEMPERATURE
The sea’s surface temperature, also known as upper ocean heat content, is 1.5 degrees (F) warmer than it was in 1950 and is projected to rise another 0.5 degrees (F) by 2050.(16) Warmer ocean temperatures fuel hurricanes by giving them more water and more strength. As a result, hurricanes can reach both further inland and further north, are more intense when they hit, and can be sustained for longer periods of time.
As a hurricane travels over the ocean it bombards the sea surface with strong winds, which makes it easier for water to evaporate and feed the hurricane. Because the evaporation process requires energy derived from heat, the warmer the sea surface, the more energy the hurricane can gather from it.(17) If a hurricane passes over a heated ocean surface, it can become supercharged and cause more destruction when it makes landfall than it would have without the extra heat. Stronger hurricanes are also able to reach further inland and cause more flooding because they sweep up more water, and push that water with stronger winds.(18)
For example, leading up to Hurricane Harvey the ocean had one of the highest heat contents on record.(19) This heat not only intensified the hurricane, but increased rain-driven flooding on land. The typical sea surface temperature needed for a hurricane to grow is 79 degrees (F). The near-surface ocean temperature, or heat content, before Hurricane Harvey passed over the Gulf of Mexico was 86 degrees (F), and after the hurricane passed it was still 83 degrees (F).(16) Even though hurricanes normally cool the temperature of the ocean, leaving a cold wake, the sea surface temperature was so hot that it continued to fuel the storm, allowing it to draw power from the ocean well after it made landfall. As the ocean continues to warm, higher sea surface temperatures will be more common, contributing to stronger hurricanes and more flooding.
As sea levels continue to rise, sea surface temperatures continue to increase, and the atmosphere continues to warm, flooding will continue to worsen. Communities are continuing to grow in low-lying and coastal areas, but public understanding of flood risk and how these factors contribute is not growing at the same pace.
Development continues in flood-prone areas with inaccurate or out-of-date flood maps being used to assess flood risk. Meanwhile, changes in drainage patterns, land use, increases in the amount of impermeable surfaces, and the reducing of many natural areas that absorb rainfall or protect against hurricane storm surge can not only worsen flooding, but leave people unprotected and unprepared. As flood risk increases, urban planning decisions need to adjust accordingly and floodplain information must be updated.
Until the growing flood risk in America is accurately understood and addressed, communities will continue to face worsening flood events without being adequately protected.
Originally published at FirstStreet.org on April 3, 2019.
