CLIMATE CHANGE & FLOODING
Climate change is increasing the risk of flooding in the US by rising sea levels, increasing extreme precipitation and total precipitation, intensifying storm precipitation, and increasing rates of snow and ice melt. Worsening floods due to climate change are putting a growing number of inland and coastal communities at risk. This year, the spring flood season is overlapping with COVID-19, multiplying the threat to communities at risk of being displaced by flood disasters.
How has climate change already worsened US flooding?
- Human-caused global sea level rise is increasing the number of coastal flood days in the US.
- In the Mid-Atlantic, climate change contributed to widespread upward trends in stream flood magnitude and frequency since 1970.
- Climate change is elevating flood risk across the US by increasing the frequency of extreme precipitation and intensifying hurricane rainfall.
- Climate change is increasing total precipitation in the Mississippi River Valley, a precursor to river flooding.
- Climate change is intensifying the extreme swings between drought and flood years in California.
What is flooding and why is it dangerous?
A flood is an overflowing of water onto land that is normally dry, and a flash flood is when flooding occurs less than 6 hours after an extreme rainfall event. Flooding can occur along the ocean, rivers, or lakes, and it can occur quickly or over a long period, lasting days, weeks, or longer. Flooding can be caused by heavy rainfall, rising sea levels coupled with high tides and/or storm surge, faster and earlier snowmelt, or when dams or levees break.
Floods are the most common and among the most deadly natural disasters in the US. Flash flooding occurs so quickly that people are often caught off-guard. People also underestimate the force and power of water: just six inches of rushing water can knock over an adult and 12 inches of rushing water can carry away most cars.
How does climate change worsen coastal flooding?
The link between climate change and coastal flooding due to sea level rise is undisputed. The combination of melting land ice and the expansion of ocean water as it warms has significantly increased the incidence of both high tide floods and the reach of storm surge - the temporary increase in sea level due to stormy conditions.
How does climate change worsen river flooding?
The influence of climate change on river flooding is more complex because of the many factors that affect riverine floods including local land use, land-cover changes, and water management. For this reason, scientists point to ways that climate change increases flood risk through changes in major flood precursors such as extreme precipitation, total precipitation, and snow/ice melt:
- Warmer temperatures increase evaporation of moisture into the air and allow the air to hold more moisture. This warm, water-laden air can then dump more precipitation, increasing runoff and flood risk.
- Observational data suggest climate change is likely increasing overall precipitation in the Midwest and Northeast, as well as storm precipitation in the Gulf of Mexico states.
- Long-term climate warming is causing shifts toward less snow, more rain, and earlier snowmelt, increasing early-season runoff and flood risk.
Event spotlight: Climate change & recent flood disasters.
The effect of climate change has been identified on many of the most infamous flooding disasters of the past few decades.
- Miami (2015): Flooding in Miami on September 27 inundated about two feet of normally dry land. Since 1994, sea level rise of 4.3 inches has increased the probability of two-foot tidal floods by 500 percent.
Storm surge flooding
Climate change driven sea level rise has increased the likelihood of storm surge flooding.
- North Carolina (2018): Approximately 1.2 million homes and 3,800 businesses were damaged by Hurricane Florence. Of the 51,000 homes that were damaged by storm surge, a report found 11,000 of them wouldn’t have been impacted if it weren’t for sea level rise.
- New York & New Jersey (2012): Human-caused sea level rise extended the reach of Hurricane Sandy by 27 square miles, affecting 83,000 additional individuals living in New Jersey and New York City and adding over $2 billion in storm damage.
- Louisiana (2005): Hurricane Katrina would have flooded 60 percent less area of New Orleans had the storm occurred in 1900, before land subsidence and climate change increased sea levels by 30 inches.
Hurricane precipitation flooding
Climate change is making tropical storms and hurricanes wetter, increasing the risk of hurricane floods. Climate change increased rainfall during Hurricanes Florence, Maria, Harvey, Sandy, Katrina, and during Tropical Storm Imelda.
Total precipitation flooding
In the Midwest and along the Mississippi River Valley, significant increases in flooding are well-documented, and the change is likely due to increases in total precipitation linked to climate change.
- Mississippi River Valley (2019): Midwest flooding and Mississippi River flooding was record-breaking in 2019. The Mississippi River Valley saw the most prolonged and widespread flooding in US history due to record total precipitation along the Upper Mississippi River, Arkansas River, and Missouri River, which all drain into the Lower Mississippi.
Extreme precipitation flash flooding
Heavy precipitation events in most parts of the US have increased in both intensity and frequency since 1901.
- Louisiana (2016): In August 2016, a historic flood resulting from 20 to 30 inches of rainfall over several days devastated a large area of southern Louisiana, causing over $10 billion in damages and 13 deaths. Climate change increased the heavy rain by 20 percent. Heavy precipitation events in Louisiana – such as the August 2016 event that led to flooding in southern – are occurring more frequently due to climate change.
- West Virginia (2016): In June 2016, a derecho brought rains so extreme they should occur only once every 1000 years. The storm caused devastating flash flooding, record flooding on several gauged streams and small rivers, and 23 fatalities. Warming in the Gulf of Mexico due to climate change likely contributed to the storm’s intense rainfall.
- Missouri (2015): At the end of December 2015, a huge storm named “Goliath” dumped 9-10 inches of rain in a belt across the central United States, centered just southwest of St. Louis, most of it in a three-day downpour. A later analysis found that human-caused warming increased the likelihood of extreme precipitation in Missouri during this event.
- Colorado (2013): In 2013, persistent, heavy rains in Boulder broke several rainfall records and led to catastrophic flooding. Climate change increased the amount of moisture in the atmosphere and likely increased rainfall by 30 percent.
Early season runoff flooding
In the Western US, increasing winter temperatures are leading to decreased spring snow cover and depth, earlier snowmelt and runoff, and increased flood risk. During the 2016-2017 winter, climate change decreased Sierra Nevada snowpack by nearly 20 percent, and increased early-season runoff by 30 percent. That same winter, runoff in the watershed supplying the Oroville Dam prior to the Dam failure was one-third greater due to global warming.