Heavy Precipitation and Climate Change
Extreme precipitation events have produced more rain (Figure 1) and become more common (Figure 2) since the 1950s in many regions around the world, including much of the United States. In particular, the Midwest and Northeast have exhibited the strongest increases in the amount of rain falling in heavy precipitation events.
Scientists expect these trends to continue as the planet continues to warm. Warmer air can hold more water vapor. For each degree of warming, the air’s capacity for water vapor goes up by about 7 percent. An atmosphere with more moisture can produce more intense precipitation events, which is exactly what has been observed, averaged over large areas of the Earth.
It is important to note that increases in heavy precipitation may not always lead to an increase in total precipitation over a season or over the year. Some climate models project a decrease in moderate rainfall, and an increase in the length of dry periods, which offsets the increased precipitation falling during heavy events.
Figure 1: The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the top 1% of all daily events) from 1958 to 2011 for each region.
Source: National Climate Assessment
Threats posed by Heavy Precipitation
The most immediate impact of heavy precipitation is the prospect of flooding as streams and rivers in the region overflow their banks. Since 2008, the United States has seen six floods costing at least $1 billion each, resulting in damaged property and infrastructure, agricultural losses, displaced families, and loss of life.
- In September 2013, Boulder, Colorado, received almost a year’s worth of rainfall (17 inches) in four days. The resulting flooding destroyed homes, shut down thousands of oil and gas wells, and damaged crops.
- In 2010, almost 20 inches of rain fell on Nashville, Tennessee, over three days. Losses in Nashville alone totaled over $1 billion.
- In 2008, floods struck the Midwest, with the worst impacts in Iowa and Wisconsin. Losses totaled $15 billion, mainly from property and agriculture.
In addition to flooding, heavy precipitation also increases the risk of landslides. When above-normal precipitation raises the water table and saturates the ground, slopes can lose their stability, causing a landslide. A particularly deadly landslide occurred in March 2014 in Washington state, where landslide risks can be relatively high. The heavy precipitation in the preceding weeks caused a landslide that buried 30 homes and killed at least 41 people
Excessive precipitation can also degrade water quality, harming human health and the ecosystem.. Storm water runoff, which often includes pollutants like heavy metals, pesticides, nitrogen, and phosphorus, can end up in lakes, streams, and bays, damaging aquatic ecosystems and lowering water quality for human uses. In the Chesapeake Bay, elevated levels of nutrients such as nitrogen and phosphorus have led to algae outbreaks, which can lower the water oxygen content, killing clams, oysters and other aquatic life.
Many cities in the United States, such as New York and Philadelphia, use a combined sewer system, where both storm water and wastewater are mixed, treated, and released. Heavy precipitation events can overwhelm such systems, sending excess storm water and wastewater directly into the environment. In the aftermath of Hurricane Sandy, overwhelmed sewer systems discharged 3.45 billion gallons of untreated sewage into New York City’s rivers, bays and canals. Even in Washington D.C., where coastal flooding was not a factor, 475 million gallons of untreated sewage was discharged into local rivers, drastically reducing water quality.
Figure 2: Relative frequency of once-in-five-year precipitation events falling over a 2-day period, averaged over the continental U.S, 1900-2011. Green bars in the recent decades show that these events have become more frequent, when averaged across much of the country.
Source: National Climate Assessment
How to Build Resilience
Communities can bolster their resilience and reduce the impacts of heavy precipitation by:
- Locating buildings and infrastructure on higher ground or areas that are less prone to flooding, raising buildings, or using flood control infrastructure.
- Limiting the use of non-permeable surfaces like pavement and concrete in developed areas, or replacing pavement with “green infrastructure” that can reduce runoff during storms.
- Separating storm water systems from wastewater systems, using holding ponds, or increasing water treatment capacity to avoid sending untreated sewage into local waterways.
Purchasing flood insurance can help families and communities recover after a flood hits. However, recent floods have put the National Flood Insurance Program billions in debt, and further reforms are necessary if public flood insurance will continue to be available in the future.
To Learn More
IPCC’s Special Report on Managing the Risks of Extreme Events (SREX)
US EPA- Heavy Precipitation
National Climate Assessment