Why is it raining so hard? Global warming is delivering heavier downpours

 hen it rains, it pours.

Today’s rainstorms give new meaning to this age-old expression.

Extreme precipitation has increased in nearly every region of the mainland United States since the start of the 20th century, according to the Fourth National Climate Assessment, a comprehensive scientific report about climate change in the U.S.

Source: U.S. Global Change Research Program

The Southwest is the only region experiencing a decrease in heavy precipitation events. But even there, the intensity of rainfall during the region’s monsoon season has increased since the 1960s, according to a study in the peer-reviewed journal Geophysical Research Letters.

One such intense monsoonal event occurred on July 15, 2017 in Tucson, Arizona, when 1.45 inches of rain fell in just one hour, an impressive total given that Tucson normally sees 2.25 inches of rainfall for the entire month of July. The same day, flash flooding caused by torrential rainfall north of Phoenix killed 10 members of a family celebrating a birthday at a swimming hole.

What causes heavier rainfall? 

Climate scientists attribute increasingly extreme precipitation events to warmer air and its ability to “hold” more water vapor than cooler air.

Since 1901, the average temperature of the U.S. has warmed by 1.8 degrees Fahrenheit (1 degree Celsius) as a result of the release of heat-trapping greenhouse gases into the atmosphere.

Higher temperatures cause more liquid water to evaporate from soils, plants, oceans, and waterways, becoming water vapor. This additional water vapor means there’s more moisture available to condense into raindrops when conditions are right for precipitation to form. And more moisture spells heavier rain, or heavier snow during winter.

How much water moves into the atmosphere when it’s hot out? The Clausius-Clapeyron equation, which relates temperature to the behavior of water vapor molecules, dictates that for every 1.8 degrees Fahrenheit (1 degree Celsius) of warming, water vapor can increase about 7%.

What are the consequences of heavy rainfall?

Under normal circumstances, more rain would generally be considered a blessing, especially for drinking water and agriculture. Excessive rainfall, however — specifically the fast rates at which it falls and the overwhelming accumulations it drops — does more harm than good. 

As rain falls at rates too fast for soil to absorb, rainwater doesn’t have enough time to soak into the ground. This water, known as stormwater runoff, instead collects and flows through yards, roadways, and over other surfaces, increasing the risk of floods and soil erosion. Localized flooding can close roads, disrupt mass transit, and damage infrastructure. Over the course of a five-day period in October 2015 in Charleston, South Carolina, 15-20 inches of rain fell across the port city, resulting in over 400 road closures, 100 bridge closures, an estimated $2.3 billion in damages, and at least 16 fatalities. 

As the stormwater rushes toward low-lying areas, it picks up sediments, chemicals, heavy metals, trash, and debris. Eventually, the stormwater drains through gutters and storm sewers and is discharged into nearby waterways. This process degrades water quality, both for human use and in ecosystems.

What’s more, much of the nation’s critical water infrastructure, including dams, levees, sewers, and wastewater treatment systems, is aging or nearing the end of its design life, which means the risk of failure from extreme precipitation events is all the more magnified.

What you can do to protect yourself from floods 

Of the 330 million Americans living in the United States, as many as 41 million may live in a 100-year floodplain or special flood hazard area, according to the Association of State Floodplain Managers. Add in those who live in a 500-year floodplain, and that number increases  to over 60 million. Although you may not be able to stop global-warming-related downpours, you can reduce the impacts of heavy precipitation by taking these steps:

• Understand your risk at a local level. How much rainfall is “normal” for your location each month? Do you live in a low-lying area? Knowing these facts allows you to anticipate your individual flood risk before heavy rain arrives in the forecast. Visit the NOAA National Centers for Environmental Information climate normals page for precipitation data for U.S. cities. You can also visit Flood Factor, a website created by a group of academics and experts, to check whether your home is likely to flood during the next 30 years.

• Purchase flood insurance. According to FEMA, just one inch of floodwater can cause up to $25,000 in damage. Flood insurance, which can be purchased whether you live inside or outside of a floodplain, lightens this economic burden by covering the cost of repairs to your home or business and by compensating you for any lost possessions.

• Plant a rain garden. Rain gardens — shallow, bowl-shaped garden beds — help capture and filter stormwater runoff by giving rainwater a place to pool and slowly seep into the ground. Slowing down the flow of stormwater limits the pollutants, such as fertilizer, motor oil, and pet and yard waste, that are washed from your yard into storm drains and ultimately into waterways. The U.S. Environmental Protection Agency offers suggestions on a myriad of other water-management strategies, ranging from harvesting rainwater in rain barrels to installing permeable pavements that infiltrate, treat, and store rainwater right where it falls.

• Stay safe during a flood. Never shelter in a basement, and never walk or drive through floodwaters. It only takes six inches of moving water to knock you off your feet, and 12 to 18 inches of water to float your vehicle, says NOAA.

According to a study in the journal Water Resources Research, a number of major U.S. cities could experience extreme precipitation events that are up to 20% more intense and twice as frequent in future years. Small but mighty acts like those outlined above will prove increasingly essential to helping hold back the downpour-caused deluges.

Also see: What you need to know about the link between sea-level rise and coastal flooding