When it rains, it pours.
Today’s rainstorms give new meaning to this age-old expression.
This past July, a slow-moving storm poured over 4 inches of rain over west Germany, in what was described as the continent’s deadliest flood since 1985. During the same month, Zhengzhou, China, saw “once-in-a-thousand-year floods” over the course of three days, displacing millions and killing more than 300 people. Just six weeks later, New York City declared its first-ever flash flood emergency as remnants of Hurricane Ida soaked parts of the northeastern US with up to 6-10 inches of rain in under 24 hours.
As historic as these rain events might be, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) warns that they’re becoming increasingly common globally. According to the 2021 draft release of the IPCC sixth assessment report — considered the gold standard in climate science — 10-year events (that is, rainfall events that have a one in 10 chance of being matched or exceeded in a given year) are now 1.3 times more likely to occur. And when they do occur, they’re nearly 7 percent wetter.
As air warms, it can “hold” more water vapor than air at cooler temperatures.
What’s causing heavier rainfall?
Scientists believe human-caused global warming is to blame. That’s because climate change is supercharging the water cycle’s evaporation process and messing with rainfall patterns.
Scientists point to a basic meteorological principle: As air warms, it can “hold” more water vapor than air at cooler temperatures. (And we know average global temperatures have risen about 1.1°C (1.98°F) from 1900-2020 due to the release of heat-trapping greenhouse gases into the atmosphere.) As temperatures rise, more water evaporates from soils, plants, oceans and waterways — this becomes vapor. Additional water vapor means there’s more water available for heavier rain and snow.
How much more? The Clausius-Clapeyron equation, which calculates how temperature affects the behavior of water vapor molecules, dictates that for every 1°C (1.8°F) of warming, precipitable water — or the atmospheric water vapor content — can increase about seven percent on a global scale, says the IPCC, but this number can vary regionally depending on relative humidity.
Based on the latest IPCC projections, precipitation may increase over high latitudes, the equatorial Pacific and various monsoon regions like the West African Monsoon’s central Sahel. One study even predicts a number of major US cities — including Salt Lake City, Utah; Nashville, Tennessee; and New York, New York — could experience extreme precipitation events that are up to 20 percent more intense and twice as frequent in future years.
Severe flooding has triggered some of the worst humanitarian disasters in history.
Ordinarily, more rain is a blessing. We need it to fill lakes and streams, which provide sources of drinking water and crop irrigation. But excessive rainfall — especially at fast rates — can do more harm than good.
When rain falls too quickly for soil to absorb, the ground can’t soak it all up. Instead, stormwater runoff collects and flows through yards and roadways, increasing the risk of floods and soil erosion. Localized flooding can disrupt transportation, damage infrastructure and cause power outages.
In October 2015, 15-20 inches of rain fell across the port city of Charleston, South Carolina, over a five-day period, resulting in over 400 road closures, 100 bridge closures, $2.3 billion in damages and at least 16 fatalities. In other regions, severe flooding has triggered some of the worst humanitarian disasters in history. In the summer of 2010, record monsoon rains and flash flooding in Pakistan affected some 20 million people and left over 1,000 dead. Around 14 million were left without homes, and the destruction of crops and other infrastructure left millions more vulnerable to malnutrition and waterborne illness.
Clean drinking water is also at risk. As stormwater rushes toward low-lying areas, it picks up sediments, chemicals, heavy metals, trash and debris on the way. Eventually, this water drains through gutters and sewers, and gets discharged into natural waterways. This process degrades water quality for both humans and nature. It also puts pressure on fragile water infrastructure, including aging levees, dams and sewers.
Finally, extreme precipitation doesn’t just cause flooding over land, but along coasts, too. Sea levels have risen an average of 8-9 inches globally since 1880. As glaciers melt and seawater continues to climb, low-lying areas will have higher background water levels, meaning it’ll take less rainfall to trigger flooding in these regions.
While some degree of heavier rainfall is here to stay, cutting greenhouse gas emissions and limiting global temperature rise is the best way to help us evade the worst effects of climate change.
How you can safeguard against floods
Globally, nearly 1.5 billion people or 20 percent of the world population have at least a moderate risk of flood exposure. Remember: after droughts, storms and floods claim more lives worldwide than any other weather disaster. While we may not be able to stop the warming-induced downpours that have already been set in motion, we can curb their impacts by taking these steps:
- Understand your risk and have a safety plan ready. How much rainfall is “normal” for your region? Do you live in a low-lying area? Knowing this can help you understand your individual flood risk before heavy rain arrives. Visit NOAA’s National Centers for Environmental Information climate normals page or the World Bank Group’s Climate Change Knowledge Portal for US and global precipitation data. You can also prep in advance by having an evacuation plan and stocking your home or car with emergency preparedness supplies in case you need to shelter in place.
- Purchase flood insurance. According to FEMA, just one inch of floodwater can cause up to $25,000 in damage. And over the past 50 years, the world has spent a collective $115 billion on floods alone. If flood insurance is available in your country, it may help cover the cost of repairs to your home or business.
It only takes six inches (less than a hand’s length) of moving water to knock you off your feet.
- Make your home, neighborhood or city more “spongey.” This could be as simple as planting native trees and rain gardens, which are shallow, bowl-shaped garden beds that capture and filter stormwater runoff by giving rainwater a place to pool and slowly seep into the ground. You can also go big by designing stormwater wetlands, green infrastructure and restoring floodplains by removing land development from areas bordering rivers and streams).
- Avoid floodwater and move to higher ground. It only takes six inches (less than a hand’s length) of moving water to knock you off your feet, and 12 to 18 inches of water to float your vehicle, says NOAA. That means you should never walk or drive through floodwaters. An upper room with a window is the safest spot during a flood, according to NOAA, and in extreme cases, seek shelter on the roof. (Never shelter in an attic without roof access — you could become trapped!)
- Don’t forget about other storm-related hazards. Handling wet electrical equipment, using power generators in unventilated areas and wading through contaminated water can lead to serious injury, including electrocution, waterborne illness and carbon monoxide poisoning.
Small but mighty acts like those outlined above will prove increasingly essential to helping hold back the downpour-caused deluges. And while some degree of heavier rainfall is here to stay, cutting greenhouse gas emissions and limiting global temperature rise is the best way to help us evade the worst effects of climate change.
Is the weather actually becoming more extreme? Watch the TED-Ed lesson to learn more: