New research showing how global warming intensifies extreme rainfall at the regional level could help communities better prepare for storms that in the decades ahead threaten to swamp cities and farms.

The likelihood of intense storms is rising rapidly in North America, and the study, published Monday in the Proceedings of the National Academy of Sciences, projects big increases in such deluges.

“The longer you have the warming, the stronger the signal gets, and the more you can separate it from random natural variability,” said co-author Megan Kirchmeier-Young, a climate scientist with Environment Canada.

Previous research showed that global warming increases the frequency of extreme rainstorms across the Northern Hemisphere, and the new study was able to find that fingerprint for extreme rain in North America.

“We’re finding that extreme precipitation has increased over North America, and we’re finding that’s consistent with what the models are showing about the influence of human-caused warming,” she said. “We have very high confidence of extreme precipitation in the future.” 

At the current level of warming caused by greenhouse gases—about 1.8 degrees Fahrenheit above the pre-industrial average—extreme rainstorms that in the past happened once every 20 years will occur every five years, according to the study. If the current rate of warming continues, Earth will heat up 5.4 degrees by 2100. Then, 20, 50 and 100-year extreme rainstorms could happen every 1.5 to 2.5 years, the researchers concluded.

“The changes in the return periods really stood out,” she said. “That is a key contributor to flash flooding events and it will mean that flash flooding is going to be an increasing concern as well.”

Better Science, Better Forecasts

The 2013 floods in Boulder, Colorado that killed nine people and caused more than $2 billion in property damage are a good example of how such climate studies can help improve flood forecasts, said Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research in Boulder, Colorado.

“That was an exceptional event and the rain was like tropical rain. The radars greatly underestimated the magnitude as a result,” said Trenberth who returned to his home in Boulder during the floods with a broken foot, only to have to climb on his roof to direct the gushing water away from his house.

A subsequent study found that the rain resulted from an unusual atmospheric brew over Colorado. Mountain thunderstorms mingled with a juicy atmospheric river from the tropics, dropping up to 17 inches of rain in a few days, nearly as much as Boulder’s annual average total. Human-caused climate change “increased the magnitude of heavy northeast Colorado rainfall for the wet week in September 2013 by 30%,” the study found.

A separate study concluded that global warming actually decreased the likelihood of the 2013 floods. The conflicting results hint at the complexities of climate research, but, since then, the influence of human-caused climate change on extreme weather has become more clear.

The risks will continue to increase as the atmosphere warms, said David R. Easterling, a climate extremes researcher and director of the U.S. National Climate Assessment. “The detection has been there for a while on a lot of extreme events,” said Easterling, who was not involved in the new study. “We’re going to see increases in extreme events, and we need to be prepared.” 

Easterling said most current infrastructure, such as dams and bridges, was designed based on rainfall values from the mid- to late-20th century and was not built to withstand the more frequent extreme rains identified by the new research.

“There are going to be much more damaging floods that are going to wash out a lot of the infrastructure,” he said. “You’ll see more floods and bigger floods and major impacts to our civil engineering infrastructure.”

According to the Environmental Protection Agency’s website, data from the National Oceanic and Atmospheric Administration indicates that the percentage of total precipitation coming from intense single day events has increased significantly since about 1980, with nine of the top 10 years for extreme one-day precipitation events occurring since 1990. The EPA’s precipitation indicator website also shows similar changes at the global scale.

Warmer Air, More Moisture and Shifting Storm Tracks

One way to visualize the planet’s climate system is as a heat-driven pump that tries to balance the planet’s energy by circulating it around the globe and cycling it from oceans, to land, to the atmosphere. Global warming puts more heat into the pump and that energy is manifested elsewhere in the system. For instance, for every 1.8 degrees Fahrenheit of warming, the atmosphere holds 7 percent more moisture that can fall as extreme rain, hail or snow. 

But global warming can increase rainfall by much more than 7 percent in individual events. In Hurricane Harvey, for example, the estimated boost in rainfall was about 30 percent, said Trenberth.

“The outcome depends on the kind of storm. If the rainfall is in or near the center of the storm, as for a hurricane, then the extra oomph from the latent heat release intensifies the storm and makes it bigger and longer lasting,” he said. “This can also happen for an individual thunderstorm.” He was not involved in the new study.

For storms outside the tropics, the most rain happens away from the center, which doesn’t necessarily make the rain more intense, but can affect the way the storms move and develop, he added.

“This is the atmospheric river phenomenon and requires the weather situation to remain stuck for a bit, as a river of moisture from the subtropics, like the pineapple express, pours into a region,” he said. A 2019 study showed that atmospheric rivers cause most of the flood damage in the Western United States already, and global warming is projected to intensify those events.

In addition to simply having more moisture in the atmosphere, global warming may also drive more extreme rainfall by shifting global weather patterns, said climate scientist Peter Pfleiderer, with Climate Analytics in Berlin. 

In a 2019 study published in the journal Nature Climate Change, Pfleiderer and other scientists looked at how global warming changes weather patterns in ways that make heat waves, droughts or rainstorms longer or more intense. With global temperature increases of 2.7 to 3.6 degrees Fahrenheit (the range to which the Paris climate agreement hopes to limit warming), periods of heavy rain would increase 26 percent—the most of all the weather phenomena studied—the research found.

Friederike Otto, acting Director of the Environmental Change Institute at Oxford, said new research showing how global warming affects extreme rain regionally complements studies that identify the effect on individual events.

As a co-investigator with World Weather Attribution, Otto has been involved in a series of recent studies looking at how global warming affects droughts, heat waves and extreme rain. The strongest signal, as she expected, was with heat waves, but she expects rain events “far outside the observations so far.”

“One thing I only started to realize in the last year, is how important attribution is for making projections,” she said. Climate attribution studies show how the warming of the planet makes some extremes more likely, and intensifies other weather events. Linking measurements of what actually happens with model predictions “gives you more confidence that the changes are because of climate change,” she said.

Escalating Impacts Require Adaptation and Resilience

Floods caused by extreme rain are among the costliest climate-related disasters. A NOAA compilation of billion-dollar disasters lists a long string of deadly catastrophes caused, at least in part, by extreme rain. These include the January 2020 floods in New York, Michigan and Wisconsin, where significant damage along the shoreline of Lake Michigan was compounded by extremely high water levels in the lake, as well as a lack of seasonal ice cover.

In 2019, extreme and persistent spring rainfall in the Midwest led to one of the costliest inland flooding events on record. Floodwaters inundated millions of acres of farms, along with numerous cities and towns and Offut Air Force Base in Nebraska—the third U.S. military base to be damaged by a billion-dollar disaster in a six-month period. In all, that wave of flooding caused $10.9 billion in damage, NOAA estimated.

Earlier this month, persistent heavy rains contributed to the failure of a dam in Michigan, and Easterling said heavy rains were also implicated in the 2017 Oroville Dam failure that cost $1.1 billion and forced the evacuation of 180,000 people. The flooding caused by record rainfall from Hurricane Harvey in 2017 was a big part of the $125 billion worth of damage caused by the storm.

Extreme rain can also have an impact on a smaller scale. In mountainous areas, heavy precipitation over even a small area can be disastrous. In the Rocky Mountains, such cloudbursts have caused toxic floods of acidic water from abandoned mines, and in the European Alps, scientists say extreme rains are unleashing larger and more destructive rockfalls and landslides.

“We are going to get more intense, extreme precipitation, this is one of the things we are sure about,” said Hannah Cloke, a University of Reading natural hazards researcher and hydrologist specializing in flood forecasting. 

The United Kingdom has been hit repeatedly by extreme rain in recent years, including Storm Desmond in 2015, which was linked with global warming and caused at least $550 million in damage, flooding nearly 10,000 homes and businesses. Cloke said the recent flooding has apparently even shaped her daughter’s world view. For a recent school assignment, the nine-year-old used plastic bottles to build a floating house reminiscent of the movie Waterworld.

“Most of the design standards for storm infrastructure are not high enough for the predictions, or even what we’re seeing right now,” she said. “We have to get away from the idea that you can just carry on business as usual. We have to adjust our expectations of what could happen. We need to get people out of harm’s way and be realistic about where we live.”

Cloke said the certainty of increased extreme rainfall means that communities have to adapt by creating or restoring natural areas that can soak up the rains in the uplands, and cities need to be redesigned with green roofs and other measures to prevent flood waters from piling up and destroying property. More and more, flood experts are thinking in terms of socio-hydrology, she said.

“You can’t just look at the water, at the heavier rain, and how fast it’s running down the rivers,” she said. “It’s about how humans and water interact at all levels, and how politics controls where the water is. It’s about who is at risk of flooding and whether those people have any agency to reduce the risk.” 

New research like the PNAS study that shows the regional fingerprint of global warming on extreme rainfall can help reduce the risk, she said, because it enables better short-term forecasts. 

“We have a lot of the right science in place but we still can’t predict the exact locations and amounts,” she said. “We don’t quite understand the development of the water cycle and we often underestimate rainfall for those reasons. But we shouldn’t be surprised that these rains are happening. We’re going to see entire cities at a standstill.”