What Used to Be 100-Year Rain Events Could Happen Every 5 Years

Just a couple weeks ago, Michigan experienced record-breaking rainstorms that broke two dams and caused widespread flooding in cities and towns downstream. They’re in line with a growing trend of heavier downpours due to climate change.

In new report published in Proceedings of the National Academy of Sciences on Monday, researchers from Environment and Climate Change Canada looked what has happened to heavy precipitation across North America—and what’s in store. The findings make it clear that global warming has made precipitation events in North America far more severe.

Since the 19th century, greenhouse gas emissions have caused the Earth to warm by an average of 1 degree Celsius (1.8 degrees Fahrenheit). That may not seem like much change, but it makes a big difference to the climate, including to rainfall. North America, for instance, saw 5 percent more rain in 2010 than it did in 1961.

“We know from physics that warmer air can hold more moisture and this will lead to an increase in heavy rainfall in most places,” Megan Kirchmeier-Young, a research scientist at Environment and Climate Change Canada and the study’s lead author, told Earther in an email.

To reach their conclusions, Kirchmeier-Young and a colleague examined examined trends in precipitation from 1961 to 2010. They then looked at how greenhouse gas emissions change precipitation “return periods,” which estimate the likelihood of how long it will be between rainfall events of a given magnitude under a worst case carbon emissions scenario. They found that rain events that occur every 20 years in a world of pre-industrial temperatures now take place every 5 years on average. Events that occurred once every 50 years in a climate free of industrial carbon pollution now happen once every 10 years. And events that occurred once in 100 years now happen every 20 years.

If the world warms by another degree—which is what the United Nation’s Paris Climate Agreement is trying to avoid—the situations will only get more dire. The former 100-year rain events would become 5-year ones and the odds of other extremes would increase as well. All this extreme rain in North America will put many people at risk, as evidenced by what just happened in Michigan.

“Heavy rainfall is a major contributor to flash flooding events, especially in urban areas,” said Kirchmeier-Young. “As we expect a continued increase in the frequency and severity of extreme rainfall events with continued warming, flash flooding may become an increasing concern.

As it stands, North American infrastructure is simply not prepared to handle all this precipitation. Kirchmeier-Young noted that most infrastructure is “designed based on historical values” that just don’t hold true anymore and certainly won’t in the future. U.S. infrastructure, for example, received a D+ from the American Society for Civil Engineers in part because it’s not resilient to the changes already in motion.

The study, which comes at the beginning of 2020’s hurricane season and the threat of massive rain-makers like Hurricane Harvey, shows how urgently we need to adapt our buildings, roads, and farms. And it’s an important warning of what’s to come if we don’t curb greenhouse gas emissions.