Climate Change Can Turn Snow Into Rain, Raising Risks in Mountain Zones

As the climate warms, mountain regions will get more extreme rainfall than previously thought, and more of the dangers that come with it, according to a study published on Wednesday in the journal Nature.

While scientists have studied how climate change may increase extreme precipitation overall, until now they hadn’t teased apart how much of the most extreme precipitation will fall as snow and how much as rain. The distinction is important because rain tends to produce more hazards for humans than snow does, including floods, landslides and soil erosion.

As the planet heats up, snow is starting to turn into rain, even in the mountains. The study found that for every one degree Celsius, or 1.8 degrees Fahrenheit, that the planet warms, higher elevations can expect 15 percent more extreme rainfall.

“This is the first time that it has ever been quantified,” said the study’s lead author, Mohammed Ombadi, an environmental data scientist at Lawrence Berkeley National Laboratory. This increase in extreme rainfall is “almost double” the increase in total extreme precipitation, including both rain and snow, that climate scientists previously expected. The precipitation finding applies only to the world’s highest regions, above approximately 2,000 meters or 6,500 feet of elevation.

But about one-quarter of the human population lives either in mountain regions or directly downstream from them, Dr. Ombadi said. While landslides don’t travel very far, flooding tends to affect people downstream more, he explained, adding that rainfall is one of the most important factors in predicting the risks of both these hazards. Soil erosion can undermine farms and natural ecosystems, and further raise the risks of floods and landslides. These threats come on top of those posed by melting glaciers in the same mountain ranges and river valleys.

Frances Davenport, a professor of civil and environmental engineering at Colorado State University, who wasn’t involved in the study, confirmed that while researchers had separately examined how extreme precipitation was increasing and how snowfall was shifting to rainfall, not much research had combined these two questions until this new study.

“It’s a nice way to put those changes together and highlight the regions where we should be particularly on the lookout for large changes in flood risk and extreme rainfall,” Dr. Davenport said.

In their study, Dr. Ombadi and his colleagues analyzed historical data from 1950 to 2019 as well as projections of climate change through the end of the 21st century. They focused on the temperate and Arctic regions of the Northern Hemisphere because data from the tropics and the Southern Hemisphere is lacking.

As they modeled different global warming scenarios, the researchers found that extreme rainfall kept increasing steadily, at the same rate, for each degree of warming. “If you have one degree of warming, then that’s a 15 percent increase. If it’s three degrees, then that’s going to be a 45 percent increase in rainfall,” Dr. Ombadi explained.

This was a surprise, as the team expected the increase in rainfall to slow down and plateau as temperatures rose more and more. They used several different climate models, with relatively consistent results between all of them. “The big message is that every degree matters,” Dr. Ombadi said. He cautioned, however, that climate models are still somewhat uncertain at more extreme temperatures.

The researchers also found that the higher the elevation, the bigger the increase in extreme rainfall. Unlike the shift that accompanied rising temperatures, this change wasn’t linear: The higher up they looked, the more they found rainfall to increase. Different mountain ranges around the Northern Hemisphere also had slightly different risks of extreme rainfall. Researchers are still trying to figure out why.

The number of deadly landslides around the world has been rising in recent decades, according to a separate 2019 study. Most of these landslides happened in places exposed to extreme rainfall.

The highest-risk areas in this older landslide study match up with the highest-risk areas in the new rainfall study, said Ubydul Haque, a geospatial epidemiologist at Rutgers University and the lead author of the 2019 paper. Dr. Haque was impressed by the scale of the data the Lawrence Berkeley team used. Their approach was “extremely novel,” he said. Dr. Haque thought the new study’s findings and underlying data could be useful for future research on the health and safety implications of extreme rainfall.

Dr. Ombadi, who has a background in civil engineering, hopes that his team’s findings will help improve risk assessment models for landslides and floods and lead to better planning and infrastructure in places vulnerable to these hazards. The research may also be useful for improving the climate models researchers rely on to predict long-term changes in rainfall.