Scientists predict more extreme rainfall in mountainous areas

A clifftop home in California is in danger of collapsing in a mudslide

A home in San Clemente, California that was evacuated due to a landslide in March 2023.Orange County/AP Fire Authority

This story was originally published by Inside climate news and is reproduced here as part of the Climate window collaboration.

A global increase of extreme rainfall, well outside the range of natural variability, has been well documented by scientists. It’s one of the hallmarks of human-caused global warming, and new research published this week in Nature shows that high-elevation areas, including most of the mountains in western North America, are especially vulnerable to floods that can trigger landslides, floods and severe erosion.

The research from the Department of Energy’s Lawrence Berkeley National Laboratory focused on precipitation extremes, due to their instantaneous triggering of runoff and association with flooding, landslides and soil erosion, the authors wrote. Previous research on extreme precipitation events, however, had not focused on whether deluges fall as rain or snow and how this affects downstream impacts.

We’ve known for decades that as the atmosphere warms, it can hold more water vapor and that water will eventually return to land as precipitation, said first author Mohammed Ombadi, who has a background in civil engineering and carried out the study of modeling as a postdoctoral researcher. at the Berkeley laboratory. Our estimate was that it would be about 7 percent for every degree (Celsius) of warming.

But the researchers found that the increase in precipitation as the warmed atmosphere becomes more saturated was much greater than they expected. What our work is showing is that if you pinpoint just the liquid portion and look at extreme precipitation separately, that rate almost more than doubled to 15 percent, he said.

Their research also suggested a linear rate of increase in extreme precipitation with future warming. The most recent estimates indicate that the world will warm up to 3 degrees Celsius by 2100, which would lead to a 45% increase in extreme rainfall in many mountainous areas.

A big message here is that every grade counts, because it comes with this extra boost, he said. We see this magnified increase in extreme precipitation over most high elevation regions and mountains of snow dominated regions in the Northern Hemisphere. And we see that the mountain ranges of western North America are more at risk than other mountain ranges.

The new research didn’t pinpoint the exact reason for the increased risk of extreme precipitation in those mountains, but Ombadi said researchers suspect that most extreme precipitation in those regions occurs during winter snowstorms at temperatures just below freezing. zero.

Last winter’s extreme snowfall in California’s Sierra Nevada Mountains clearly showed the growing threat as streams and rivers flowing west from the mountains into the Central Valley swelled and filled Lake Tulare, north of Sacramento. Weather and climate experts have been on tenterhooks for several weeks in midwinter, warning that a warm, rainy storm could lead to catastrophic flooding.

A worrying aspect of the new study is that it supports a growing body of scientific evidence that extreme rainfall may exceed projections, said Hayley Fowler, a climate extremes researcher who was not an author on the new paper.

He said that, until recently, the intensification of extreme precipitation was thought to be indeed quite limited by the well-known relationship between the temperature of the atmosphere and the amount of moisture it can hold. Scientists call it the ClausiusClapeyron report, which shows the water-holding capacity of the atmosphere increasing by 7% for 1 degree Celsius of warming.

It is pointing to the fact that, potentially, we will see much bigger changes than we think in extreme rainfall, he said. Some of his research on extreme short-term precipitation suggests that local conditions can lead to increases of more than 7%, such as when rising air over extremely warm areas of land draws more moisture from and around it.

You get more moisture convergence from these local processes and therefore much larger increases can be achieved, he said. But then you also get the transition from snow to rain. So it’s kind of bringing together composite factors that together can have a much bigger effect than we thought.

When we look at the average over a large region, what we tend to see is an increase approaching 7% in the ClausiusClapeyron report, he said. But then what can happen, and what seems to be happening more and more with climate change, is that these increases are concentrated in smaller areas. This means that the impact in smaller areas is much larger than you are potentially planning for. But you don’t know where these events will fall.

It’s not always possible to predict what impacts will trigger the intensified rains, but the threats are many, ranging from glacier collapses and glacial lake flooding to giant wet snow avalanches, rockfall, landslides and massive erosion in fire-scarred areas.

Ombadi said more heavy rains may exacerbate all of these, suggesting that global warming is not only intensifying the Earth’s water cycle, but could also speed up parts of the geological cycle, breaking down more rock faster and moving more sediment and debris through rivers and streams. lakes and oceans. . Another hypothesis, he said, is that more warming causes soil moisture to evaporate, which further intensifies rainfall.

This tells us we should be more aware of and invest more in adaptation and mitigation strategies to really prepare communities living in the Western US for those types of extreme rainfall and the associated risks of flooding, landslides and so on, Ombadi said. , noting that civil engineering may not keep pace with climate change. We need to take these findings into account in how we design and build infrastructure in these mountainous regions so they can withstand the negative consequences of increasing extreme rainfall.

Engineers typically design infrastructure using estimates of extreme precipitation events over the past 50 to 70 years, he said, which don’t account for changes in precipitation intensity and extreme precipitation. New model studies like the one he conducted can help refine the range of potential threats to help civic planners prepare.

The risks are not limited to people living in the high mountains. About a quarter of the world’s population lives in areas directly downstream from mountains, and all will be affected by the increase in extreme rainfall, including many people in developing countries who don’t yet have much access to climate data.

While some of the extreme rainfall hotspots are in developed areas, he said, a significant portion of the developing world, including mountain valley regions in countries like India, Pakistan, Nepal and other Asian countries, will also be affected. from these risks.

He said the new study was limited to looking at the northern hemisphere due to a lack of observational data from places in the global south like the Andes.

This is especially concerning because those countries often lack the resilience and resources to effectively mitigate these short-term extreme events, he said. Thus, it becomes crucial to prioritize long-term investments in resilient infrastructure in these regions.

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