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By Roger Highfield on

Extreme rainfall could affect a quarter of the global population

Mountains across the Northern Hemisphere will be hotspots for extreme rainfall that could affect a quarter of the world’s population, reports Science Director Roger Highfield.

Climate warming is causing a decrease in snowfall and increase in rainfall at high altitudes in the Northern Hemisphere, triggering more floods, landslides and soil erosion, a new study reports today.

Scientists already expect climate change to increase the volume of precipitation during extreme events but this new research by a team at the Department of Energy’s Lawrence Berkeley National Laboratory in Berkeley, California, marks the first time that researchers have looked at whether that extreme precipitation comes as rain or snow.

Map showing changing rainfall patterns across the northern hemisphere.
Map showing changing rainfall patterns across the northern hemisphere.

‘One quarter of the global population lives in or downstream from mountainous regions,’ said Mohammed Ombadi, first author of the paper published in Nature. ‘They are going to be directly affected by this risk.’

To uncover this hazard the team focused on the ‘extratropical’ Northern Hemisphere, ranging from 20N to 90N, because of limitations of computer modelling in the tropics and lack of data on the southern hemisphere.

They combined data from climate observations from between 1950 and 2019 with future projections, up to 2100, taken from computer models, and found that as the fraction of water falling as snow decreased in mountainous regions the rainfall rose.

For every 1 degree Celsius increase in the global temperature, the researchers expect an average of 15% more rain at high elevations. ‘One degree of warming causes 15% more rain, while 3 degrees leads to a 45% increase in rainfall,’ Ombadi said.

‘This increase in rainfall extremes is not only something that is going to happen from now until the end of the 21st century – we’re already seeing it,’ Ombadi said. ‘Rainfall extremes in mountains have already been increasing and will continue to change with that 15% rate.’

Regarding European mountain ranges closer to the UK, the effect could be seen in the Alps and Kjölen, for example. However, those at greatest risk of extreme rainfall events are the North American Pacific mountain ranges (the Cascades, Sierra Nevada, and coastal ranges from Canada to Southern California), the Himalayas, and high-latitude regions.

Peaks in the Hamla pass, Himalayas
Photograph of peaks in the Hamla pass, Himalayas. Captured 1850-1900.

‘We think that North American Pacific mountain ranges are more susceptible to the risk of rainfall extremes than other mountain ranges because a significant portion of snowfall in this region typically occurs at temperatures just below zero degrees Celsius,’ Ombadi said. ‘The slightest change in air temperature will shift this snowfall to rainfall. This is unlike other mountain ranges where snowfall may occur at very low temperatures below zero degrees.’

The team hopes that fellow climate scientists will incorporate the distinction between snowfall and rainfall to improve the predictions of global climate computer models, and that civil engineers and planners will use the data to better prepare for intense downpours. ‘We need to factor these results into how we design and build the infrastructure in these mountainous regions,’ Ombadi said.

He added: ‘It is important to note that the majority of inhabited land, where the impacts of this phenomenon are most relevant, lies in the northern hemisphere. However, we encourage other researchers to build upon our work and explore mountain ranges in the southern hemisphere, such as the Andes. By doing so, we can gain a more complete understanding.’

Explore more about the science of climate change in our animated video series.