Warming World worsens extreme rain, study finds

A warming world is transforming some major snowfalls across mountains into extreme rain, worsening both dangerous floods such as the one that devastated Pakistan last year and long-term water shortages, a new study has found.

In a study published in Wednesday s journal Nature, scientists calculated that for every degree Fahrenheit the world warms, extreme rainfall at higher elevations increased by 8.3% 15% for every degree Celsius.

The heavy rain in mountains causes more problems than large snow, such as flooding, landslides and erosion, scientists said. And the rain doesn't conveniently store away like snowpack that can recharge reservoirs in spring and summer.

It is not only a far-off problem that is projected to happen in the future, but the data is actually telling us that it s already happening and we see that in the data over the past few decades, said lead author Mohammed Ombadi of the Lawrence Berkeley National Laboratory hydrologist and climate scientist.

As the world has warmed to the brink of the 1.5-C 2.7-F globally agreed threshold to stem the worst effects of climate change, this study shows every degree Celsius matters because it comes with an additional 15% increase in extreme rain over mountains, Mr. Ombadi said. That per-degree rainfall boost in the mountains is more than twice the increase the rest of the world gets from warming air holding more water.

The study found that as altitude increased, so did the turbocharger of rain, resulting from the heaviest rains every year over six decades in the northern hemisphere. The biggest increase in rains was noticed at about 10,000 ft 3,000 meters, which includes much of the American west, where Ombadi said it s very pronounced as well as parts of the Appalachian Mountains. A major hotspot in Asia is the Himalayas, Tian Shan and Hindu Kush mountains, which are also affected by the Alps.

About one in four people on Earth live in an area close enough to the mountains or downhill that extreme rain and flooding would hit them, Ombadi said.

It means more of the type of floods off the mountains that killed more than 1,700 people in Pakistan and put one third of the country underwater, Mr. Ombadi said. He also said that they have not studied Pakistan's 2022 floods exactly so there may be some small differences.

The study makes sense and the implications are serious, said University of California climate hydrologist Park Williams, who was not involved in the research. Scientists expect more precipitation with higher temperatures, but heavy snow's flooding impact is lessened because it takes time to melt and it's easier to monitor snowpack to see what's happening.

As the proportion of mountain precipitation falling as snow decreases, flood hazards may enhance especially rapidly, Williams said.

It hits hard in the American west in two different ways, said a study co-author, Charuleka Varadharajan, a lab climate scientist and hydrologist.

This kind of extreme rainfall is going to make the floods worse. Where is the water going? And then you have to figure out where it's going? She said she would not comment on the situation, but said that there was a need to improve. We have this situation right now in the Sierras with the Tulare Lake flooding, and such a serious issue relating to that. The floods can also harm food production, Mr. Ombadi said. He referred to California Department of Agriculture estimates of $89 m in crop and livestock losses from this year's torrential rains.

In the long term, however, there is another problem with the water supply. When the American West experiences heavy snowfall in the winter, that snow melts slowly in spring and summer, filling reservoirs where it can be useful when it is needed later.

It s going to decrease your snow, your water supply in the future, Varadharajan said. You're going to have more short-term runoff, leading to more floods and less snowpack that recharges the groundwater and the groundwater is ultimately what helps maintain stream flows.