Global warming is making extreme rain and catastrophic flooding more likely

A warming world is transforming some major snowfalls over mountains into extreme rain, worsening both dangerous flooding as well as long-term water shortages, a new study has found.

Using rain and snow measurements since 1950 and computer simulations for future climate, scientists calculated that for every degree Fahrenheit the world warms up, extreme rainfall at higher elevation increases by 8.3%, according to a study in the journal Nature.

Heavy rain in mountains causes a lot more problems than big snow, including flooding, landslides and erosion, scientists said. And the rain is not conveniently stored away like snowpack that can recharge reservoirs in spring and summer.

"It is not just a far-off problem that is projected to occur 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, a Lawrence Berkeley National Laboratory hydrologist and climate scientist.

The study looked at only the heaviest rains each year over six decades in the northern hemisphere, finding that as altitude rose, so did the turbo-charging of rain. The biggest increase in rains were noticed at about 10,000 feet (3,000 meters).

That includes much of the American West, where Ombadi said “it’s very pronounced”, as well as parts of the Appalachian Mountains. Another big hotspot in Asia is the Himalayas, Tian Shan and Hindu Kush mountains, with the Alps also affected.

About one in four people on Earth live in an area close enough to mountains or downhill that extreme rain and flooding would hit them, Ombadi said. The flooding also can hurt food production. He pointed to California department of agriculture estimates of $89m in crop and livestock losses from this year’s torrential rains.