Changes that begin in the ice at the tops of high mountain ranges are cascading down to lower altitudes. As the world warms, they are changing borders, livelihoods and the shapes of mountains along the way.

It is a sunny day in autumn, and I am walking up a rocky slope next to a glacier at around 3,000m (9,800ft) above sea level, on the border between Austria and Italy. Next to me is Paul Grüner, the owner of a mountain refuge on the Italian side, which overlooks the glacier. Two brown ibexes – a type of wild mountain goat – graze peacefully by a turquoise glacial lake, their long, curved horns peeking out between the rocks. At our feet, a southern slope leads down into Italy, and on the other side, a northern slope faces Austria. Nearby, a weather-beaten wooden signpost with an arrow reads “Grenze / Confine” – meaning “border”, in German and Italian, both of which are spoken in this multilingual area.

Grüner, who has been running the refuge since the 1980s, has invited me up here to show me how much the glacier, called Hochjochferner has dwindled due to global warming. One startling consequence: its meltwater, which used to flow into both Austria to the north and Italy to the south, now just flows into one country, Austria. That’s because the southern part of the glacier has retreated much more dramatically than the northern part, and is now gone, those familiar with the glacier say. It’s just one example of how profoundly climate change is reshaping the mountains – with far-reaching consequences for everything from border relations, to hazards such as rockfall, to Europe’s water supply.

“When I was a child, the glacier covered this entire ridge, and the meltwater on that side flowed down to Italy,” says Grüner, who grew up in the area, gesturing at the south-facing slope. That slope is now rocky and bare. “And now it flows down here, and into Austria,” he adds, and gestures at the slope below our feet, which points down towards that country.