Mountains have for millennia been a symbol of immovable stability. A warming planet is undermining that, leaving landslides, floods, and avalanches in its wake.

Global warming is making high-altitude regions unstable, and could trigger more frequent landslides and floods.

WASHINGTON, Aug 27 — The cause of the deadly landslide and flood along the Nepal-Tibet border isn’t yet certain, but experts say the dangers associated with such events are...