When Tashi Lhazom was 11, a glacial lake overflowed and inundated her home in a high Himalayan valley. Of her family’s hundreds of shaggy yaks, only two survived. In 2025, another flash flood cascaded into the village, the result, scientists say, of melting permafrost on a warming planet. Such natural disasters spurred Ms. Lhazom’s political awakening. When Nepali young people protested a year ago, she joined other climate activists on the forefront of a Gen Z revolution that eventually ushered in a youth-dominated government in March. Last month, when a…