India, May 28 -- For the past week, the air quality index in Delhi has mostly hovered in the low- to mid-200s, even reaching 300 one morning -- levels the city routinely records in its smoke-choked winters but rarely in summer.

The culprit, according to meteorological experts, is dust lifted from the deserts and semi-arid regions of Rajasthan by the hot, dry winds blasting through northwest India --- a weather phenomenon that, decades ago, would not have manifested with such ecological severity.

The spike in pollution brings back into focus the destruction of the Aravalli ranges, which for centuries acted as a natural barrier. Oriented almost precisely across the corridor through which northwesterly winds carry Thar dust toward the capi...