BEIJING—China’s capital and a wide swath of the country’s north were enveloped on March 15 in the worst sandstorm in a decade, leading to the cancelation of hundreds of flights. Skyscrapers in the center of Beijing appeared to drop from sight amid the dust and sand. Traffic was snarled and more than 400 flights out of the capital’s two main airports were canceled before noon. Such storms used to occur regularly in the springtime as sand from western deserts blew eastwards, affecting areas as far as northern Japan. Massive planting of trees and bushes in fragile areas has reduced the storms’ intensity, but the expansion of cities and industries has put constant pressure on the environment throughout China. The National Meteorological Center forecasted the sand and dust would affect 12 provinces and regions from Xinjiang in the far northwest to Heilongjiang in the northeast and the eastern coastal port city …