It's easy to imagine new snow so bright that we must avert our eyes even while wearing sunglasses. What scientists are discovering, though, is this brilliant whiteness of snow and ice is increasingly being dimmed by air pollution.

From Greenland's ice sheets to Himalayan glaciers and the snowpacks of western North America, layers of dust and soot are darkening the color of glaciers and snowpacks, causing them to absorb more solar heat and melt more quickly, and earlier in spring.

This trend toward darker snow from soot and dirt has been observed for years. Sources vary from dust blowing off deserts and snow-free Arctic land, to soot from power plants, forest fires, and wood-burning stoves. But now soot and dust are taking