It turns out soot, a by-product of burning fossil fuels, is to blame for an average temperature rise of about 1.2 degrees Fahrenheit. While that's not quite enough to melt a mogul, it is doing demonstrable damage (from the ski bunny's point of view, at least). When soot falls, it darkens the snow it lands upon. In turn, the darkened snow absorbs more of the sun's energy than whiter, cleaner snow would. Absorbing more sunlight results in a thinner snowpack, which then reflects less sunlight back into the atmosphere, leading the area to heat up even more. So, soot actually darkens snow in two ways, from its own dark color and by shrinking the snowpack to expose the dark natural ground beneath. This "soot-snow cycle" causes snow to melt up to a month earlier, and furthermore, it leads to reduced run-off in late spring and summer. Says atmospheric scientist Yun Qian, "If we can project the future -- how much water we'll be getting from the rivers and when -- then we can better plan for its many uses. Snowmelt can be up to 75 percent of the water supply, in some regions. These changes can affect the water supply, as well as aggravate winter flooding and summer droughts."