This glacier wasn’t supposed to shrink. In the late 1800s, retreating glaciers throughout the Alps marked the end of a centuries-long cold spell that climatologists have dubbed the Little Ice Age. Austria’s Hintereis Glacier (above), which retreated more than 1 kilometer between 1860 and 1930, is one of the most dramatic examples. But the meltback has bewildered researchers for decades. Weather data from that era, including trends in precipitation, suggest that glaciers in the area should have been expanding, or at least holding their ground. Indeed, at that time elsewhere in the world, glaciers were advancing, and carbon dioxide concentrations hadn’t risen enough to strongly affect global climate. Now, researchers have come up with a possible explanation for the curious shrinkage: It’s as simple as soot. By darkening the insulating layer of snow over the glacier and causing it to melt earlier each year, soot exposed glacial ice to the sun’s melting rays for a longer period every summer, they explain. Layers of ice in the upper reaches of glaciers provide a year-by-year chronicle of soot emitted by local industry and by the coal and wood burned to heat homes in the valleys nearby. In many places, these emissions more than doubled between the 1850s and the 1880s. The researchers combined that data with modern weather patterns to estimate the amount of soot deposited on glaciers in the 1800s. According to their models, soot could have caused about 1 meter’s worth of additional ice melt each year despite cool temperatures and steady precipitation, they report today in the Proceedings of the National Academy of Sciences. By 1900, increased emissions of soot could have triggered the loss of more than 15 m of ice from a glacier’s surface; by 1930, the loss could have totaled 30 m or more—magnitudes and timing that can easily account for the Alpine glacial retreat, the scientists contend.