Humanity is upsetting not just levels of carbon in the air but those of nitrogen as well. Although the burning of fossil fuels is known to release nitrogen oxides that can excessively fertilize ecosystems or react with other compounds to form smog and acid rain, researchers have had difficulty pinpointing the extent to which people have disrupted nitrogen levels in the atmosphere. To investigate, scientists at Brown University and the University of Washington analyzed an ice core from Greenland, which trapped nitrate deposits over the past three centuries. They found that levels of the rare nitrogen 15 isotope had plummeted over the past 150 years when compared with the more common nitrogen 14. This skewing likely results from an influx of nitrogen oxides from fuel combustion, which for uncertain reasons generates nitrogen oxides depleted in nitrogen 15. The shift, described in the June 5 Science, also coincides with the industrial age frut—indeed, the greatest rate of change happened between 1950 and 1980, after a rapid increase in fossil-fuel emissions.