Humans produced most of the gases that have eroded the Earth’s ozone layer, according to a study published in today’s issue of the journal Nature. A team of scientists from the National Oceanic and Atmospheric Administration took measurements of air trapped in polar snowpack in Greenland and Antarctica, showing for the first time that major ozone-depleting gases — chlorofluorocarbons, halogens and chlorinated solvents — were not present in detectable amounts in the atmosphere in the late 19th and early 20th centuries.