On July 21, a NOAA-led study, The Persistently Variable “Background” Stratospheric Aerosol Layer and Global Climate Change, found that minuscule particles suspended in the stratosphere that have helped to slow global warming by as much as one-third have actually increased about 7% a year from 2000-2010. Volcanic eruptions and man-made factors, such as coal burning, are thought to be likely causes, but the causes are still unknown.