Voice of America: Two American scientists have received a major environmental award, the Tyler Prize, for their work in tracing global warming and chronicling the effects of pollutants on the atmosphere and ice sheets. Richard Alley of Penn State University has traced climate change through the record left in the ice sheets of Greenland and Antarctica. Indian American scientist Veerabhadran Ramanathan of the Scripps Institution of Oceanography has studied the effects of pollutants in shifting …