The way clouds cover the Earth may be changing because of global warming, according to a study published Monday that used satellite data to track cloud patterns across about two decades, starting in the 1980s.Clouds in the mid-latitudes shifted toward the poles during that period, as the subtropical dry zones expanded and the highest cloud-tops got higher.These changes are predicted by most climate models of global warming, even though those models disagree on a lot of other things related to clouds, says Joel Norris, a climate scientist at the University of California, San Diego.