CBS News: Americans are now almost evenly split over what they believe to be the major contributor to global warming, according to a new Gallup Poll released Thursday. Fifty percent of respondents believe human activities are causing a rise in the earth's temperature, while 46 percent say the rise is due to natural causes. Gallup reports that as early as 2003, 61 percent of Americans believed global warming was due to human activities and 33 percent to natural causes.