Humans have altered the world"s climate by (1)_____ heat-trapping gases since almost the beginning of civilization and even prevented the start of an ice age several thousand years ago, a scientist said. Most scientists (2)_____ a rise (3)_____ global temperatures over the past century (4)_____ to emissions of carbon dioxide (5)_____ human activities like driving cars and operating factories. Dr. William Ruddiman, a professor at the University of Virginia, said at a meeting of the American Geophysical Union (6)_____ humans" effect (7)_____ climate went back nearly 10, 000 years (8)_____ people gave up hunting and gathering and began farming. In a commentary accompanying the article, Dr. Thomas J. Crowley of Duke University, said he (9)_____ Dr. Ruddiman"s premise at first. "But when I started reading, Dr. Crowley wrote, "I could not help but (10)_____ whether he just might be (11)_____ something." The climate of the last 10,000 years has been unusually stable, (12)_____ civilization to flourish. But that is only because people chopped down swaths of forest in Europe, China and India for croplands and pastures. Carbon dioxide (13)_____ by the destruction of the forests, plus methane, another heat-trapping gas, (14)_____ by irrigated rice fields in Southeast Asia, trapped enough heat to (15)_____ an expected natural cooling. Levels of carbon dioxide and methane rise and fall in natural cycles (16)_____ thousands of years, and both reached a peak at the end of the last ice age 11;000 years ago. Both then declined (17)_____ expected. Both (18)_____ declining through the present day, leading to lower temperatures, and a new ice age should have begun 4,000 to 5,000 years ago, Dr. Ruddiman said. Instead, levels of carbon dioxide reversed 8,000 wears ago. The decline (19)_____ methane levels reversed 5,000 years ago, (20)_____ with the advent of irrigation rice farming.