17（）

A．aggravate
B．intensify
C．offset
D．provoke

Humans have altered the world’s climate by (1) heat-trapping gases since almost the beginning of civilization and even pred 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.