Text 1
Global warming
On Feb. 2, 2007, the United Nations scientific panel studying climate change (01) declared that the evidence of a warming trend is “unequivocal”, and that human activity has “very likely” been the driving force in that change over the last 50 years. The last report by the group, the Intergovernmental Panel on Climate Change, in 2001, had found that humanity had “likely” (02) played a role.
The greenhouse effect has been part of the earth’s workings since its earliest days. Gases like carbon dioxide and methane allow sunlight to reach the earth, but prevent some of the resulting heat from radiating back out into space. Without the greenhouse effect, the planet would never have warmed enough to allow life to form. But as ever larger amounts of carbon dioxide have been released along with the development of industrial economies, the atmosphere has grown warmer at an accelerating rate. Since 1970, temperatures have gone up at nearly three times the average for the 20 century.
(Global Warming Science - The New York Times, May 6, 2007)
According to the 2nd paragraph,