Of course, the earth's climate has always been changing due to “natural” factors such as volcanic eruption (熔化) or changes in solar, or cycles concerning the Earth's going around the sun. According to the scientific research, however, the warming observed by now matches the pattern of warming we would expect from a build-up of greenhouse gas in the atmosphere-not the warming we would expect from other possible causes.
Even if scientists did discover another reasonable explanation for the warming recorded so far, that would give birth to a difficult question. As Robert Henson puts it: “If some newly discovered factor is to blame for the climate change, then why aren't carbon dioxide and the other greenhouse gases producing the warming that basic physics tells us they should be?”
The only way to prove with 100% certainty that humans are responsible for global warming would be to run an experiment with two identical Earths-one with human influence and one without. That obviously isn't possible, and so most scientists are careful not to state human influence as an absolute certainty.
59.In most scientists' opinion, the global warming is mainly caused by________.
A. solar activity
B. volcanic activity
C. natural factors
D. human factors
60.The text is developed by________.
A.giving typical examples
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