Just as the world’s most respected scientific bodies have confirmed that the world is getting hotter, they have also stated that there is strong evidence that humans are driving the warming． Countless recent reports from the world’s leading scientific bodies have said the same thing． For example, a 2010 summary of climate science by the Royal Society stated that: “There is strong evidence that the warming of the Earth over the last half-century has been caused largely by human activity．”
The idea that humans could change the planet’s climate may be counter-intuitive（与直觉不符的）, but the basic science is well understood． Each year, human activity causes billions of tons of greenhouse gases to be released（释放）into the atmosphere． As scientists have known for years, these gases hold heat that would otherwise escape to space, wrapping the planet in an invisible blanket．
Of course, the planet’s climate has always been changing thanks to “natural” factors（因素） such as changes in solar or volcanic（火山的）activity, or cycles relating the Earth’s going around the sun． According to the scientific literature, however, the warming recorded to date 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 observed so far, that would beg a difficult question． As Robert Henson puts it in The Rough Guide to Climate Change: “If some newly discovered factor can account for the climate change, then why aren’t carbon dioxide（二氧化碳）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．
In most scientists’ opinion, the global warming is mainly caused by      ．

A．solar activity	B．volcanic activity
C．the Earth’s going around the sun	D．human activity

The text is developed by      ．

A．giving typical examples	B．following the order of space
C．analysing a theory and arguing it	D．comparing and finding differences

The underline word “identical” means      ．

A．totally different	B．exactly the same
C．extremely important	D．relatively independent

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