The idea that humans could change the planet’s climate may be counter-intuitive, but the basic science is well understood. Each year, human activity causes billions of tons of greenhouse gases to be released 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 such as changes in solar or volcanic 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 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.

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