Just as the world's most respected scientific bodies have confirmed that world is getting hotter, they have also stated that there is strong evidence that humans are driving the warming. The 2005 joint statement from the national academies of Brazil, Canada, China, France, Germany, India, Italy, Japan, Russia, the UK and the US said:

"It is likely that most of the warming in recent decades can be attributed to human activities."

Countless more recent statements and 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, such as the burning of fossil fuels and changes in land use, including agriculture and deforestation."

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 tonnes of greenhouse gases to be released into the atmosphere. As scientists have known for decades, these gases capture heat that would otherwise escape to space – the equivalent of wrapping the planet in an invisible blanket.

Of course, the planet's climate has always been in flux thanks to "natural" factors such as changes in solar or volcanic activity, or cycles relating the Earth's orbit 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 plausible explanation for the warming observed to date, 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. Nonetheless, the evidence is now extremely strong.

We'll look at more specific questions about the links between greenhouse gases and global warming separately. Check back soon for those.

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