It's true that quite a few most respected scientific authorities have confirmed that the world is becoming hotter and hotter. There's also strong evidence that humans are contributing to the warming. Countless recent reports have proved the same thing. For instance, a summary of the climate science made by the Royal Society notes, "Global warming over the last half-century has been caused mainly by human activities."
You may not believe that humans could change the planet's climate, and the basic science is well understood. Each year, billions of tons of greenhouse gases are released into the atmosphere because of human activities. As has been known for years, these gases hold heat that would otherwise escape to space, wrapping the planet in an invisible blanket.
Of course, the Earth's climate has always been changing due to natural factors such as volcanic eruptions or changes in solar system, 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 gases in the atmosphere—not the warming we would expect from other possible causes.
Even if scientists have discovered 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 factors are to blame for the climate change, then why aren't carbon dioxide and the other green-house 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, so most scientists are careful not to state human influence as an absolute certainty.
