The rub is in the definition of terms. I have an even higher degree of certainty that humans can change the climate: There are small regions of the earth's surface which for decades have had hotter temperatures than they did previously and higher than surrounding regions due to human activity (they're called "cities" and the higher temperatures are called "the urban heat island effect"). Temperatures in the American Great Plains used to have much larger swings between summer heat and winter cold before the Corps of Engineers built lots of lakes for flood control.
I'm even think it likely that the warming trend since the end of the Little Ice Age (though largely caused by variations in solar irradiance and solar magnetism -- see work of Svensmark on the latter -- has been exacerbated by human activity. The problem is that the IPCC models and all the other alarmist models all assume that human contribution to warming is mediated entirely by CO2 and CH4 emissions, when in fact, besides these there are the already mentioned urban heat island effect and changed to Arctic albedo due to the deposition of soot from coal and wood burning (both of which, unlike the greenhouse gas contribution explain the asymmetry between the Arctic and Antarctic, which cannot be explained by greenhouse gas warming, as greenhouse gasses, esp. CO2, are well-mixed in the atmosphere).
