Global warming is only one aspect of a larger phenomenon called Climate change. That includes more extreme weather, not only temperatures, but long periods of drought or floods, more /larger occurrences of hurricanes, el nino , tsunami etc.
Another evidence for climate change may be migration of certain species to areas where they were not common before. For example in my area, mosquitoes are becoming more common compared to 10 years ago.
What about weather extremes that affect horticulture? This year in Europe, there was an unusually warm weather in early spring followed by frost that damaged a large amount of flowers on trees which resulted in lower production of fruit.
Don't get me wrong. These things did happen in the past and they are nothing new. What is unusual is frequency and scale of these events.
On the other side, it is true that in prehistoric era, there was a period of warmer climate. So this is not the first time that the temperature rises. The problem is that it happens quite quickly in geological terms.
There's also a positive feedback. For example, as the ice melts, the albedo (ratio of reflected and absorbed light on the surface) changes which results in larger absorbtion of heat and further warming.
There are huge amounts of methane below surface in siberia under frozen ground. As the permafrost thaws, methane is released and the temperature increases, causing higher release of methane.
So no, not whole climate change is caused directly by human activity. But human activity serves as the well known movement of butterfly wings that enhances natural cause and effect cycle of positive feedback in the atmosphere.
It is honest to say that a few studies of GW were manipulated and were found not to describe the actual state of the problem.
However, it is still evident that the climate changes and people will have a hard time in the future to get used to it.
To be honest, I don't quite understand the American problem with accepting the issue because the rest of the world accepts it as a fact. (in fact, yes, I do understand the economical motive of certain industries :-p)
Source : me, environmental science graduate
(disclaimer to prevent possible misunderstanding : this post was not written to defend some stupid and contraprodictive "ecological" practices that are presented as a solution. I'm not a fan of "green washing" in marketing either)