The fact that the earth has gone through warming and cooling cycles in the past is certainly true, but that fact does not necessarily preclude humans from having an effect on the process. That would be like saying that since dinosaurs went extinct long before man was on the scene, we can't be accused of playing a role in the extinction of any specie since then. In any case, too many people take pre-determined stances on this, and on other issues, based on their political ideology rather than on what they've researched on the subject. When they claim their stances are based on facts or research they mean they have selectively chosen sources that support their stance and discounted those that didn't. Some believe global warming is happening just because Al Gore says it is, and others claim it can't be happening, just because Al Gore says it is. It is unfortunately human nature to look at things so subjectively.
I can't claim to know whether or not humans are hastening the process but I have seen some pretty drastic changes first-hand in the last few decades. More and more southern species which were once rare in my area are now commonplace, such as Brown Pelicans and Glossy Ibises. I am seeing the treeline at the edge of saltmarshes all along the local coast dieing back and receding, being replaced by stands of Phragmites. And one of my favorite winter activities of my youth, which was ice-fishing, can rarely be done in my area with any degree of safety. Any of these can have other explanations, but similar things are happening all over the world, seemingly sharing a common explanation.
I don't know if it is even possible to prove with 100% certainty that humans are speeding up the warming process or not. But should the difficulty of discerning this proof alone be reason to insist it can't be happening? My feeling is that most of the actions suggested to reverse the possible effect humans are having are not bad things to do anyway. Is it a bad thing to slow down the consumption of non-renewable resources? Is it bad to use more modern or alternate technology and end up spending less money driving your car or heating your home? Depending on what the costs of doing so are, there is much to be said for playing it safe when unsure of the correct course in life. Boy, that was wordier than I had planned. Perhaps my wife is right???????