The climate of our planet in the last 4 billion odd years has changed more times than anyone will ever know. We werent there on nearly all of them.....yet it still changed. It will change again...and again....and again. Its nothing to do with us, at least the climate change is not because of our actions. Ours isnt the only planet on which changes have been noted either.
I'm not aware that any serious scientist is claiming that the climate isn't changing, the issue is the rate at which it is changing. The current rate of change is off the scale compared to the changes in deep-time. So something must be different this time. One of the things we know for sure that is different this time is that man is around and is polluting the planet like never before. One of the main reasons why we can't predict what is going to happen this time is that we have nothing to compare it with. All the other significant changes in the climate have happened at a much, much slower rate.
There is a very strong correlation between CO2 in the atmosphere, and global temperature. CO2 concentrations are rising rapidly (search for "Keeling Curve") and so it follows that global temperatures are also going to rise. The main contributor of CO2 to the atmosphere at the present time is man.
To say other planets have undergone climate change is irrelevant, as their atmospheric conditions are nothing like ours. If we found another planet that was currently undergoing rapid climate change, and it hosted a bacteria that was producing huge amounts of greenhouse gases, would you assert that it can't be down to the bacteria because we've undergone climate change and yet we don't have those bacteria?
Graham