> My understanding is [...] As far as I can tell [...]
Kudos for being honest that those parts are just your opinion / your understand. So I skipped those bits, as I doubt you're qualified to reliably inform me on the topic.
> No one is denying the scientific effect of atmospheric CO2, or that the Earth is warming - what is under debate is the balance between natural climatic variation and human CO2 emission
So you concede the reality and importance of climate change, and that CO2 in the atmosphere is one of the major contributors, yet somehow want to debate "cause". This implies your only difficulty is with the idea that human activity has been the major contributor to CO2 in the atmosphere, right? Well, that's easily fixed, allow me to direct you to the simple answer:
TL;DR: Yes natural CO2 is emitted, but it also *naturally absorbed*. It is a cycle, called the "Carbon Cycle". The problem is this: *Human emissions create an IMBALANCE between what is emitted what is absorbed*. Get it now? It's not about "how much is sent UP by us vs nature", it's about how we send up an EXCESS that cannot be absorbed back into the cycle.
> what is under debate is the balance between natural climatic variation and human CO2 emission
As you can see, no; it is not under debate. Like most in your camp, it is only under debate in your own mind, probably because you have only heard the rhetoric from other sources and not really understood the issues at all. Someone probably said to you "this is crap because nature sends up FAR more CO2 than humans do!" and that's all you heard. But it's a false, deceptive, pathetic argument, meant to mislead in order to allow companies to keep polluting. As such, it is also an immoral argument and those pedalling it should feel ashamed for contributing to ignorance and quite possibly future misery.