The position on the part of Climate Change Supporters is that we are releasing more gas emissions than would normally be present in our atmosphere (which is a fact since we burn fossil fuels) while simultaneously removing the natural checks for those emissions (deforestation and sea plant destruction coupled with mass farming, since those organisms would normally intake the extra CO2 and other gases we're pumping into the atmosphere).
Now, we don't put out the levels that, say, a volcanic eruption does, but over time we are altering the chemical balance of our atmosphere (more methane and CO2) and could, if left unchecked, cause a runaway greenhouse effect (the gas balance reaches a point where Earth traps more heat). The argument is really about how far out we are from that occurring, if at all. I don't think it's a big threat for the next 100 years, if not longer, and the planet has been hotter, more oxygen rich, and had more volcanic activity in the past and supported life. It's really about how we can try to maintain hospitable conditions for humans for as long as possible within the factors under our control. We might not kill off all life on Earth or turn Earth into Venus atmospherically, but we could f*ck up the food table on this planet so much that eventually things get arid and food becomes scarce for a population of billions upon billions, though we have millions starving as things stand now. IMO, that's a legit issue, primarily because I actually care about my fellow man and not just my own interests, though I don't think we're gonna have "The Day After Tomorrow" like weather catastrophes occurring, and that's where the scare tactics and BS starts to come into play. At the end of the day we DO need better, cleaner technology and better resource management, and in my opinion, that's a priority.