Current beliefs on climate change: I would defer to the IPCC.
I would have first came across the subject while I was at school about 25 years ago (probably not at school, or at least only in passing). I think I accepted the idea as plausible based on a basic understanding of the physics and on scientific authority (probably of science popularisers). I don't remember anyone mentioning quantitative warming estimates, or anyone being particularly alarmist or alarmed.
My current views aren't based on detailed investigation. I would say they are based mostly on (a) things on climate change I come across in general scientific reading e.g. I have read most abstracts of articles and letters (and try to struggle through full text of all climate-related ones) in Nature in the last 7 years or so, and more sporadically in the last 20 years; and (b) the Azimuth blog of John Baez et al, who seem genuinely interested in what you can and can't get out of climate models.
One thing I learned on reading your posts (if I got the right impression) is that "respectable skeptics" agree about the C02 increase, that it is human-caused, that there is some warming, that the warming is C02 caused, but they disagree about the magnitude and confidence of future projections. I did not know this.
"respectable skeptics" agree about [...]
My impression (which is only a handwavy impression, and I'll be happy to be corrected) is that climate-change "skeptics" used to say that there was no global warming, then that there was some but it probably wasn't anthropogenic, then that there was some and some of it was anthropogenic but that it was a good thing rather than a bad thing, and now that there is some and some of it is anthropogenic and it's probably a bad thing but the cost of stopping it would outweigh the benefits. In other wo...
Note: Please see this post of mine for more on the project, my sources, and potential sources for bias.
I have written a couple of blog posts on my understanding of climate forecasting, climate change, and the Anthropogenic Global Warming (AGW) hypothesis (here and here). I also laid down the sources I was using to inform myself here.
I think one question that a number of readers may have had is: given my lack of knowledge (and unwillingness to undertake extensive study) of the subject, why am I investigating it at all, rather than relying on the expert consensus, as documented by the IPCC that, even if we're not sure is correct, is still the best bet humanity has for getting things right? I intend to elaborate on the reasons for taking a closer look at the matter, while still refraining from making the study of atmospheric science a full-time goal, in a future post.
Right now, I'm curious to hear how you formed your views on climate change. In particular, I'm interested in answers to questions such as these (not necessarily answers to all of them, or even to only these questions).